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Growers using greenhouses in which temperature, light and relative humidity are controlled have relied for many years on releases of natural enemies to manage aphids, thrips and two-spotted spider mites. However, many of the natural enemies used to manage these pests in heated structures are too sensitive to swings in air temperature and relative humidity to be used in cool structures such as minimally heated greenhouses and unheated high tunnels. Because these season extension tools are widely used by organic and sustainable vegetable growers, SARE funded a project to study the efficacy of biological insect control in minimally heated greenhouses and high tunnels. Researchers conducted 23 case studies involving tomatoes, cucumbers, eggplants, winter greens and peppers grown in greenhouses and high tunnels at nine locations in upstate New York from 2007 to 2009. This fact sheet reports the results and provides detailed advice on how growers can use natural enemies to manage insect pests in minimally heated greenhouses and unheated high tunnels.
Biological control—the suppression or eradication of crop pests using their natural enemies—is a tool used by growers to manage plant pests. Biological control offers many benefits:
• Compared to repeated applications of organic sprays, releasing natural enemies requires less time.
• Once established, natural enemies work around the clock.
• When using natural enemies, re-entry and pre-harvest intervals are unnecessary.
In this study, researchers found that it was important to select the correct species of natural enemy, because most have narrow preferences for air temperature, relative humidity and pest hosts. The categories of natural enemies used in this study were predatory mites, parasitic wasps and the predatory minute pirate bug (Orius insidiosus).
Management of Thrips (Introduction)
The most common thrips in greenhouses and high tunnels in the Northeast are flower thrips (species of Frankliniella) and onion thrips (Thrips tabaci). Flower thrips damage flowers and leaves. Onion thrips are pests on several hundred host plants, including cucumbers, tomatoes, eggplants and peppers. Thrips reduce the yield of crops by puncturing plant cells with their mouthparts. They damage the leaves and blossoms of host plants as well as transmit viruses. Thrips like dry conditions, so keep plants well watered and relative humidity high. Researchers examined the efficacy of two generalist predators, A. cucumeris (a mite) and O. insidiosus (the minute pirate bug), on controlling thrips on tomatoes and cucumbers in greenhouses and high tunnels.
Management of Aphids (Introduction)
Many species of aphids can be found on vegetable crops in the Northeast. Low to moderate numbers of aphids are usually not harmful to vegetables.
However, large populations cause curling and yellowing distortion of leaves and stunting of shoots. Aphids can also transmit viruses to vegetable crops. A. colemani and A. ervi, two pest-specific wasps that parasitize aphids, were used to manage a broad range of species on winter greens and peppers in high tunnels at several locations.
Management of Multiple Pests
To manage multiple pests, researchers recommend two approaches: releasing multiple species of natural enemies at one time or releasing multiple natural enemies over the course of the growing season. For heavy infestations of pests, it may be necessary to remove the most infested plants or spray them with an insecticide to knock down the pest population before releasing the natural enemies. When using multiple controls, consult with Cooperative Extension personnel or commercial distributors of natural enemies to make certain the chosen tools work well together. Insecticides vary in their impact on natural enemies; some insecticides are toxic to natural enemies, even in residual form. SARE Research Summary In 2007, 2008 and 2009, SARE funded a research project, Natural Pest Management in New York High Tunnel and Greenhouse Vegetables (LNE07-262), to study the efficacy of releasing natural enemies to control insect pests on vegetables grown in minimally heated greenhouses and high tunnels. The researchers recruited vegetable growers to host the on-farm trials, conducted a poll to determine their most problematic pests, established randomized research plots on each farm, and monitored pest numbers before and after releasing the natural enemies.
In total, this included 23 case studies involving tomatoes, cucumbers, eggplants, winter greens and peppers grown at nine locations in upstate New York. In several case studies, a pesticide was applied in addition to releasing the natural enemy. The size of the plots, densities of the crop plants and methods of pest monitoring varied by crop, site and year. Researchers monitored pest populations before and after applying the natural enemies by counting the numbers of each pest and/or rating their damage every one to two weeks. In addition, the overall success of the control method at each case study was rated on a scale of 0 to 5 as failed (0 rating), controlled (3 rating) or eradicated (5 rating). Pest population data was entered into spreadsheets and the mean population density or damage rating was calculated.
Thrips were monitored by counting their numbers on 12 leaves per block. Two-spotted spider mites were monitored by rating their damage to leaves on a scale of 1 (leaves are free of damage) to 10 (leaves are dead). Aphids and whiteflies were monitored by counting their numbers on approximately three to five leaves or leaflets per plant.
Further details on scouting procedures are available in the Farm-by-Farm Demonstration Trial Results in Reid, J. 2010b. Trial data and input from growers indicated that while biological pest control with natural enemies is difficult to quantify economically in field settings, it can be the difference between a crop and a failure, and for many organic growers it is their only option.