Some growers shut down their greenhouses for a few weeks during the winter expecting the cold temperatures to be sufficient to freeze and eradicate all of their pest populations. However, some pests can survive and continue on your spring crops, especially if infested debris is left in the greenhouse.
- Aphids may overwinter in some greenhouses. In cold climates, aphids generally develop males and females in late summer to early fall. The females mate with males, then lay their overwintering eggs on woody plants or weeds depending on the species. For example, green peach aphid overwinters as eggs on Prunus species. In the spring, winged females start new colonies on herbaceous (or non-woody) plants. Depending upon greenhouse temperature, if you have weeds or pet plants in your greenhouse, ensuring a continuous food supply, non-mated female aphids may continue to give birth to living nymphs rather than laying eggs.
- Two-spotted spider mites (TSSM) enter hibernation (diapause) in the fall during short days, and low temperatures with fewer food sources. During diapause, the spider mites change color, turning orange to orange-red. They leave host plants to hibernate in cracks and crevices in the greenhouse where they overwinter. As soon as temperatures are favorable in the spring, they slowly come out of diapause and move to the nearest plants. This is why growers may have a spider mite problem in the same areas each year.
- Whiteflies and thrips do not go through diapause, but may survive cold periods in unheated greenhouses, especially during mild winters. Although greenhouse whitefly has no hibernation stage, the egg stage is highly tolerant to low temperatures and can survive up to 15 days at 27°F, and up to five days at 21°F. As long as green plant material exists in the greenhouse, the whiteflies have a good chance of surviving relatively cold conditions in their egg stage.
- Adult Western flower thrips (WFT) may also survive in unheated greenhouses. One study showed that adult Western Flower thrips (from California) survived for three days, and four hours at 14°F and for 6 days, and 14 hours at 23°F.
Crop residues and leftover plants left in the greenhouse or in areas surrounding the greenhouse provided suitable overwintering sites and can contribute to spring infestations.
The best way to prevent spring pest problems is to thoroughly clean greenhouses and not rely on freezing temperatures to eradicate pest populations. Remove all weeds, unsold plants and plant debris in greenhouses and around the perimeter of the greenhouse (if possible). Be sure cull piles are located as far away from greenhouses as possible.
A fallow period (with greenhouses completely empty) for at least four weeks may help to reduce pest pressure for the upcoming growing season. Even a break in production of as little as two weeks can help reduce pest pressure.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
Ferguson G. Low Temperatures and Pest Populations. 2009. Greenhouse Vegetable IPM, OMAFRA.
Casey C. Use of Temperature and Moisture Extremes to Manage Western Flower Thrips in Greenhouses. 1995. Cornell Cooperative Extension.
Cold hardiness of adult western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). 2007. Applied Entomology and Zoology.