High tunnel winter lettuce

Dec 7, 2020
Curb lingering pests to maximize winter production

Cold weather stops bugs but winter green production in high tunnels still faces challenges from insect pests. The pests multiply quickly on sunny days and controls don’t work as well in the winter because of cold temperatures and dense canopies.

“It’s far harder to get control of a situation in the winter that gets a little out of hand,” said Elizabeth Buck, Cornell Cooperative Extension vegetable specialist and a speaker at the 2019 Great Lakes EXPO in Grand Rapids, Michigan.

Grower education is the key first step in managing winter pests and higher revenue is the result. That’s the conclusion of a recent Northeast SARE-funded (Sustainable Agriculture Research and Education) project on sustainable pest management when growing winter greens in high tunnels. The study was conducted by Cornell Cooperative Extension.

Cornell educators worked with 24 high tunnel growers who were producing winter greens and helped them adopt biological or biorational pest control methods. The increased awareness of insect pests when growing winter greens and better scouting skills increased revenue by an average of nearly of $2,500 per tunnel.

Summer pests remaining in the high tunnels when going into winter is the core problem. The SARE project showed significantly fewer problems in high tunnels that started winter green production with low pest numbers compared to tunnels with established pest populations.

Simple practices like cleaning up weeds will help.

Sowthistle, lambsquarters and pigweed can all attract aphids into a high tunnel. “Chickweed is a terrible weed to have in a high tunnel in winter production,” Buck said. “It creates a thick, moist, canopy environment that’s ideal slug habitat. Caterpillars, aphids and slugs are the most common pests you see in winter production.”

Remove all dead plant material before starting winter greens and look for any volunteer plants growing under the benches or in unused areas. Know the insect pests’ alternate hosts. “Find their favorite habitats,” Buck said. “Find any reservoirs of pest infestation.”

Proper ventilation and increased airflow can help. “Winter humidity can be a problem,” Buck said. “Cutting back disease-forming humidity also cuts down on slugs.”

Be committed to scouting and scout weekly until the second hard frost. “The first hard frost doesn’t get all of the pests,” Buck said. Then scout at least once every two weeks through mid-February. “Go out and scout on any day it’s sunny,” Buck said.

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Aphids blend in with the foliage and are especially hard to find in winter greens. Look for them on leaf undersides and protected areas of the plants like along the midribs and near petiole bases and hearts. “Aphids will be in the curls of the leaves so you’ll have to flip them over and unfurl them,” Buck said. The action threshold is typically one aphid per leaf.

Caterpillars of the “cabbage” type – diamond backs, imported cabbage worms and loopers – tend to be in the upper two-thirds of the canopy on upper or lower leaf surfaces. These caterpillars blend in well and can be quite small when young. Armyworms and cutworms are the caterpillars that hide in the soil and can devastate young plantings.

It’s more likely to see caterpillar feeding or large frass pellets than the actual insects. Caterpillar frass pellets may fall to the base or center of a leaf or plant.

The action threshold for “cabbage” type is the presence on more than one plant and more than one caterpillar per plant. Any presence of armyworms or cutworms is the action threshold.

Slugs are particularly tricky. They like to hide in dark, damp places during the day and the “look for a slime trail” method can be unreliable.

A better scouting technique is to put a small piece of shingle or scrap of wood in a few areas that stay moist and check underneath them. These moist areas can be along the floor edges, in dense plantings, under plastic or in weedy patches. Check the crop itself, looking for stringy, messy frass and “layer by layer” feeding patterns.

The best action threshold is to assume you’ll have slugs. Apply slug bait at planting when the crop is small and easy to treat. This will reduce the slug population. Resupply the bait as needed.

“Successful biocontrol works because of strong IPM (integrated pest management),” Buck said. “It’s based on knowledge and preventative care. It’s knowing what’s going on in the high tunnel and identifying pests early.”

— Dean Peterson, VGN columnist

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