Management of Swede Midge on the Farm

The swede midge is an unwelcome guest, that once established on your farm will be virtually impossible to eradicate. However, with the use of best management practices you should be able to keep insect populations low enough to avoid economic damage. Proper management incorporates as many strategies as possible and should minimally include:

1) Use of clean transplants
2) 2- to 3-year rotation to non-crucifer crops
3) Post-harvest crop destruction
4) Swede midge detection and monitoring
5) Insecticide applications as needed
Details on these and other management practices are provided below:

Clean Transplants
Always start off with clean transplant material. This will not only help your operation but will reduce the movement of swede midge to other areas. Be confident about the source of your transplants.

If transplants come from an infested area or are possibly infested by swede midge, insecticide treatments should be applied before transplanting. Check what insecticides are labeled in your area. The effectiveness of different foliar sprays for control of swede midge-infested plants will vary depending on the time when plants became infested with swede midge. For example, in trials conducted at Cornell, the efficacy of acetamiprid (Assail) on swede midge using foliar sprays was 99.5, 100, and 99.8% when cauliflower seedlings were sprayed before inoculation with swede midge, at inoculation and 4 days after inoculation, respectively. The efficacy of acetamiprid was reduced to 69.9% when seedlings were sprayed 8 days after inoculation, and swede midge larvae could successfully pupate and emerge after the spray. Based on these results, foliar sprays on transplants at the early stage of infestation and before shipment of seedlings from swede midge-infested areas are recommended in New York. In the United States, Assail 30SG is not labeled for greenhouse use, thus plants must be removed from the greenhouse prior to treatment.

Refer to the label for specific product information. Inspect all transplants prior to planting, particularly in known areas of SM infestation. When hardening off young seedlings outside, cover them with fine netting or a floating row cover to prevent any egg-laying activity.

Crop Rotation
Crop rotation is perhaps the single most effective way to reduce swede midge populations in the field. With multiple generations and a high reproductive potential, swede midge populations can build up very quickly under continuous production of a host crop. A small number of swede midge adults in the first generation can give rise to tens of thousands by the third generation in July! By late September, high numbers of swede midge drop to the soil to pupate and can create challenging management issues for the following growing season when they emerge in very high numbers in the spring.

To investigate whether swede midge would emerge from soils with or without host plants being present in the soil, Cornell researchers conducted a series of trials in the laboratory. They found that plant type (host or non-host) did not significantly affect the swede midge emergence pattern from soil, i.e. swede midge emergence time and emergence rate from soil that was previously planted to a suitable host plant (cauliflower), alternative host plants (shepherd’s purse and wild mustard) and non-host plants (sweet corn and kidney beans) were very similar to those from fallow soil. This information is important for swede midge management since swede midge has been reported to survive in the soil for 2 or more years and can emerge from soil if climatic parameters (such as temperature and moisture) are suitable, regardless of the presence of host plant. Thus, rotating out of cruciferous crops and weeds can go a long way towards reducing a swede midge population. The longer the rotation period between crucifers the better it will be for managing swede midge.

To further evaluate the effectiveness of crop rotation on swede midge control, Cornell University researchers used 11 simulated cauliflower-sweet corn and cauliflower-kidney bean crop rotation systems, with and without the presence of cruciferous weeds as alternative hosts under controlled laboratory conditions. Their results indicated that the cauliflower-sweet corn, and cauliflower-kidney bean rotation systems could provide full control of swede midge. The effectiveness of one cycle of non-host crop rotation was reduced when cruciferous weeds were present; however the swede midge population in a one-cycle non-host rotation system with cruciferous weeds present was significantly lower than that in a non-rotation system. Two consecutive cycles (simulating a cropping season) of non-host plant crop rotations provided full control of swede midge, regardless of the presence of the cruciferous weeds, which suggests that 1) crop rotation can be a very effective tool for swede midge control in the field and 2) cruciferous weeds are not very suitable host plants for swede midge, although weeds can sustain swede midge population for a limited time.

A field survey of swede midge occurrence on cruciferous weeds in western New York by Cornell scientists indicates that swede midge populations may be maintained to the next season on cruciferous weeds in fields without crucifer crops being available. Trials in Europe indicate that high infestations were reduced to economically acceptable levels for several years following a 2-year crop rotation. Much higher damage has been observed in multi-cropped cole crop fields. Similarly, on a small farm in New York, a once economically damaging swede midge population became practically non-existent just two years after not growing any crucifer crops, despite prevelance of crucifer weeds. Depriving the swede midge of its preferred host plant material (crucifer crops) through crop rotation provides an effective and ecologically acceptable management technique.

Swede midge from the previous year’s cruciferous fields may present a season-long threat to neighboring cruciferous crops. Preliminary work in Ontario (2004) illustrates the importance of a host-free period to swede midge population dynamics. In areas previously under continuous production of susceptible cole crops and then planted to soybeans or corn (non-host crops), swede midge continued to emerge throughout the following year.

We suggest a minimum 3-year crop rotation, recognizing that for many growers, this presents a major challenge from the standpoint of available land base and continued access to markets. However, even a single year’s rotation with careful cruciferous weed management practices will reduce the population of swede midge and should be encouraged. More research needs to be devoted to this area.

How far away does the field need to be? Some European data suggests a minimum of 600 to 1,000 ft between sites; distances of up to 0.6 miles have been proposed. Distances of 50 feet are not at all a deterrent for the previous year’s swede midge to find the current year’s crucifer crops. We lack the data to provide a science-based answer, but caution dictates that new plantings should be as far away as possible from the previous year’s planting to reduce the possibility of swede midge being carried into the field by a light wind. Without a host crop nearby, the swede midge female will not be able to find a place to lay her eggs in her short 1-5 day life span.

Post-Harvest Crop Destruction
The second most important thing that you can do to manage swede midge is to destroy your cruciferous crop as soon as possible after harvest. After the marketable portions (i.e. broccoli crown, cabbage head) are harvested, the plants sprout several secondary side shoots, which are ideal sites for swede midge to lay their eggs. Optimum host conditions in combination with the absence of insecticide sprays make post-harvest cruciferous plantings ideal for swede midge to thrive and build up very large populations - the larvae of which will drop to the soil to pupate, and then emerge in droves to find the next susceptible crucifer planting within the same season or the following spring. Leaving harvested cruciferous crops in the field post harvest is especially favorable for swede midge when the weather is still warm in July, August and September. In late October, November and December, swede midge activity drops considerably. Once you are finished harvesting a planting, it should immediately be chopped, disked or plowed, so that it no longer provides living tissue for swede midge to prosper. In western New York, pheromone trap catches tend to drop drastically after crop destruction. On the other hand, trap catches tend to steadily increase when a crucifer planting is left unattended after harvest.

Soil Manipulation
Since swede midge pupate in the soil, soil manipulation can be an important aspect of control. Experiments conducted at Cornell indicate that swede midge can infest crops in many different soil types, but that varying the moisture level or depth at which pupae reside can make a difference in their ability to emerge. Laboratory results indicated that extremely dry and extremely wet soil hinders swede midge emergence. Optimal moisture content for swede midge emergence was from 25 – 75 %, and varied in different soils. The distribution of swede midge pupae was also studied in different soils. Under laboratory conditions, most swede midge pupated within the top 1 cm of soil, regardless of soil types. In a follow-up set of experiments, we covered swede midge pupae with 2, 5, 10 and 15 cm depths of soil, and then checked the emergence number and timing. We found that 2 cm depth of soil cover had no negative impact on adult emergence. However, more than 5 cm depth of soil cover greatly reduced the emergence number and delayed the time of emergence.

These results suggest that cultural practices, such as flooding fields during non-cropping periods to achieve 100% soil moisture level or even drying the soil, may be viable methods to reduce swede midge emergence. Similarily, swede midge populations and damage are expected to be reduced when saturated soil or drought conditions occur. However, tillage trials in Canada indicate that swede midge emergence can be enhanced by deep plowing of soil, since that brings some of pupae to the soil surface. Therefore, until further work is conducted it is not recommended that growers in affected areas use deep plowing in the spring.

Field Selection
Swede midge is considered a poor flier. Growers with swede midge populations have found less damage when they plant crucifers in open fields. Damage symptoms are typically first observed in the field along tree-lines, buildings and hedge rows, because the insect is easily blown into these areas. Planting in isolated areas, up from prevailing winds, may help decrease the risk of spread. Rotating crucifer crops upwind from previous swede midge infested sites may also interfere with them finding their host.

Crop Selection
The swede midge will attack most members of the Brassicaceae family. Some studies in Canada suggest that the highest levels of damage occur on collards, broccoli, Chinese broccoli (gai lan), Brussels sprouts, cauliflower, and Chinese cabbage (choy sum). The broccoli variety “Paragon” appears to be highly susceptible to swede midge damage, while “Everest” and “Triathlon” are less susceptible than some other broccoli varieties. Canola is readily attacked. Cruciferous weeds including field, wormseed, dog and wild mustards, yellow rocket, shepherd’s purse, field pennycress, field pepper grass and others are also hosts and may act as reservoirs for swede midge populations in the absence of crucifer crops or canola. However, under laboratory conditions, when cauliflower and weed plants (wild radish, field pennycress, shepherd’s purse, field pepper grass, wild mustard and yellow rocket) were simultaneously exposed to swede midge adults, significantly greater number of swede midge larvae were found on the cauliflower plants 8 days after oviposition, suggesting that cruciferous weeds are less preferred by swede midge.

Field Sanitation
In studies conducted at Cornell from 2005 to 2007 in western New York, more than 3,000 cruciferous weeds were sampled in different fields in which pheromone traps had caught swede midge adults. Weeds were separated according to species and then placed in emergence cages and the number of adults that emerged was calculated per 100 grams of dry weight. Swede midge adults were found in yellow rocket, wild mustard, field pepper grass, field pennycress, hedge mustard and shepherd’s purse. In 2006, the weed species that yielded the highest number per 100 grams of dry weight was shepherd’s purse. These studies suggest that cruciferous weed hosts of swede midge may serve to sustain a swede midge population to some extent and should be managed whenever possible.

Planting and Harvest Dates
Planting only early season crucifer crops is another control strategy to reduce damage levels and population growth. The amount of damage is directly related to the plant growth stage at the time of attack. The younger the plant when attacked, the more severe the damage. As plants grow, damage becomes increasingly evident. The first emergence of swede midge occurs from mid to late May. Damage to early plantings will be less severe than to late plantings because the plant development will be advanced by mid-July when high populations occur. Harvesting of early season crucifers begins in July, which minimizes the amount of damage to the crop. Avoidance of late season crops will also help to reduce the size of the overwintering population in your fields. This strategy will not work for long season crucifer crops like Brussels sprouts. Also, an early planting may have high levels of swede midge damage if it is located at the same site as last year’s crucifer crop, because it can be subjected to the emergence of a large overwintering population.

Monitoring and Scouting
Do not assume that you have an established population of swede midge on your farm, but be on the look-out for signs and symptoms, particularly if you are located within an infested county. Early detection of swede midge is key to maintaining pest populations at manageable levels. Familiarize yourself with the information provided in the section on this website resources.

Insecticides and Spray Timing
Well-established thresholds have not been determined for swede midge. Preliminary research results from Ontario, Canada indicate that growers may have opportunities to optimize their spray applications for swede midge by monitoring pest pressure with pheromone traps.
Growers with swede midge populations capable of causing economic damage are advised to initiate a pest control program incorporating both cultural and chemical management. Transplants should be protected as soon as they are removed from the greenhouse, or immediately after planting. Management of early swede midge populations will reduce the potential for a population explosion by the end of the season.

Once plants are in the field, monitoring swede midge population with pheromone traps and timing insecticide sprays are the keys to achieve satisfactory control of swede midge. In a greenhouse study conducted at Cornell, Assail 30SG provided very effective control of swede midge on cauliflower plants as foliar spray regardless of plant sizes (if coverage was good) and swede midge densities (related to population pressure). However, Assail 30SG provided only up to 9 days of control, after which time, control decreased significantly. When swede midge pressure is high, susceptible crucifer crops need to be protected at all times.

In greenhouse trials conducted at Cornell in 2005, most pyrethroid, carbamate and organophosphate insecticides that are commonly used to control caterpillar pests in crucifers were toxic to swede midge. However, in field situations where swede midge are protected from direct contact with insecticides, many of these materials provide minimal control. No materials that meet organic standards provided effective control of swede midge in laboratory trials.

Based on laboratory and field studies, Cornell was able to provide data to New York regulators to have several products labeled in New York State (link to the page of insecticides). However, check the label before any applications. In areas where swede midge is known to be present and abundant, the safest strategy would be the use of imidacloprid soon after transplanting to provide an estimated 3-5 weeks of control, followed up by foliar sprays of another insecticide.

Insecticides should be wisely used in conjunction with accurate swede midge population monitoring tools to ensure optimal control. However, with very high populations, insecticides will not be able to prevent injury. With a limited tool-kit, the development of resistance to the available insecticides is a serious concern. Choose and use products wisely. Research teams in New York and Ontario, Canada are working collaboratively towards the evaluation and registration of additional insecticides for use in both the field and greenhouse. Check with your Cooperative Extension Service for registration updates. Read labels carefully!

Final Thoughts and Take Home Messages
No single strategy will provide 100% control of swede midge. However, using as many of the management practices described above as possible will help to keep swede midge populations manageable and reduce economic damage.

Swede midge tends to be particularly problematic when the pest is not detected early and builds up to high populations. Factors that contribute to high populations include lack of rotation to non-crucifer crops and leaving the stumps of cruciferous plants in the field after harvest. Insecticides can be an important tool in swede midge management, but may not be effective if populations are too high. Currently, there are no effective products that meet organic standards. Small farms that grow multiple crucifer crops are particularly at risk because they often have too small a land base for adequate crop rotation.

While we have learned much about swede midge in the last several years, much remains to be studied about its biology and management. Considerable research efforts are needed to identify less susceptible varieties and to document the ways in which swede midge spreads over short and long distances.

Swede midge is becoming an increasingly important pest of crucifers in North America, and it is imperative that management strategies be implemented while it is still a relatively new pest. Assume that you have swede midge, or that you will be getting it. Practice excellent crop rotation and immediate post-harvest crop destruct, and be sure you know how to identify swede midge damage.



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