Root Maggot Management Research

Root maggots represent a serious problem for growers of cruciferious vegetables like radish, cabbage, and rutabaga. A series of studies were conducted to evaluate the incidence and severity of damage, chemcial, biological, and integrated management options for this pest.

The articles are available in HTML or as PDF files.

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2010 Root Maggot Control Trial PDF
Root maggots (Delia radicum) represent a major problem for growers of cole crops in Saskatchewan. Maggots damage to the roots renders them less capable of delivering the water and nutrients required for optimal growth. In vegetable crops grown for their roots (rutabaga and radish), the scars left by the feeding maggots make the roots unmarketable. Biological and cultural control practices may reduce problems with maggots but some form of chemical control is usually required.

Control of Flea Beetles and Root Maggots in Diakon Radish and Rutabagas PDF
Root maggots and flea beetles are some of the most significant pests for growers of cruciferous vegetables like radish, rutabaga, and cabbage. This study evaluates a range of the presently approved products for control of flea beetles and root maggots, and examines the efficacy of alternative control options.

Evaluating New Products for Root Maggot Control PDF
Admire (imadochloprid) is a new systemic pesticide presently registered in Canada for the control of Colorado Potato beetles and aphids in potatoes. This study evaluates the effectiveness of Admire for root maggot control in cruciferous vegetables.

Management of root maggots in vegetable crops PDF
Saskatchewan vegetable growers are finding it increasingly difficult to control damage by root maggots (Delia sp.) utilizing presently registered pesticides or standard production practices. This project examined a range of chemical and cultural control strategies for root maggots in rutabaga.

Powerpoint Presentation: Root Maggot - Biology and Management Options (2.4 MB)
The life cycle andvegetable host range of the root maggot is discussed. Integrated pest management options are also presented.

Root Maggot Control Trial

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Root Maggot Control  (2009 and 2010)

Root maggots (Delia radicum) (Fig. 1) represent a major problem for growers of cole crops in Saskatchewan.  Maggot damage to the roots renders them less capable of delivering the water and nutrients required for optimal growth.  In vegetable crops grown for their roots (rutabaga and radish), the scars left by the feeding maggots leave the roots unmarketable (Fig 2).   In Canada, the standard approach to controlling maggots is to apply the insecticide chlorpyrifos (Lorsban) as a root drench on several occasions through the growing season.  The timing of chlorpyrifos application is targeted to correspond to the development of the maggot population.  Applications of chlorpyrifos must cease 30 days prior to harvest for rutabaga.   As organophosphates like chlorpyrifos are relatively toxic to both the applicator and the environment this type of pesticide has been targeted for phase out as soon as effective reduced-risk alternatives are identified.  

The objectives of this project were to;

1. test new reduced-risk insecticides for potential use in root maggot management
2. look for alternatives to insecticides as a means to control root maggots    

Insecticide Testing

The insecticide testing project was conducted in 2009 and 2010 at the U of S Vegetable Crops Research Station in Saskatoon.  Because of heavy root maggot pressure, production of vegetable cole crops is difficult at this site – even with repeated applications of pesticides.   
            Each treatment consisted of two adjacent rows of rutabaga (cv. Laurentian) spaced 60 cm apart, with each treatment replicated four times in a randomized complete block design.   The plants were hand thinned to a 10 cm spacing soon after emergence.  The plot was kept weed free by hand weeding.  Overhead irrigation was used to maintain optimum soil moisture levels throughout the growing season.  No problems with crop development or health were observed, beyond the expected problems with root maggots.  Cool growing conditions in both years were favorable for rutabagas.    
            The root maggot control treatments tested in 2009 and 2010 are outlined in Table 1.  Except for Trmt 1, the control products were applied three times over the season, starting at the first true leaf stage and then repeated at two week intervals (late June, mid-July and late July).   In all cases except Trmt 9 the control products were applied using 80-08 flood nozzles to deliver the product in the equivalent of 1000 L/ha of water over a 15cm wide spray band.   Trmt 9 was applied to the foliage in 100 L/ha of water using 8002 flat fan spray nozzles.   Trmt 10 involved application of just water and served as the control.  Only Trmt 6 (chlorpyrifos) is presently registered for root maggot control on rutabaga in Canada.   
            The efficacy of each round of spraying was evaluated 2 weeks after treatment.  For the first 3 rating dates 10 plants from each treatment replicate were evaluated for root damage using a semi-quantitative 0-5 rating scale (0=no damage : 5=severe damage)( Dosdall et al. 1994)(Fig. 3).  A 0-4 damage scale was used for evaluations made at the final harvest (King and Forbes 1954)(Fig. 4).  This method of evaluation is based on the number of knife cuts required to remove the maggot damaged tissues from mature roots (maximum of 4).   Roots requiring 2 or fewer cuts to remove the maggot damaged tissues were considered marketable.

Results
Root maggot damage was more severe in 2009 than in 2010.  In 2009 none of the roots in the untreated controls were marketable by the end of the growing season (Table 1), but in 2010 about 1/3 of the roots in the untreated control treatments were still marketable (Table 2).  In both years maggot damage to the roots increased as the season progressed.   The chlorpyrifos treatment provided a significant degree of maggot control in both years, resulting in a relatively high proportion of the crop meeting grade standards at the final harvest.  The only other treatment that provided any consistent degree of control was the high rate of the presently unregistered product CCCCC.    

Table 1.  Influence of pesticides on root maggot damage rates in rutabaga (Saskatchewan 2009). 

1. For Jul 8, Jul 23 and Aug 7 … a damage rating of 0=no damage : 5 = severe damage.
2. For Aug 25 … a damage rating of 0 = no damage : 1 = damage removed with one cut, 2 = two cuts,  4 = maximum.  
3. Roots with a damage rating of 2 or less were considered to be marketable.   

Table 2.  Influence of pesticides on root maggot damage rates in rutabaga (Saskatchewan 2010).  

1. For Jul 7, Jul 23, Aug 6 and Aug 26  … a damage rating of 0=no damage : 5 = severe damage.
2. For Oct 6 … a damage rating of 0 = no damage : 1 = damage removed with one cut, 2 = two cuts,  4 = maximum.
3. Roots with a damage rating of 2 or less were considered to be marketable.  

This trial illustrates the severity of the maggot problem facing growers in Saskatchewan.    Chlorpyrifos, which presently represents the standard treatment for maggots across Canada, provided a high level of maggot control in both years.   While several of the new unregistered reduced risk insecticides provided an acceptable degree of protection when pest pressure was relatively light they did not provide sufficient protection when pest pressure was high.   This suggests that while these new products may be useful as part of a maggot management program there is still a need for Lorsban.          

Root Maggot Interference Trials

Alternative (non-chemical) methods of root maggot management are clearly desirable – especially as the most effective chemical control product (Lorsban) is under regulatory review.  One possible alternative approach to maggot control is to physically restrict the pests’ access to the crop.    Potential options for interfering with access to the crop include;

a) covering the crop with a semi-transparent field cover.  Field covers are already extensively used in production of high value vegetable crops – primarily because the covers produce a warm, sheltered growing environment.   Field covers may also protect the crops against insect pests.  

b) fencing.   The adult root maggot fly is know to prefer to fly less than a meter above the ground when seeking egg laying sites at the base of its host plant.    It is therefore possible to use relatively short fences to protect the crop – assuming that the fence is designed from material that is too finely woven to allow the root maggot flies access to the crop.

In 2010 we assessed field covers and fencing as a means of protecting a rutabaga crop.  The trial was conducted utilizing the production practices previously outlined.  Each treatment consisted of 15 rows (8 m long) with a single plot for each treatment.   As soon as the crop was seeded the “interference” treatments were put into place.   The field cover was Agryl P-17 (1.7 oz/m2)(Fig 5).  This material is thin and light weight, allowing about 80% of the sunlight to pass.  The mesh for the woven polyester is far too small to allow passage by root maggot flies.   The entire plot was covered with a single sheet of the cover, with the edges sealed with soil.   The covers were only removed briefly to allow for thinning, weeding, and crop evaluation. 
            The maggot fence was constructed by sandwiching a sheet of finely woven row cover (Agryl P-17) between two layers of standard plastic snow fence.    The fence stood about 1.3 m in height and was supported at the corners by angle iron fence posts (Fig. 5).
            While the field covers and fences may be effective at preventing access to the crop by passing maggot flies, they cannot protect the crop against any flies that may emerge from pupae that overwintered within the protected area.   For that reason half of each plot was treated at regular intervals with the standard insecticide (Lorsban).
            A late planted crop was also tested to determine if it might be possible to reduce crop damage by avoiding at least some of the peak egg period.   In previous trials we had found that egg laying commenced in late May and peaked in mid-late June.  The late planted crop was therefore planted in the 3rd week of June. 
            Maggot damage was assessed at intervals over the course of the 2010 growing season using the previously described damage rating system.  A standard non-protected plot was used in all comparisons.
The plot was harvested once the crop matured (Oct. 6) at which time the weight and degree of root maggot damage was evaluated.  At the final harvest the degree of root maggot damage was evaluated based on the number of cuts required to remove visible maggot damage from the roots. 

Results
Crop growth - within 3 weeks of crop emerge it became clear that both the field cover and fencing treatments were enhancing crop growth relative to the non-covered treatments - this beneficial effect persisted through the growing season for the fenced trial.   It seems likely that the crop covers and fences were protecting the crop from wind damage.   

            By mid-summer more leaf disease (Cercospora sp) was present in the covered crop than for any of the other treatments.  This disease did not appear to progress much over the course of the growing season and was unlikely to have affected yields.   However by mid-summer the covered crop began to take on an abnormally “leggy” appearance – the tops were taller and thinner than normal and development (swelling) of the tap root appeared to stall.  
            Heavy rainfall interfered with pre-plant tillage operations for the late planted crop and the crop was seeded into sub-optimal soil conditions.   This resulted in an inferior stand relative to the early planted crop.  Otherwise, the late planted crop appeared to do well. 

Treatment management – the field cover was easy to install and maintain.  The cover stayed in place through several storm events and was easily repositioned if it did blow loose.   The fencing required much more work to install and maintain.  The fence tended to blow down during storm events despite efforts to re-enforce the supporting posts.   The fence also interfered with the machinery used in crop maintenance – by contrast the field covers were easily removed and replaced once tillage or spraying was completed. 

Maggot control – root maggot damage to the roots increased in all treatments as the season passed.    The field cover and to a lesser extent the fence provided some degree of maggot protection over the duration of the growing season (Table 3).  Combining the field cover or fencing treatments with Lorsban treatments provided a high level of crop protection but it is unlikely that the observed additional degree of protection achieved with the cover or fence would have warranted the associated costs of materials and labor required to construct and maintain these crop protection systems.  
            Delaying planting by … weeks did not appear to mitigate the maggot pressure (Table 3) – perhaps an even later planting date is required in order to avoid the first egg laying generation but this will necessarily delay crop development – leading to a loss in yields and also increasing the risk of damage to the crop by the larger 2nd generation of maggot flies that emerges in late summer.   

Yields – root weights prior to grade out due to maggot damage were increased by the Lorsban treatments (Table 3).  This would be expected if the Lorsban treatments had protected the feeder roots from damage by the maggots.   Average size of the roots was larger in the fenced treatments than for other treatments planted at a comparable time.   We had noted that the fencing treatments appeared to be enhancing crop growth.   The average root size for the late planted crop was also large – but this likely reflects the fact that the stand for this crop was quite thin – so each plant was provided with more abundant growing space than for the earlier planted crops.   The Lorsban treatments substantially reduced the amount of root maggot damage at the final harvest and this resulted in a substantial increase in the proportion of the crop that would have met grade standards for freedom from excessive maggot damage.  None of the maggot interference treatments substantially increased the proportion of the crop that made grade standards – except that when Lorsban was not used, having the crop protected by a field cover did increase the proportion of roots that met grade tolerances for maggot damage (Table 3).

Table 3.  Root maggot damage and root yield data for rutabaga over the course of the 2010 growing season as influenced by various crop protection systems, with or without supplemental application of Lorsban
           

1 Root damage graded according to method of Dosdall et al; numbers represent means of four 10-plant samples
2  Root damage rating based on number of cuts required to remove maggot damage (0 to maximum of 4)
3  Roots considered marketable if maggot damage could be removed with a maximum of 2 cuts 

While the use of crop covers or fencing to protect crops from maggot damage makes sense in principal – in practice neither technique provided any consistent or economically significant degree of protection.   Both fencing and covers involved considerable additional costs and tended to interfere with other aspects of normal crop maintenance.   The crop covers also appeared to compromise crop health late in the season.    

Companion Cropping Trial

Companion cropping involves mixing two or more crop species within a field in an effort to enhance productivity – with the companion crops “helping” the other crop by repelling insect pests or by providing nutrients. 
            Root maggot flies seeking an appropriate site for egg laying identify suitable hosts by a combination of visual cues and odor.  The presence of non-host plants in the immediate vicinity of the potential host tends to “confuse” the flies, reducing egg laying.   The flies are attracted by the volatile sulphur-based compounds that give members of the Brassicacea their characteristic odor. 

This study sought to determine whether root maggot damage to a high value vegetable crop (rutabaga) could be mitigated by inter-planting the rutabagas with either a non-host “companion” crop like wheat that would physically interfere with egg laying or a faster growing and stronger smelling (more attractive) brassica crop like oilseed mustard.

The trial was conducted at the University of Saskatchewan Vegetable Crops Research site in Saskatoon (see previous for site details).  The rutabagas (cv. Laurentian) were planted in rows alternating with rows of;
            a) rutabaga (standard) 
b) oilseed mustard 
c) wheat
The wheat and oilseed mustard companion crops were planted 1 week ahead of the rutabaga so that these companion plants would be larger than the rutabaga.  It was anticipated that the efficacy of these treatments in either or confusing or diverting the maggot flies would be enhanced by increasing plant size. 
            Each plot row was 8 m long and each treatment was replicated four times in a randomized complete block design.  The plot was kept weed free by hand weeding.   Due to abundant rainfall in 2010 no supplemental irrigation was required.  
            One half of each treatment row of rutabaga (4m) was treated at regular intervals with Lorsban (see above for description of treatment method).  This allowed for comparison of the companion treatments with and without supplemental control via insecticides. 
            At regular intervals over the growing season root samples (n=10) were evaluated for root maggot damage using the previously described rating system.   The plot was harvested once the crop matured (Oct. 6) at which time the weight and degree of root maggot damage was evaluated for 20 randomly selected roots from each treatment replicate.  At the final harvest the degree of root maggot damage was evaluated based on the number of cuts required to remove visible maggot damage from the roots (0 to a maximum of 4).   Roots requiring 2 or less cuts were considered to be marketable. 

Results
Crop growth - both the oilseed mustard and wheat established quickly and, as designed, were well advanced prior to the emergence of the rutabaga (Fig. 7).   Both of the “companion” crops grew more quickly than the rutabaga – this resulted in considerable shading of the rutabaga crop – especially by the sprawling oilseed mustard plants.  To reduce this competition, both companion crops were “topped” to the height of the adjacent rutabaga plants in mid-July and again in mid-August (Fig. 8).  Despite this effort to control competition by the companion plants, the root size of the rutabaga plants growing next to the oilseed mustard plants were smaller than normal, with the largest roots occurring when rutabaga was grown adjacent to other rows of rutabaga (Table 4).

Maggot control – the extent of maggot damage to the roots increased as the season progressed, except towards the end of the season, when the roots grew more rapidly than the maggot damage.    
Neither of the companion crops had any significant impact on the intensity of root maggot damage to the rutabaga crop at any of the early sampling dates (Table 4)   As seen in the other experiments, the multiple Lorsban treatments again provided a significant degree of maggot protection throughout the growing season.        

Yields – both companion crops reduced yields of the adjacent rutabaga plants (Table 4).  This occurred despite the fact that the companion plants were pruned back to the height of the adjacent rutabagas on two occasions during the growing season.   Competition is one of the major limitations of using companion planting to limit pests in relatively non-competitive horticultural crops.   While the damage rating taken early in the growing season had shown little consistent benefit of the companion plantings, at the final harvest having mustard as a companion plant significantly reduced root maggot damage to the adjacent rows of rutabaga.   The degree of root maggot protection was equivalent to the degree of benefit achieved with multiple applications of Lorsban.    When considering the proportion of the crop that would have been considered to be marketable based on freedom from excess root maggot damage (damage rating <2.0), the combination of Lorsban+mustard as a companion crop was clearly superior (78% marketable), while rutabaga grown without either insecticide or a companion crop had the highest rate of grade-out due to excessive maggot damage (23% marketable).     

Table 4.  Root weights, root damage ratings and % marketable roots as influenced by various companion crops with or without supplemental application of Lorsban
                  

1 Root damage graded according to method of Dosdall et al; numbers represent means of four 10-plant samples
2  Root damage rating based on number of cuts required to remove maggot damage (0 to maximum of 4)
3  Roots considered marketable if maggot damage can be removed with a maximum of 2 cuts 

Establishing and maintaining the companion crops required some additional effort.  The yield data suggests that the companion plants need to be managed in a manner that more effectively limited their ability to compete with the rutabaga crop (ie; more frequent/aggressive pruning).  This management would need to be achieved without compromising the value of these crops as means of protecting the target crop from insect damage.   Another possibility would be to select companion plants with a less aggressive growth habit (ie; kale as a mustard crop and spring planted winter wheat as a cereal).  However it is thought that larger plants are more effective as an attractant/distraction or more effective as a screen – therefore a less vigorous companion crop may be undesirable.   While the companion crops appeared to have little consistent beneficial effect on root maggot damage ratings taken over the duration of the growing season, by the final harvest having mustard as a companion significantly reduced the severity of maggot damage and increased the proportion of the crop that would have made grade standards for freedom from excessive maggot damage.    The validity/basis of this apparent anomaly  needs to be explored by further testing.    It is noteworthy that the increase in marketable % achieved using mustard as a companion plant almost exactly offset the loss in total yield caused by competition between the rutabaga crop and the companion mustard planting.  

Literature Cited

Dosdall, L. M., M. J. Herbut, and N. T. Cowle. 1994.  Susceptibilities of species and cultivars of canola and mustard to infestation by root maggots (Delia spp.) (Diptera: Anthomyiidae).  Can. Entomol.  t 126: 251-260.

King, K. M. and A.R. Forbes (1954).  Control of root maggots in rutabaga. J. Econ. Entomol. 47:607-615.

THIS RESEARCH WAS SUPPORTED BY THE AGRICULTURE DEVELOPMENT FUND
OF SASKATCHEWAN AGRICULTURE

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Control of Flea Beetles and Root Maggots in Diakon Radish and Rutabagas

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Growers of cruciferous vegetables like radish, rutabaga and cabbage face many insect pests - but flea beetles and root maggots seem to be causing the greatest problems in Saskatchewan. It takes relatively few beetles feeding on the emerging seedlings to seriously delay crop development or even kill the young seedlings. Root maggot damage occurs later in season. The developing maggots feed on the roots, weakening the plants, slowing crop development and leaving the roots open to invasion by rot-causing diseases.

While problems with both flea beetles and root maggots seem to be on the rise, the options available to the grower to control these pests continues to dwindle. As early control of flea beetles is critical, treatments that protect the emerging seedling are desirable. However, the range of products approved for use in the furrow or as seed treatments for vegetable crops is limited and the efficacy of the approved products is questionable. Chemical control of root maggots is already difficult and the potential loss of chlorpyrifos may make this even more challenging. The objective of this study was to evaluate a range of the presently approved products for control of flea beetles and root maggots and to examine the efficacy of several alternative control options.

The trial was conducted at the Vegetable Crops Research plots of the University of Saskatchewan. This site has a long history of vegetable crop production and is heavily infested with root maggot. Populations of flea beetles in the plot area were not as high as situations where vegetable fields are surrounded by canola. Diakon radish (cv . April Cross) and rutabaga (cv. Laurentian) were chosen as the test crops as they experience both root damage due to maggots and shoot damage due to flea beetles. As diakon develops more rapidly than rutabaga, selection of these two crops should provide information on how insect damage is influenced by the rate of crop growth and duration in the field.

The crops were seeded in early May using a small plot seeder. There were a total of 6 different insecticides tested. Each treatment plot consisted of a twin 4.5 m long rows of test plants. Each plot was replicated 4 times in a randomized complete block design. The insecticides were applied as seed treatments or in the furrow adjacent to seed at the label specified rate. Flea beetle damage to the emerging seedlings was visually evaluated 10 days after the crops emerged. One half of each plot was treated with a soil drench of chlorpyifos in early June as the maggots began to develop.

The diakon were harvested at maturity on July 23. The rutabagas were harvested October 9. Root yields and the incidence and severity of maggot damage to the roots were evaluated at harvest. The roots of both crops were considered marketable if less than 15% of the surface area of the root showed maggot damage.

Results

The warm dry conditions in the spring of 2004 were near ideal for emergence of the crops, but were also very favorable for flea beetles. Flea beetle damage was more severe in the slower growing rutabaga than in the diakon. Although several of the test treatments reduced damage by flea beetles relative to the untreated controls - carbofuran (furadan) was the only treatment that provided an adequate level of control (Table 1). Few of the seedlings actually died due to flea beetle damage but at 6 weeks after seeding, the carbofuran-treated plants were twice the size of the other treatments. This illustrates the growth-retarding effects of flea beetle damage.

Maggot damage was not as severe in this trial as has been observed in previous years. Whether this reflects growing conditions or some site-specific factors could not be determined. In the diakon crop, terbufos, chlorpyrifos and tefluthrin applied as seed treatments increased marketable yields relative to the controls (Fig. 1). Some of this yield response was unrelated to protection from root maggot damage, as the diazinon and tefluthrin treatments actually produced the highest % of roots free of excessive root maggot damage. In the slower growing rutabaga crop, none of the seed treatments provided any significant degree of protection against root maggots through to the final harvest (Fig. 2).

Table 1. Influence of seed or seed row applied pesticides on flea beetle damage to diakon and rutabaga.

Ratings : 0 = no damage, 5 = dead
Values within columns followed by the same letter are not statistically different (p=0.05)

Fig.1.Impact of seed applied insecticides on root yields and quality of diakon.	Fig.2.Impact of seed applied insecticides on root yields and quality of rutabaga.
	In both the diakon and rutabaga crops, treatment with chlorpyrifos as a soil drench was quite effective at reducing losses to root maggots and improving the overall appearance of the crop (Fig. 3). The soil drench provided a greater degree of root maggot protection to the diakon as the interval between treatment and harvest was quite short.
Fig. 3. Impact of root drench with chlorpyrifos on daikon quality.

Conclusion

Although carbofuran provided the best protection of the emerging crop against flea beetle damage, this protection did not provide any longterm benefit in terms of higher yields. Greater benefits could be expected if growing conditions and pest populations were more severe. Some of the seed-applied pesticides helped protect the fast maturing diakon roots against maggot damage - but unfortunately none of the effective products are registered for use on this crop.

None of the seed-applied pesticides provided any significant degree of root maggot protection to the rutabaga crop. This reflects the greater length of the period from treatment to harvest in this crop. Soil drench application of chlorpyrifos provided effective protection against root maggots in both crops ... but care must be taken to conform with the pre-harvest interval for this product.

NB - many of the products included in these trials are not approved for use in vegetable crops and their inclusion in these trials does not represent a recommendation for their use.

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Evaluating New Products for Root Maggot Control

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Root maggots (Delia sp.) attack the root systems of a range of cruciferous vegetable crops (cabbage, radish and rutabaga). Maggot damage is of importance if;

• a) it impairs the function of the root system
• b) if the damage mars the appearance of the marketed portion of the crop (ie; damage to rutabaga and radish roots).

Traditionally, Saskatchewan's vegetable growers have utilized a range of pesticides to minimize losses to the root maggot. However, growers are finding it increasingly difficult to control damage by root maggots utilizing presently registered pesticides or accepted production practices. This may reflect higher maggot populations as a result of extensive cultivation of canola and/or increasing resistance to pesticides.

Admire (imadochloprid) is a new systemic pesticide which can be applied through either the foliage or via soil application. Admire is presently registered in Canada for the control of Colorado Potato beetles and aphids in potatoes. In this trial, Admire was applied as either an in-furrow treatment at planting or as a foliar treatment to rutabagas. Lorsban (chlorpyrifos) was selected as the chemical check treatment. The trial was conducted in Saskatoon and Outlook. Both sites have a long history of vegetable crop production and have heavy maggot pressure.

The in-furrow Admire treatment was applied at seeding in a band at the rate of 0.1 g a.i/m of row. The foliar Admire treatment was applied at 0.08 L/a in the first week of July. Lorsban 10 G was applied as an in-furrow granular treatment at seeding (0.8 kg/1000 m of row) followed by foliar application of Lorsban 4E (84 ml/a) in the first week of July and again in early September. The foliar treatments were timed to coincide with the larval stage of the pest. Maggot damage was evaluated at three times during the growing season by harvesting 10 plants from each treatment row. Roots with more than 25% of the root surface damaged were considered non-marketable.

Results

The intensity of root damage increased with time at both sites, although maggot pressure was consistently more severe in Saskatoon than in Outlook. The Admire treatments provided little protection from the maggots at either site. Lorsban provided quite effective control early in the season, but as time passed damage began to accumulate even in this treatment. By the final harvest, none of the rutabagas at the Saskatoon site were marketable. However, in Outlook over 60% of the roots in the Lorsban treatments were marketable at the final harvest - this compares to less than 10% marketable in the control treatments.

NB- Admire is not presently registered for use for maggot control in vegetable crops.

Figure 1 & 2. Influence of several insecticides on marketable yields of rutabagas in Saskatoon and Outlook.

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Management of root maggots in vegetable crops

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Saskatchewan vegetable growers are finding it increasingly difficult to control damage by root maggots (Delia sp.) utilizing presently registered pesticides or standard production practices. This project examined a range of chemical and cultural control strategies for root maggots in rutabaga.

This project comprises 3 sections:

• Trial 1. Time of planting
• Trial 2. Chemical control
• Trial 3. Crop covers

Trial 1. Time of planting

Adults of the root maggot emerge in the spring, lay their eggs at the base of cole crop seedlings and then die. The time of emergence if the adults is determined by weather conditions. By delaying planting, it may be possible to avoid having eggs laid in the emerging crop. However, delayed planting may also reduce yields. This study examined the influence on yields and incidence of root maggot damage in rutabagas of varying the planting date.

The trial was conducted from 1995 – 1997 in Saskatoon on a heavy clay soil with a long history of root maggot problems. The seed was treated with Vitavax RS (Carbathiin + Thiram + Lindane) to provide early control of flea beetles. This seed treatment does not protect against root maggots. Lorsban 15G was applied in each seed row at the recommended rate to provide a baseline level of root maggot control. Twin rows of rutabagas (cv. Laurentian) were seeded in early May, late May, early June, or late June. Early May is the normal planting time for rutabagas. The in-row spacing was 10 cm, with 50 cm between rows, and there were four replicates of each treatment. No insecticides were applied after seeding. The crop of overhead irrigated at regular intervals during the growing season. The crop was harvested in mid-September and rated for yields and root maggot damage.

Results

In the 1995 trial, delaying planting from early May until late May had no significant impact on final yields, but substantially reduced the incidence of maggot damage to the roots. (Table 1). Further delays in planting reduced yields without providing any further reduction in maggot damage. In the 1996 trial, each delay in planting resulted in a substantial reduction in total yields (Table 1). However, grade out to excess maggot damage was substantial in the early planted crops and consequently, marketable yields for the first three planting dates were not significantly different. Maggot damage was much more severe in the 1996 trial than in the previous season. In 1997, the earliest planting produced the highest yields, however, grade out to excessive maggot damage was also high in the early planted crop (Table 1). Consequently, marketable yields for the first and second planting were comparable. Delaying planting until June resulted in exceptionally poor yields with no significant reduction in grade out to maggot damage. Maggot damage was less severe in 1997 than in 1996.

Table 1. Influence of planting date of yields and grade out to maggot damage for rutabagas (Saskatoon 1995-1997).

*values within columns followed by the same letter are not significantly different (p=0.05).

Trial 2. Chemical control

Chemical control of root maggots involves seed treatments, row treatments, or soil drenches or some combination of the above. In trials conducted from 1994 – 1997, a range of approved and experimental products were evaluated for the efficacy in controlling root maggot damage in rutabagas.

N.B. Inclusion of a product in this test does not imply a recommendation for its use. Several of the granular products tested in 1994 (Guthion, Thimet) were dropped from subsequent trials as they were no longer commonly available.

Trials were conducted at the Saskatoon site described above in 1994 – 1997 and on a sandy loam site near Outlook in 1996 and 1997. Rutabagas (cv. Laurentian) were seeded in late May in twin 10 m long rows for each treatment, with 50 cm between rows, 1 m between treatments, and 10 cm between plants within the row. Each treatment was replicated four times.

The in-furrow seed treatments were applied to the rows immediately prior to seeding. Half of each plot received recommended rates of Lorsban 4E as a soil drench in late June and again in mid-August. The other half of each plot received only the seed row treatments. This allowed for determination of the relative importance of early versus late season control of the maggots. The second Lorsban treatment was too close to harvest for safe consumption of the crop, but was included to determine if the crop could be safeguarded chemically against the late generations of maggots that cause losses just prior to harvest. The Lorsban soil drench was applied in 1000 L/ha water using flood nozzles. The drench treatments were followed with a light irrigation. The plots were harvested in mid-September, graded, and evaluated for maggot damage.

Trial results can be summarized as:

1994 - None of the chemical control products tested provided a satisfactory level of maggot control (>80% marketable). Counter 5G produced the highest marketable yields. Lorsban 15G provided excellent maggot control, but had suppressed yields (Table 2).

1995 - Lorsban 15G significantly reduced root damage relative to the controls. Post-planting application of Lorsban 4E increased marketable yields and the % marketable, while also decreasing the average extent of damage on the marketable roots (Table 3).

1996 - None of the products tested as a seed treatment provided a significant degree of maggot control through until the final harvest. Post planting application of Lorsban 4E increased marketable yields and the % marketable, while also decreasing the average extent of damage on the marketable roots (Table 4).

1997 - Total yields for all the various seed treatments were similar (Table 4). Grade out to maggot damage combined with growth abnormalities averaged 70% of the crop. None of the treatments reduced grade out to excess maggot damage relative to the non-treated controls. The post-planting application of Lorsban also had no effect at the Saskatoon site. Yields at the Outlook site were slightly lower that in Saskatoon. There were again few differences in total yields for the various seed treatments. The controls did produce lower total yields than some of the in-furrow treatments. This likely reflects some loss of stand to heavy flea beetle pressure early in the season. None of the treatments reduced grade out to maggots relative to the controls. One of the experimental products (EXP-4) had the lowest grade out to maggot damage and highest yields after grade out. The mid-season applications of Lorsban resulted in increased total yields, reduced losses to excessive maggot damage, and resulted in a 43% increase in marketable yields at the Outlook site.

Table 2. Yields and root maggot ratings for rutabagas treated with in-furrow and post-planting insecticides.

* The difference between + Lorsban Spray and - Lorsban Spray is significant at p=0.05.

Table 3. Yields and root maggot ratings for rutabagas treated with in-furrow and post-planting insecticides.

* The difference between + Lorsban Spray and - Lorsban Spray is significant at p=0.05.

Table 4. Yields and grade out to maggot damage for rutabagas with in-furrow and post-planting insecticides.

* The difference between + Lorsban Spray and - Lorsban Spray is significant at p=0.05.

Conclusion

Although several of the in-furrow granular insecticides tested provided effective short-term control of root maggots, none provided the consistent, season-long maggot control required for crops such as rutabaga. Product efficacy seemed to vary between seasons; likely in response to differing environmental conditions and pest population levels. Multiple post-planting applications of Lorsban reduces losses to maggot damage, but again did not provide complete control when pest pressure was high. Multiple applications of systemic products like Lorsban is not recommended due to problems with residue levels at harvest.

Trial 3. Crop covers

Transparent coverings constructed of woven polyester can be used to protect crops from insects. This trial examined the potential for using crop covers to protect rutabagas from root maggot damage.

The trial was conducted in 1995 and 1996 at the Saskatoon site described above. Four plots consisting of 4, 4 m long rows of the cv. Laurentian were seeded in mid-May. The seed was treated as described in Trial 1. The crop was thinned at emergence to 10 cm, with rows spaced 0.5 m apart. Two of the four plots were then covered with Agryl P-17 row covers. This material is lightweight and transmits 85% of the available light. The edges of the covers were buried 4 cm underground. Daytime temperatures under P-17 covers range from 5-8°C warmer than the surroundings. The covers were lifted briefly on two occasions to remove weeds. Two rows were harvested from each plot in late September for assessment of yields and maggot damage.

Results

The covers stayed in place will throughout the season, suggesting minimal opportunities for invasion by adult flies. The limited number of replicates prevented statistical analysis of treatment effects; the data presented represents trends. In mid-July, the non-protected plots had a high incidence of root maggot infestation (60-70%), while maggots were completely absent from the covered plots. The degree of damage to the roots in the non-covered plots was light, but it suggested the potential for more significant damage later in the season.

In both years, total yields at the final harvest were higher in the uncovered plots than under the covers (Table 5). The crop under the cover looked more vigourous early in the season, but lacked vigour later in the season. Covering the crop substantially reduced both the incidence and severity of maggot damage to the roots. There was some damage to the roots in the covered plots; we were unable to determine how the maggots gained access to the covered plants.

Table 3. Yields and maggot infestation levels for rutabagas covered with Agryl P-17 throughout the growing season.

*values within this column are significantly different at P=0.05

• Maggot severity:
• 0-4 = no trimming
• 5-9 = 1-3 cuts required to trim off surface maggot damage
• 10-20 = >3 cuts required to trim off surface maggot damage

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Related Papers

Waterer, D. 1998. Integrated management of root maggots in vegetable crops. ADF Project No. 94000077 Final Report