Caterpillar Management for Brassica Crops
By Jude Boucher, UConn Extension Educator, Commercial Vegetable Crops
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There are now four important caterpillars that attack Brassica crops: imported cabbageworm (ICW), diamondback moth (DBM), cabbage looper (CL) and cross-striped cabbageworm (CSCW). Most of these insects overwinter here in CT, but the CL generally migrates into the state from the south, and usually arrives after mid-summer. The first three all lay their eggs singly, so you tend to find a single caterpillar, or just a few on a plant. However, the CSCW lays batches of eggs, so you may come across scattered plants that are completely skeletonized with 10 or 20 caterpillars on them. This is obviously an important difference when it comes to deciding when to take action (action threshold) against the pests.
The populations of all these pests are moderated to some extent by natural parasites and predators, and you can also purchase some biological agents to help manage some of the pests. You can read more about these beneficials in the IPM Cole Crop Guide or the on-line article mentioned above. Perimeter trap cropping, using collards around cabbage, can limit the number of DBM caterpillars, even without pesticides, due to high parasitism rates by the naturally occurring wasp, Diadegma insulare. However, perimeter trap cropping doesn’t work on the other three Brassica caterpillar pests. You can also exclude all of these pests from your Brassica plantings by covering them with row covers just after planting or transplanting. In my experience, you must bury the edges of the covers to successfully exclude the pests. Weighing down the edges with rocks, stapes, sandbags or irrigation pipe have all proven to be less than fully successful.
Scouting for caterpillars
If you plan on spraying, scouting your crop weekly, especially after crops such as cabbage, broccoli, cauliflower and Brussels sprouts begin to form heads or sprouts, is important to minimize pesticide use and damage. Scout 25 plants per field for larger plantings and just 10 plants if you only have a few rows of Brassica plants. Scout each type of Brassica plant separately for larger blocks, but just make sure you scout some of each type for smaller plantings. All of these caterpillars tend to feed on the underside of the leaves. You should scout by lifting and inspecting the underside of first the outside, lowest or oldest leaves and moving toward the younger or center leaves or head, until all the leaves have been checked. Then multiplying the number of infested plants by either 4 or 10, depending upon whether you scouted 25 or 10 plants, respectively. This will give you the percent infested plants per field (i.e. the number of infested plants out of 100 plants). If you know how to tell the different caterpillars apart (see NEVMG, IPM web site or Cole Crop Guide color pictures), then you should note which species you are finding too.
For most heading Brassica, treat only after heading begins, and when 20% of the plants are infested with caterpillars. For other Brassica crops and cauliflower, treat if 10% of the plants are infested, especially before tying if using cauliflower varieties that don’t self-blanch. You don’t want to tie caterpillars under the leaves where they are protected and will then defecate on the curd. These thresholds are designed to provide 99% clean heads or marketable plants at harvest.
Many organic farmers find that they can tolerate much more injury to the crops before spraying is necessary. I usually suggest that organic farmers start by using a threshold of 50% infested plants. Most seem to find that 50% will provide a product quality level that they can direct-market, while others use a slightly lower or higher threshold.
Obviously, if CSCW is going to completely defoliate the plant so that it is unmarketable, it requires a lower threshold than the other three pests, which usually just put small holes in leaves. Treat or spot treat immediately if you notice that 1-5% of the plants are infested with multiple CSCW.
Resistance Management and Products
Some pest species, such as the ICW and CSCW, are susceptible to all the products mentioned in the New England Management Guide, while others like the DBM and CL tend to become resistant to products quickly. While scouting, once you start finding more DBM or CL than other caterpillars it becomes crucial to alternate between products when you spray, or resistance will quickly become a problem and you will lose some of the tools to fight these pests.
One of the more effective programs for organic growers would be to alternate between a B.t. aizawai strain product (i.e. XenTari) and spinosad (i.e. Entrust). The aizawai strain of B.t. provides better control of resistant DBM than the older kurstaki strain found in products such as Dipel. B.t. products spare all beneficials but only have a 24-48 hour residual period, so usually require reapplication at least weekly, if moths continue to lay eggs. Spinosad is effective for up to 7 days.
Conventional growers now have many product choices that help spare beneficial insects and some that have extended periods of effectiveness too. Coragen can provide up to three weeks of caterpillar control before you have to worry about re-infestation (only 14 days for CL). Intrepid is an insect growth regulator that only works on caterpillars and lasts for 10 to 14 days. Other products that are soft on most natural enemies include Assail, Avaunt, Proclaim, Synapse, Rimon, Radiant, Confirm and the B.t. products. A single application each of Coragen and then Intrepid should provide control all the way through to harvest for most Brassica crops, while sparing most beneficials.