Greenhouses in transition

Some growers shut down their greenhouses for a few weeks during the winter in northern states 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.

Two-spotted spider mites (TSSM) populations are often high in the fall after the warm dry summer conditions that favor their development. TSSM enter hibernation (diapause) in the fall during short days, low temperatures and less food. During diapause, the mites change color, turning orange to orange-red and walk off plants to hibernate in cracks and crevices in the greenhouse. 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 have a mite problem in the same areas each year. In a diapause state, spider mites are more resistant to chemicals and are not as attractive to most beneficials. It is important to manage TSSM on plants prior to closing the greenhouse for the winter. In greenhouses with host plants and subtropical climates such as Florida, TSSM are active throughout the 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 (originating in 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. In another study, a temperature of 104°F and low relative humidity of 10 percent for four days killed 100 percent of WFT (plants not watered were dead by the end day two). WFT are susceptible to very dry or very wet conditions and these conditions likely cause more deaths than low temperatures. It may be best to maintain a warm and dry greenhouse during fall cleanup to reduce WFT populations, versus attempting to freeze them out.

WFT pupae are the non-feeding resting stage, often found on greenhouse floors under the groundcover and well-hidden. Warm temperatures promote the transition from pupa to adult stage, when WFT is much more susceptible to clean-up practices. The time taken for completion of the pupal stages is three days at 86°F, about five days at 68°F, and 8½ days at 59°F.

Aphids overwinter outdoors. Those found in the northern U.S. generally develop males and females in late summer to early fall. The females of most species mate with males, then oviposit the eggs, which overwinter on woody plants or weeds. In the spring, winged forms are produced that are generally females that start new colonies on plants in spring.

In greenhouses, aphids overwinter on weeds or pet plants. In this case, with continuous food, they are generally all females, and each female gives birth to more females without the need to mate. Aphid females give birth to living nymphs rather than laying eggs.

Important – clean greenhouses thoroughly. The best way to prevent spring pest problems is to thoroughly clean greenhouses in the fall 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.

Be sure cull piles are located as far away from greenhouses as possible. Clean now and save money and time next spring.

Smith is a frequent contributor to Greenhouse Management and an extension floriculture specialist for the UMass Extension Greenhouse Crops and Floriculture program.

Have a question? You can write Tina at tsmith@umext.umass.edu.