Insects that attack young brassica crops early in the season

Just as a reminder, there are a few pests including flea beetles, cabbage maggot, Swede midge and cutworms that can affect transplanted and direct seeded brassica crops early in the season. Once the season starts moving along be aware! Please see OMAFRA Pub. 363 for control recommendations of the pests.

Flea beetle adults can cause enough foliage damage in seedlings to result in death. The adults overwinter in debris/leaf litter to emerge and start feeding in late April or early May. Plants can tolerate no more than 1 flea beetle/plant up to the 6th leaf stage. Feeding damage incurred in later growth stages may negatively impact the quality and thus the marketability of the crop, particularly in the “leafy” brassica vegetables. Brassica transplants seem to be less susceptible to flea beetle damage compared to direct-seeded crops. Speciality Brassica crops, like Chinese cabbage and bok choy, are preferred over the “traditional” cole crops.

Cabbage maggot adults emerge during the spring and lay eggs, which look like small grains of rice, on the stems of seedlings or on the ground around brassica seedlings. There are 3 generations per season in Ontario and the level activity of each generation varies according to a number of factors including temperatures, soil types, and local conditions. Typically, the first generation tends to be the biggest problem, but in cooler regions and in Brassica root and ethnic crops all three generations can be a problem. Based on degree day accumulations in various regions, we haven’t reached the emergence of the first generation adults as of 24 May 2011, but egg-laying is expected from mid-May until early-June. Upon egg hatch, the larvae burrow and tunnel into the roots causing damage and creating entry points for secondary pathogens like soft rot. From the seedling stage until a month after transplanting, plants are particularly susceptible to cabbage maggot attacks. Note that cabbage maggot controls are primarily preventative in nature.

Swede midge, which many of you are familiar with, can be a problem all season long. There is a relationship between the severity of damage and stage of development at which the crop is attacked. For instance, plants attacked in early development stages (before buttoning) typically have high levels of damage (blind head formations). First generation adults start emerging from mid-May until mid-June and here we see 4-5 overlapping generations per season. Crop rotation, use of clean transplants, post-harvest crop management, field sanitation and in some cases planting only early season crops to avoid population growth are all methods that can help to manage Swede midge. Intercept 60 WP Greenhouse Insecticide was recently registered for use on brassica transplants prior to transplanting in the field. Please see label label for precautions and directions for use.

In late April and early May, early season cutworms can attack seedlings by clipping the stems at the soil surface. Damage is likely to occur in weedy fields or along the field edges. Many cutworm species attack vegetable crops; however, the black cutworm is likely the culprit in brassica crops. The adult moths are blown into Ontario in late March and early May. This pest may be a problem this year due to a couple of factors. First, due to the cool, wet conditions we’ve seen lately, weed management may be a challenge. Weedy fields are attractive to female cutworms looking to lay eggs. Second, again due to the weathers conditions, the push back of planting dates may mean that plants are in the susceptible seedling stage when the cutworm larvae are larger.

For more information on these pests and others, refer to the OMAFRA Vegetable Production Recommendations (Pub. 363) and/or Ontario CropIPM.