By Meredith Swett Walker
If you live in a temperate climate, you probably think of greenhouses as a tool to produce fruit, vegetable, and flower crops out of season. But these structures don’t just trap heat; they can also keep pests out. In fact, greenhouses are commonly used in warmer climates but primarily for pest control. Instead of solid, translucent walls, greenhouses in southern temperate, subtropical, and tropical climates typically have walls made of screening or netting. Organic growers, who must adhere to a limited list of approved pesticides, often find these “screenhouses” an effective pest control tool.
In a guide published last week in the open-access Journal of Integrated Pest Management, Phyllis Weintraub, Ph.D., of Israel’s Gilat Research Center and colleagues from Israel, Argentina, and the United Kingdom review integrated pest management (IPM) methods in organic greenhouses. While a greenhouse structure affords growers pest control options that are not available for field crops, such as physical exclusion, it can also create challenges. Weintraub and colleagues discuss methods to prevent pests and pathogens from colonizing the greenhouse, as well organic-compliant pesticides and biocontrol methods to control pests if they do get in.
Biocontrol, the use of a predatory or pathogenic organism to control a pest, can be particularly effective in the greenhouse. Even in warm climates where greenhouses have walls made of netting, they do moderate temperature. This allows cold-sensitive biocontrol organisms to be active for a larger portion of the growing season than they would be if exposed in an open field. Also, because the biocontrol organism is contained inside the greenhouse, there is little risk of it dispersing away from the crop.
But that containment can have a downside. Growers typically do not wait until they have a full-blown pest infestation before employing a biocontrol organism. However, in the absence of a large population of pests, what will the biocontrol organism eat or parasitize? Will the population of biocontrol organism starve or die out before the pests arrive?
In an open-field situation, nearby non-crop plants provide resources such as shelter, food, and prey to biocontrol organisms such as parasitoid wasps. Some growers even plant “insectary” or “banker” plants near crops and deliberately infest them with an alternative host, such as an aphid species that does not feed on the crop plant, to ensure a robust population of parasitoids. But there may not be space available for banker plants in the confines of a greenhouse.
Researchers have explored alternative methods to provide supplementary food to biocontrol organisms in greenhouses. These include feeding brine shrimp cysts to certain pirate bugs (Orius spp.), which are used to control thrips, or providing supplemental pollen to the predatory Swirski mite (Amblyseius swirskii) which is used to control whitefly and other pests.
But finding practical supplemental diets can be tricky. Some formulations may leave unwanted residues on plants. Other diets may become an additional source of food for the very pests growers are hoping to control. Weintraub and her coauthors conclude that development of practical, effective supplemental diets for biocontrol organisms would significantly enhance the use of biocontrol in greenhouses.
Greenhouses can not only boost the effectiveness of biocontrol; they may also provide growers with an opportunity to “blind” pests. Unlike humans, most mites and insects can see light in the UV range. Pests’ ability to see is important for finding host plants, dispersing and other behaviors. If UV light is blocked, for instance by the materials covering the greenhouse, the ability of pests to reproduce and damage crops may also be restricted.
Many of the plastics used in greenhouse netting materials absorb some UV light, and previous research by Weintraub and other scientists demonstrated that netting with enhanced UV blocking ability can be effective in controlling pests in the greenhouse. However, “the problem with the UV absorbing nets is that they don’t last as long in the field as the regular nets,” says Weintraub. Also, excluding too much UV light can inhibit proper development of the crop plant. The strategic blocking of UV light is not yet widely used as a pest-control strategy in greenhouses, but as industry develops new, sturdier, UV-absorbing netting materials, growers may be able to leave the pests “in the dark.”
Weintraub says that the IPM techniques described in this review would apply equally well to solid-walled greenhouses in cooler climates and could be used in conventional, non-organic greenhouses as well. The ability of greenhouses to physically exclude pests, as well as contain beneficial biocontrol organisms, makes them effective pest management tools.
Journal of Integrated Pest Management
Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.