ALM, Flea Beetles, and Seedcorn Maggot Are Active

A couple weeks ago we mentioned growing degree days (GDD) indicated common early spring insect pests were on their way, and sure enough, they have arrived. We received reports this week that allium leafminer, flea beetles, and seedcorn maggots are active and feeding right now. See descriptions below for more information on what you can do to continue to protect your crops and mitigate damage from these pests.

Allium Leafminer

A report came in from a farm in Enfield, CT earlier this week regarding allium leafminer (ALM) activity. Adult ALM lay eggs in the top of an allium leaf making punctures. Scout for characteristic oviposition marks, as displayed in the image on the right. The larva mine the leaves, creating tunnels of damage as they eat. These tunnels provide good entryways for fungal and bacterial pathogens to cause more damage to the plant.

Foliar chemical applications have been shown to be effective for reducing ALM damage. Two or three well-timed applications of an insecticide should provide an acceptable level of ALM control as long as the first application is 2- 3 weeks after initial detection of ALM flies and subsequent sprays are timed at 1–2-week intervals. Examples of highly effective insecticides applied as a foliar spray include dinotefuran (e.g., Scorpion), spinetoram (Radiant) and cyantraniliprole (Exirel). One OMRI- labeled option that works well is spinosad (Entrust). Other insecticides such as abamectin (Agri-Mek) and cyromazine (Trigard) can also help reduce ALM densities.

Flea Beetles

Brassica flea beetles have started their search for host crops, feeding on leaves and stems and leaving behind small round pits and holes. Crops with waxier leaves like cabbage, broccoli and kale are less attractive than crops with glossier leaves such as bok choy, Napa cabbage and mustard greens. The latter crops will remain susceptible to flea beetle damage from planting to harvest.

Scout across the field by counting beetles from above, then under the leaves, and estimating the percent of leaf damage. Because brassica crops differ greatly in susceptibility and attractiveness there is no fixed economic threshold that applies to all crops and crop stages. A working threshold of 1 beetle per plant or >10% average leaf damage on 50% of the plants has proved effective in leafy greens and early stages of heading brassicas.

Several synthetic pyrethroids, carbamates, neonicotinoids, and diamides are labeled for flea beetles in brassicas. Repeated applications may be needed if pressure is high.

Seedcorn Maggot

Seedcorn maggot flies are the first maggot flies to become active in the early spring. The adults look like small, gray houseflies with a slightly hump-backed shape. The seedcorn maggot larva is yellow-white, up to 1/4" long, legless, and has a wedge-shaped head. Pupae are oblong, brown, and about 4-5 mm long. The larvae feed on the seeds and early seedlings of large-seeded crops such as corn, beans, and peas, and are attracted to other crops such as spinach, onion, brassica, tomato, and cucurbit seedlings as well.

The first symptoms are usually poor emergence of seeds or wilting of transplants that have lost their roots to feeding. Look for maggots and feeding tunnels inside seeds or stems to help distinguish damage from wireworm feeding or damping-off. Crops that are planted in wet soil, or soil that is too cool to support quick germination and seedling growth, are especially susceptible to damage. Warmer soils and moderate moisture will help seeds germinate faster and get established before damage can occur.

Check soil temperatures and use forecasts before direct seeding in the field. If you need to replant, check the size of the maggots to determine how long to wait. If maggots are a quarter inch long, wait at least 5 days. If they are smaller than a quarter inch, wait at least 10 days to ensure existing maggots will not damage new seeds. Preventive chemical treatments include commercially applied systemic seed treatments and in-furrow applications of insecticides.

See the New England Vegetable Management Guide website for more information on insect controls by crop type. 

Keep the Row Cover On!

As we flip the calendar into the month of May, it can be tempting to get warm season crops planted out in the field. However, prior season experience reminds us that a mid- to late-May frost event is always a risk. A cold snap can significantly stress a warm season plant, halting any early growth the plant put on and slowing the plant's growth moving forward. Plants protected from temperature stress grow much more consistently and will be healthier overall. Our caution to enthusiastic growers: don't put warm season crops out yet unless you will be protecting them with row cover.

Covers vary in their insulation characteristics, but they can generally add 3 to 5 degrees to the ambient air temperature, helping to protect plants against low temperatures. Medium and lightweight row cover can be used during warmer periods when coverage is needed for pest control. However, be mindful of the potential for excessive heating on warm days if the row cover isn't opened. Row covers must also be removed at bloom for fruiting crops to encourage bee or wind pollination.

The efficacy and durability of row cover is contingent upon its proper use. Row cover can tear or rip easily or get picked up by the wind if not properly secured. Pests can also become trapped beneath cover or overwinter in the soil and emerge without you being able to notice. Weeds similarly benefit from the use of row cover. Removal and replacement of the cover to scout for pests or hand weed is necessary for sufficient control.

IPM in High Tunnels: Biological Control Options

Dr. Ana Legrand, UConn Department of Plant Science and Landscape architecture, explores the use of beneficial insects and other biocontrol agents for insect pest management in high tunnels. This presentation reviews arthropod predators and insect parasitoids available for growers to purchase and release against key pests in high tunnels. She also addresses the benefits and limitations of these biocontrol agents and mentions other tools from the IPM toolbox including microbial pesticides.

Watch here.

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