Coping with root maggots in Prairie canola
By Juliana J. Soroka Agriculture and Agri-Food Canada Saskatoon Research Centre
Crucifer root-feeding maggots occur in field Brassica crops across the Prairies, causing the greatest damage to canola fields in central and northern Alberta. No chemical options are available for their control in canola. Seed treatments currently utilized for flea beetle control are not efficacious against root maggots, and flies emerge over extended periods, making application of insecticide sprays impractical. Biological and cultural practices are the primary methods of root maggot management in canola. Fall cultivation, the use of vigorous canola cultivars, increased seeding rates, wider row spacings and suitable fertilization regimes will aid in diminishing root maggot damage. Because natural enemies are an important means of root maggot control, research is needed into co-ordinating cultural management practices with biological control strategies for maximum effectiveness of both strategies and a truly integrated crop management approach to maggot control.
Biology and damage
The crucifer-feeding Delia root maggot complex that attacks canola on the Prairies comprises five species. Cabbage maggot D. radicum (L.), turnip maggot D. floralis (Fall.), and seedcorn maggot D. platura (Meig.), are frequently encountered, while bean seed maggot D. florilega (Zett.) and D. planipalpis (Stein) occur in fewer numbers.
Delia spp. overwinter in cigar-shaped tobacco-coloured puparia five to 20 cm below the soil surface. Adults emerge from mid-May to mid-July. Shortly after emergence flies mate and females lay eggs, singly or in batches of up to 25 to 30, near the base of host plants, usually in cracks or under a thin layer of soil. The eggs hatch in three to 10 days and the white legless maggots burrow into the soil to feed on root hairs and on secondary roots. Developing through three instars, older maggots tunnel into the taproot of the plant. Maggots feed for three to four weeks and pupariate in the taproot or in the soil nearby. Pupation lasts two to three weeks or the entire winter. Root maggots have one or two complete generations per year in canola, depending on species, geographic latitude and local conditions.
In canola, severe maggot infestations can cause plant wilting, stunting and reduced flowering, decreased seed weight, and lower seed yields. If feeding tunnels are extensive and girdle the root, plant lodging and death can occur. Roots damaged by Delia spp. are more susceptible to invasion by root pathogens such as Fusarium than intact roots. Yield reductions from root maggot damage of 50 and 19 percent for crops of Brassica rapa L. and B. napus L., respectively, have been reported.
In a four-year survey of nearly 3000 canola fields across Western Canada, the greatest degree of damage over the largest area was found in central, western and northwestern Alberta, although localized areas with severely damaged roots occurred along the northern edge of the entire Parkland ecoregion. As well as ecoregion, soil type can play a part in degree of root maggot infestation of canola. Dosdall and co-workers found that the highest infestation rates in canola roots by maggots in Alberta occur in areas of black chernozemic soils, with plants in lighter soils receiving less root damage.
Seasonal weather affects Delia population densities. In Alberta canola root damage from root maggots is positively correlated with rainfall received during the growing season. Similarly, infestation rates in Manitoba have been correlated with average daily air temperature and total precipitation for June and July. Soroka and associates found that the severity of maggot infestation is decreased if the previous summer was warm and dry.
Natural enemies arguably provide the greatest control of root maggot populations in Prairie canola. Predators attack all stages of Delia spp. development. Rates of parasitism of Delia puparia of up to 95 percent have been recorded in the field. Studies are underway to examine the feasibility of introducing a European staphylinid species, A. bipustulata L., as an additional mortality factor to enhance the effectiveness of Aleochara control of root maggot populations throughout the summer.
Several species of entomopathogens, including the fungi Entomophthora muscae (Cohn) Fesenius and Strongwellsea castrans Batko and Weiser and the bacterium Bacillus thuringiensis Berliner exert some control of Delia spp. populations. A commercial formulation of Metarhizium anisopliae (Metsch.) Sorokin, a soil-dwelling fungus that is pathogenic to specific host insects, recently received Canadian registration for control of root weevil species. The fungus holds promise for control of D. radicum and other soil-dwelling pest insects.
However, because the reproduction of Metarhizium conidiospores is reliant upon infection of a suitable host under conditions of high humidity, use of the pathogen for root maggot control in Prairie canola fields will depend on discovery of the most efficacious fungal strain and appropriate application techniques.
At present there are no insecticides registered for root maggot control in canola in Canada. Seed treatments currently used for flea beetle control do not appear to last long enough for effective control of maggots. In Germany, producers are utilizing a new seed treatment from Bayer CropScience containing the insecticides clothianidin and betacyfluthrin as a means of managing Delia spp. during oilseed rape establishment in the fall.
Because there are no registered insecticides for root maggot control in canola crops in Canada, cultural measures to decrease maggot damage are of primary importance. Cultivar selection is an important first step for root maggot management in districts prone to severe maggot infestations. Brassica rapa cultivars are more susceptible to root maggot injury than are B. napus and B. juncea (L.) Czern. (brown mustard) entries, with Sinapis alba L. (yellow mustard) cultivars least damaged in choice tests.
Increased seeding rates, with resulting increases in plant density, reduce root maggot infestations.
Wider row spacings can decrease root maggot damage levels to canola. Research determined that canola grown at spacings of 20 and 30 cm had less root damage and higher yields than canola grown at 10 cm row spacings.
Tillage, or the lack of it, can also influence root maggot damage to canola. Root maggot egg numbers and canola root injury are greater under zero-till systems than under conventional tillage, in part because the increased soil moisture levels in zero-till fields are conducive to root maggot abundance. Despite higher numbers of maggots under zero tillage, yields in zero-till fields usually exceed those under conventional till, and the benefits of practising zero-till usually outnumber the drawbacks. In regions of high root maggot populations, fall cultivation could be used to decrease subsequent root maggot damage without sacrificing all of the benefits of no tillage in the next spring.
Soil fertility can have varying effects on root maggots. Addition of sulphur alone in either elemental or ammonium sulphate form did not consistently decrease root maggot oviposition or root damage, and is not recommended as a maggot management method. Increasing levels of all four principal soil nutrients from 0.5 to 2.0 times the recommended rate had no effect on egg deposition but actually increased degree of root damage by the maggots, suggesting that the maggots performed better on plants growing vigorously than on stressed plants. However, vigorously growing plants were able to better compensate for maggot damage than were plants receiving less fertilizer, and seed yields also increased. Therefore, adequate fertilization for optimum crop health is important for many reasons, including increasing the ability of plants to compensate for damage by root maggots.
Female root maggot flies go through an elaborate sequence of behavioural activities in host selection prior to oviposition, and contact with a non-host plant causes the flies to restart the selection process from the beginning. Therefore, increasing vegetation diversity by intercropping or delayed weed control could decrease the number of maggot eggs laid in a canola field. Delaying weed control from the two- to the four-leaf stage, if not agronomically detrimental to the crop, may minimize opportunities for females of Delia spp. to complete the behavioural sequence required for oviposition and result in fewer eggs laid.
Host plant resistance
Species and cultivars of canola and mustards vary in their susceptibility to root maggot infestation. Intergeneric crosses between S. alba, which has a high degree of resistance to the maggot, and B. napus resulted in lines with heritable resistance at levels similar to those of S. alba. Substantial progress has been made in developing Brassica germplasm resistant to root maggot injury, but it may be some time before canola cultivars with resistance to crucifer- feeding root maggots will be available commercially. Canola, especially B. napus, has a tremendous ability to compensate for root damage. Producers can capture this ability by growing vigorous canola cultivars and implementing cultural practices that encourage healthy crops and that discourage root maggot infestation.
Increases in the area devoted to the production of canola in Western Canada since the 1970s have been accompanied by increased abundance levels of several insect pests, including root maggots. The focus on increasing canola production in future years ensures that this insect pest complex will continue to pose a serious threat to sustainable production of the crop across the prairies, and especially in the Aspen Parkland and Peace River regions of Western Canada. Over the past 20 years, research to identify strategies for minimizing root maggot damage has proceeded along several fronts, and has resulted in a number of recommendations that can be applied by canola producers.
Among these, selecting the least susceptible canola varieties, seeding at recommended rates and with relatively wide row spacings, and applying recommended levels of fertilizer, can combine to provide a competitive advantage to the crop and facilitate plant compensation for insect attack. Recent research investigating the influence of increases in crop diversity on root maggots and their natural enemies, and studies on the biology of A. bilineata, the most important natural enemy of these pests, has shown promise. Further research is needed to identify cropping practices that best enhance populations of A. bilineata and other natural enemies. If research to develop root maggot-resistant canola is successful, an additional new tool would be added to manage root maggot infestations.
The complete, unedited version of this journal article can be found at www.prairiesoilsandcrops.ca . Prairie Soils and Crops: Scientific Perspectives for Innovative Management is a “peer reviewed” e-journal that provides agronomists, producers, agrologists and certified crop advisors with current perspectives on various issues pertaining to soil and crop management on the Prairies.