Cultural Management of Swede Midge

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.

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.

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.

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.

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. Similarly, 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.

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 prevalence 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.