Beware the brassica buggers

Crucifer flea beetles on cabbage. Photo courtesy of Dan Gilrein

by Sonja Heyck-Merlin

The Brassica Pest Collaborative is a multi-year project funded by Northeast SARE, bringing together Extension educators and researchers from UMass, UConn, UNH and Cornell Cooperative Extension of Suffolk County. Four of the project’s participants discussed practical strategies for controlling common brassica pests – cabbage maggots, flea beetles, worm species and cabbage aphids – in a recent webinar.

Cabbage maggots were discussed first. Growers can scout for the fly using yellow sticky cards and also look for eggs. “The cabbage maggot fly resembles a house fly,” said Cornell entomologist Faruque Zaman. “The flies lay their eggs, which are about half the size of a rice grain, at the base of brassica stems or on the soil near the stem.” When the eggs hatch, the larvae begin feeding on the below-ground portion of the stem and roots. Yellowing, wilting or dead plants may provide an indication of the damage.

On Long Island, where Zaman conducts most of his research, there are typically four generations of cabbage maggot flies per season. For each generation, there is a flowering plant indicator species. “The first generation of flies appear when yellow rocket blooms; a second generation during daylily bloom; a third generation with Canada thistle and early goldenrod; and a fourth when New England asters bloom,” said Zaman.

Cabbage maggot management is primarily preventative. Zaman did not provide a specific threshold for when control is needed, but growers should assume that at the seedling stage, plants are at risk of damage. To prevent this, Zaman suggested a pre-transplant drench with an appropriate insecticide, followed by direct applications two and four weeks post-transplanting.

“After four weeks, it may not be necessary to continue treating if you have good control, appropriate moisture levels and the cabbage maggot population is no longer a threat,” he said. Floating row cover or exclusion netting are also effective non-chemical controls for cabbage maggot and other insect pests, provided the sides are securely closed.

Flea beetles were covered second. “You need to scout regularly and keep on top of flea beetles because once they’re present, they’re present all summer long,” said Sue Scheufele of UMass.

Flea beetles overwinter in field edges, becoming active in early May, when the larvae pupate in the soil and reemerge as adults. “Rotate your spring crop as far as possible from last fall’s crop. Two hundred meters is the minimum distance you want between the new and old crops,” said Scheufele. “If you can, use distance or a physical barrier such as a hedgerow or a road.”

In addition to crop rotation, Scheufele recommended controlling brassica weeds such as shepherd’s purse, yellow rocket and wild mustard, since all act as flea beetle hosts. Floating row covers and exclusion netting are also effective controls.

Flea beetles have a preference for the non-waxy Brassica rapa species (bok choy, tat soi, Chinese cabbage, etc.). Waxy brassicas (B. oleracea), including cabbage and broccoli, are less but still susceptible.

According to Scheufele, growers can use this distinction to their advantage by growing B. rapa species as trap crops. For example, a grower could interplant a row of bok choy into a broccoli cash crop. “Then, you can spray the trap crop to reduce the flea beetle population rather than spraying the whole block,” Scheufele said.

When spraying for this pest, Scheufele recommended using a sticker spreader and hollow cone spray nozzle, which creates a cloud of material and better coverage. B. rapa species will need to be sprayed more often, while B. oleracea species generally only need insecticide as they are getting established.

“If you’re on the fence about controlling flea beetles,” Scheufele said, “also consider that controlling them can reduce the spread of alternaria and black rot.”

Dan Gilrein of Cornell discussed three types of worms that impact brassicas: imported cabbage worm, cabbage looper and the diamondback moth. The imported cabbage worm is one of the few butterfly crop pests; the looper and diamondback are moths. Of the three, the diamondback can sometimes be the most difficult to control because of insecticide resistance and its habit of feeding on the undersides of leaves where insecticide coverage is difficult.

It’s important to scout for both eggs and caterpillars, although it’s difficult to see diamondback eggs. “Scout your crop at least weekly – four plants at 10 spots,” Gilrein said. For cabbage looper and diamondback, Gilrein suggested using pheromone traps for an early warning they are in the area (cabbage looper migrates in each year; diamondbacks can overwinter in some locations).

According to Gilrein, the thresholds for spraying for worms on cabbages are 20% infestation on young plants, 30% from early vegetative stage through cupping and 5% through harvest. Growers can adjust these cabbage thresholds to help decide when to spray other brassica species.

In summary, Gilrein said, “Use clean transplants; in the fall, till under brassica residue that acts as an overwintering site for the diamondback moth; use row cover or exclusion netting; start with a weekly BT spray, and then rotate to other products, if needed, later in the season. Coverage is an issue, especially on the undersides of leaves, so consider increasing the gallonage, adjust nozzles and pressure and include an adjuvant.”

Becky Sideman of UNH covered the final pest: cabbage aphids. These aphids are distinctive, covered in a waxy protective coating that gives them a white to matte-gray appearance. The eggs are laid on wild and cultivated brassicas in autumn, and the eggs hatch in spring. The wingless females feed on host plants and reproduce asexually until overcrowding and stress leads to the production of winged females. The winged females then make it very difficult to predict the movement of the aphids.

“Cabbage aphids love new growth,” Sideman said. “They are particularly problematic on fall crops where the edible portion of the crop is that new growth – Brussels sprouts, cabbage, kale and broccoli.”

An early warning sign of these aphids is a vague yellowing on brassica leaves. The undersides of these yellow spots are where the aphids are often found. “The key is to scout regularly and remove heavily infested plants,” Sideman said. “Treat when 10% of the plants have more than one aphid. Early and repeated pesticide applications are necessary to control cabbage aphids, which can mean six to eight applications per season.”

Knowing the type of brassica pest, their lifecycle and effective controls are all key elements in managing these brassica pests. Frequent scouting is critical. “Use a scouting program if possible. The reduced costs for insecticide costs will usually more than cover the expense of scouting,” said Gilrein.

2021-05-26T10:40:31-05:00May 31, 2021|Grower, Grower East, Grower Midwest, Grower West|0 Comments

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