Q: Last summer, my Brussels sprouts, collard greens, and broccoli plants were decimated by cabbage butterfly caterpillars. I’d like to plant Brussels sprouts this year but don’t want to lose them again to cabbageworms. How can I protect my garden from these voracious pests?
—John Barbo, Geneva, Ohio
|cabbage looper||diamondback moth||cross-striped cabbageworm|
A: Very hungry caterpillars are indeed a scourge of cabbages and related crops in the genus Brassica. In addition to the velvety pale green imported cabbageworm—offspring of those fluttering white butterflies—your sprouts and broccoli may also harbor striped green cabbage loopers, diamondback moth larvae (pale green and segmented with a light brown head), and cross-striped cabbageworms (chunky with a blue-gray back and a lengthwise yellow stripe on each side). All four of these caterpillars reach about an inch long, and all four riddle cole crops’ leaves and heads with holes and droppings.
Protecting your garden from these pests takes a mix of preventive measures, persistent monitoring, and appropriate use of controls. Start with good garden sanitation. Clean up residues of all cabbage-family crops as soon as they are harvested to remove pest eggs, pupae, and larvae from the garden. Control weedy relatives such as wild mustard (Sinapis arvensis subsp. arvensis) and shepherd’s purse (Capsella bursa-pastoris) that may also harbor pest caterpillars.
Secure floating row covers over young plants immediately after transplanting to exclude egg-laying adults. “You must bury the edges of the covers,” says University of Connecticut extension educator Jude Boucher, vegetable crops IPM coordinator. “Folks often think that you can just weigh down the edges with rocks, staples, sandbags, or irrigation pipe, but these techniques have all proven to be less than fully successful at excluding the caterpillars,” he warns. Row covers can keep plants worm-free as long as they remain in place and without holes or gaps where pests can enter.
Encourage tiny wasps that parasitize caterpillars by planting sweet alyssum and other small-flowered ornamentals around your garden. If you see caterpillars on your cabbage crops, removing and squashing them by hand may be the quickest and simplest solution. In larger gardens where hand-squishing is impractical, consider spraying cole crops with BTK (Bacillus thuringiensis var. kurstaki), BTA (B. t. var. aizawai), or a product containing spinosad. BTK and BTA target caterpillars specifically, while spinosad will kill a broader range of insects. “It turns out that spinosad is tough on some of the parasitic wasps and flower flies, so you are more likely to cause an aphid outbreak after repeated applications of spinosad than with BTK,” explains Boucher.
With either product, it is important to apply sprays to cole crops infested by caterpillars and not on any nearby flowers to avoid affecting desirable butterflies, bees, and other beneficial insects. Boucher notes that diamondback moths in some areas have shown resistance to BTK. Spinosad remains effective for up to 7 days and kills caterpillars by both contact and ingestion. BTK and BTA affect caterpillars that ingest them; their protective effects last for only 24 to 48 hours.
|cabbage looper||diamondback moth||imported cabbageworm|
Ask Organic Gardening is edited by Deb Martin
Photography by David Cappaert; Frank Peairs; Clemson University; Nigel Cattlin; Bill Johnson; Blickwinkel
Originally published in Organic Gardening magazine, June/July 2014