However, there are two points that work against this assumption. Firstly, insects and mites have been extremely successful at adapting to the cold temperatures of winter. For example, pests such as spider mites, Lygus bugs, and European corn borers enter the state of diapause during fall. In this physiological state, they slow down considerably and undergo various internal changes that enable them to survive the winter. Secondly, temperatures even within an unheated greenhouse are usually quite a few degrees warmer than outdoor temperatures, and therefore, likely never quite reach freezing point. It is for this reason that even those insects that are not specifically adapted to the cold, and are in fact found mainly in warmer climates, can still survive in the greenhouse during the winter months.
|Mature cabbage looper on tomato leaf.
|Large numbers of whiteflies infesting eggplant leaf. (photos courtesy Gillian Ferguson)
Greenhouse whitefly – Although the greenhouse whitefly has no stage specially adapted for hibernation, its egg stage is highly tolerant to low temperatures. For instance, eggs can survive up to 15 days at -3ºC, and as many as five days at -6ºC. As long as cold-tolerant green plant material exists in the greenhouse, the greenhouse whitefly has a good chance of surviving relatively cold conditions via the egg stage.
Thrips – Most commonly, female adults survive winter conditions. During the fall months, thrips usually move away from the light and seek cracks and crevices. Many species can overwinter just beneath the soil or on plant debris. Any crop residues left in the greenhouse or in areas surrounding the greenhouse also provide suitable overwintering sites and can contribute to large infestations the following spring. Survival of individuals in litter can be considerably reduced if it’s buried sufficiently deep to prevent adults from reaching the surface. Thrips are very susceptible to very dry or very wet conditions. Such conditions probably cause more deaths than low temperatures.
Moth pests – The cabbage looper, a common moth pest, does not tolerate prolonged cold weather and usually reinvades all of Canada in late spring or early summer after overwintering in southern latitudes. However, the lower limit for its development is about 10 to 12ºC. If the cleanup period is short and days are sunny providing relatively warm conditions, the pupal stages of this pest can very likely survive in cryptic areas to re-emerge relatively soon after the new crop has been planted.
The tomato pinworm is another moth pest that thrives in warmer climates, which can also develop under cooler conditions. At lower temperatures, the insect simply slows down its metabolic rate and takes longer to complete its life cycle. For example, whereas it completes its life cycle in 26 days at 24-26ºC, it takes about 100 days to complete it at 10-13ºC. Thus, although the tomato pinworm is reported to be unable to survive the winter outdoors in Canada, this strategy of lowering its metabolism can enable it to survive in protected locations. This survival mechanism applies broadly to those insects normally found in warmer areas but which were somehow brought to colder climates.
MOST GREENHOUSE PESTS CAN OVERWINTER IN CANADA
In general, most of the pests that afflict greenhouse crops are able to survive in the greenhouse between crops during the winter period, particularly if infested debris is left in the house. The fact that temperatures may not even reach freezing point because of the buffering effect of the structure means that even those pests that don’t have a specific overwintering stage can survive.
Instead, we need to capitalize on the higher activity and metabolic rate of insects at higher temperatures and use this to our advantage. Despite the energy cost, maintaining a warm greenhouse during and after cleanup will contribute to hastening the completion of their life cycles, and particularly, in making them more vulnerable to applied controls. We have much more to gain in reducing pest populations by combining warmth with dry conditions, versus attempting to freeze them out.