As the growing season comes to an end, extending the season by using hoop houses or high tunnels of a variety of sizes and shapes has become an option for many small growers. While unheated growing spaces might not extend the season all the way until spring planting, they do allow for cold season crops to produce well long after the first frost.
But growing in covered conditions doesn’t come without pest pressures. And pests often mean disease pathogens can readily enter plants. In some cases, insects directly serve as vectors for pathogens, which they then spread while feeding on crops.
Aphids are a cold weather pest and a common cause of concern for winter production. Their short life cycle, particularly for asexual production – one week from being born live to birthing the next generation – combined with their ability to pierce plants during feeding, causing injuries which in turn attract other pests, and their role as virus vectors means that control of aphids is a top priority, no matter the season.
There are many aphids that affect one or more families of vegetables, weeds and ornamental plants. Some are winged; some are not. Some aphids are host-specific, others have a wider ranging palate. All will cause damage to the plants they consume, both directly and indirectly.
Cornell Cooperative Extension Vegetable Specialist Judson Reid conducted research on pests in high tunnel winter green production from 2010 – 2014. The project, funded by a Sustainable Agriculture Research and Education (SARE) grant, involved 24 successful case studies of New York farmers growing high tunnel winter greens, including spinach, lettuce, Swiss chard, kale and Asian greens. Aphids were among the pests causing concerns in the crops.
Aphids were found in high populations on winter greens. Biologic and biorational control measures were found to decrease aphid damage and increase crop yields and revenue on the farms in the study.
Biological controls of aphids include parasitoid wasps. These wasps will lay their eggs inside aphids, killing them. However, the wasps prefer warmer temperatures, while aphids thrive in colder temperatures. Lady beetles, however, were found to be cold-tolerant and provide good aphid control throughout the winter.
Biopesticides also provide growers with aphid control. Beauveria bassiana is a fungal pathogen which attacks aphids. There are commercial biopesticides which use the fungus, and application of these sprays was shown to reduce aphid populations below threshold levels. However, the sprays are hindered by canopy density, so growers need to plan plant spacing accordingly, leaving the canopy less full, which also helps with high tunnel disease control.
Aphids do have weed hosts, and those who prefer Aster family weeds were found to move to lettuce grown in high tunnels during the winter once the weeds died back. Weed control throughout the season is an important pest reduction measure.
Often, pest pressures exist from previous tunnel crops. Crops grown in the tunnels during warm weather can harbor pests, including aphids, whose populations can be numerous enough to make winter control difficult. Scouting in the high tunnel is imperative in order to find and treat any pest populations before they reach levels at which control becomes difficult. Treating for pests prior to the winter crops being introduced is critical.
Researchers concluded that biological control measures which impact multiple aphid species, applied early before high threshold levels were reached, offered the best control of aphids in high tunnel winter greens. Regular scouting was performed, and aphids were counted across three leaves per plant. When the average number of aphids exceeded 1.0 aphid per leaf, control measures were taken. For more information see the report at www.bioworksinc.com/products/shared/aphid-management-in-high-tunnel.pdf.
Caterpillars are also a concern in the winter high tunnel. Researchers found that caterpillar frass caused damage to winter tunnel greens. Caterpillars will survive the winter in high tunnels, although they were found to decrease feeding below 50 degrees. Once temperatures warm, they become very active again.
Innovation in high tunnel aphid control
Researchers at the University of Vermont Entomology Research Laboratory are using plant-based controls to decrease aphid damage in both summer and winter high tunnels. The winter tunnels are planted to leafy greens. Alyssum, beans, marigolds, calendula and viola are planted in the winter tunnels, along with the crop, and serve to offer food and shelter to aphid predators.
These plant-mediated IPM systems include indicator plants, trap plants and banker plants. Indicators show the presence of pests early, while trap plants attract the pests away from the crop. Banker plants host the natural enemies, allowing them to reproduce on site. Habitat plants are those that provide food and shelter for natural enemies of crop pests and are planted in proximity to the crop.
Parasitic wasps and orius and syrphid flies were prominent in both winter and summer seasons high tunnel production. Lady beetles and spiders were also found to stay active during the cold weather. Alyssum was found to have good cold tolerance and to provide the longest bloom, attracting predators during both seasons. The “guardian plant” system was promoted to customers and well-received.
Aphids not only weaken plants, allowing for disease pathogens to enter the crop – they also serve as vectors for various viral diseases. By transmitting these viruses from plant to plant when feeding, aphid populations can exacerbate disease concerns exponentially. Controlling aphid populations in winter high tunnels can help growers to reap the benefits of winter season vegetable production.