Vegetable Notes 2024 Vol. 36:23

We’re at the point in the season where there are few new diseases and pests to report. Foliar diseases of tomato (early blight, Septoria leaf spot, and Stemphylium, and gray mold and leaf mold in tunnels), brassicas (Alternaria leaf spot, black rot, downy mildew), and cucurbits (downy mildew, powdery mildew, Plectosporium) are all out there, continuing to develop. Caterpillars in brassicas continue to be active, as well as squash bugs, striped cucumber beetles, and the corn caterpillars. We’re keeping our eyes peeled for signs of allium leafminer and will put out an alert as soon as we see it—let us know if you see oviposition marks in your leeks (umassveg@umass.edu or 413-577-3976).

We are seeing cabbage aphid populations building up in some brassica fields now. This pest has been a significant problem in some years and less so in others, typically becoming an issue in late-season brassicas. Infestations can stunt plants and reduce yield but the physical presence of the aphids in Brussels sprout buds or on leafy greens is often the biggest concern. The best control is achieved by scouting early in the season and treating crops when populations are small and isolated. Cabbage aphids will feed on any brassica crop, but Brussels sprouts are hit particularly hard as they provide aphids with hiding spots among the buds where they can’t be reached by insecticide sprays and plenty of time to build up. 

Identification 

Both nymphs and adults are grayish-green with short, dark-colored cornicles, but they usually appear dusty and grayish-white due to a fine, white, powdery secretion that covers their bodies. Dense colonies may develop, particularly on the undersides of leaves. 

Life Cycle 

Cabbage aphids only infest crops and weeds in the brassica family. Cabbage, cauliflower, broccoli, and Brussels sprouts are most severely affected, but other brassica crops may become infested. We usually start to see small numbers of cabbage aphids in brassicas mid-summer, but populations grow slowly, initially, and often don’t reach damaging levels until early fall. Cabbage aphids overwinter as eggs which are laid on the undersides of leaves of the same plants that were fed on during the summer. This differs from the life cycle of many aphid pests of vegetables, where eggs are laid and overwinter on alternate hosts outside the field. Eggs typically hatch in April. Nymphs feed and develop into reproductive females who can produce live young without mating. Winged adults disperse with wind and infest new crops. There are multiple summer generations and potential for huge population growth, especially where long-season crops (e.g. Brussels sprouts) are infested early. 

Damage 

Cabbage aphids prefer to feed on young leaves, flower buds, or seed stalks in the upper part of the plant, as well as in developing Brussels sprout buds. Heavy feeding injury results in wrinkled and downward-curling leaves, yellowing, and reduced growth. Colonies often develop on the undersides of leaves, and the corresponding area on the top of the leaf turns a blotchy yellow. The biggest issues requiring control tend to be contamination of marketable parts of the plant with aphids or honeydew, a sugary, sticky secretion produced by the aphids. Cabbage aphids can also transmit cauliflower mosaic and cabbage ring spot virus, among other viruses; transmission is non-persistent, with virus particles passed to new plants by aphid probing.  

Management 

Biological Control 

Natural enemies can suppress cabbage aphid populations but may not be able to control high densities that can occur in cool fall weather. Predators of cabbage aphids include syrphid fly larvae and ladybugs. The tiny ichneumonid wasp, Diaeretiella rapae, is a parasitoid of cabbage aphid—the female wasp lays an egg within an aphid and the resulting larva consumes the aphid from within, creating a round, bronze-colored “mummy”. These natural enemies can be attracted to brassica fields by planting flowers that provide them with nectar and pollen. Research done at both UMass and the University of New Hampshire in 2018-20 found that the flowers of sweet alyssum, dill, cilantro, and false Queen Anne’s lace (Ammi majus) attracted a high number of syrphid flies and D. rapae.

Cultural Control 

If cabbage aphids are present on your farm, each of these tactics alone will likely not prevent infestation of your crop. If these cultural control practices are implemented along with early, regular crop scouting, outbreaks can be controlled. 

• Incorporate crop residues immediately after harvest or, for late-fall brassicas, as early as fields allow the following spring. 

• Control brassica weeds in or near fields. 

• Inspect transplants to be sure they are aphid-free. Early-season aphid infestations may originate from greenhouse infestations. 

• Use reflective mulch to repel aphids. 

• Scout for cabbage aphids weekly beginning in early June. If isolated patches of infested plants are found, removing those plants from the field and destroying them can help avoid aphid spread. 

Chemical Control 

Use selective products when controlling other brassica pests (e.g. Bt products to control caterpillar pests) to conserve beneficial insects including predators and parasitoids of cabbage aphid. If cabbage aphids are a consistent problem on your farm, systemic insecticides applied at planting or sidedressing may eliminate early infestations. Scout weekly to determine the percentage of infested plants, starting before harvested portions of the plant form. Treat if >10% of the plants are infested with aphids, especially after heads or sprouts begin to form. Alternatively, select 10 leaves at 10 sites for a total of 100 leaves per field, and treat if >20% have aphids. Spray coverage of all leaf surfaces, buds, and new growth is key—hollow cone nozzles can help achieve good coverage of the undersides of leaves. Include a spreader-sticker to prevent insecticides from beading up and rolling off of waxy brassica leaves. Waiting until there are heavy outbreaks or until just before harvest makes it difficult to prevent loss of marketable yield. 

Research from the University of New Hampshire showed that cabbage aphid control using OMRI-listed materials can be achieved, but you must spray early and often while population levels are still low. Azadirachtin (e.g. AzaGuard, Azatin, Azatrol) + pyrethrins (e.g. PyGanic) + insecticidal soap (e.g. M-Pede)  used in a tank mix or in rotation has been found to be the most effective organic program for controlling cabbage aphids. Azadirachtin + pyrethrins was previously available as the pre-mix Azera; this product is no longer on the market, but the two individual materials can be tank mixed. Horticultural oils (e.g. SuffOil-X, JMS Stylet Oil, others) will also function to smother aphids—Dan Gilrein of Cornell Cooperative Extension recommends a solution of 2% horticultural oil + 0.5% M-Pede. All of the above insecticides work by contact, so thorough coverage is essential. 

For conventional products, Dan Gilrein writes: “Movento was particularly effective in one cabbage trial at the Long Island Horticultural Research and Extension Center. Fulfill and Beleaf are also highly effective and aphid-specific materials—all three have helpful translaminar or systemic activity. Alternatives include Assail and Admire Pro/generic spray. Note that the Bravo Weather Stick label prohibits tank mixing with DiPel, Latron B-1956, or Latron AG-98 and warns about combinations with other pesticides and surfactants unless a test shows the mix is safe. Cabbage aphids affected by treatment may not immediately appear dead—check after 2 days to verify efficacy.” 

--Adapted from the New England Vegetable Management Guide by the UMass Extension Vegetable Program