We generally observe that our orchards are free from insect pests in winter. The question is that where they go? They appear again in spring till the whole summer. The second question is; if they disappear from orchards in winter then where from they come again in spring. Insects can survive in the cold winter by many ways. In general, insects have ability to stay alive in cold temperatures, when the temperatures are steady, not variable. Many insects get protection and food in the winter in a mixture of micro-habitats, including harboring around, in, and under our crops and orchards. In these micro-habitats, it seldom gets below freezing. For instance, crawl spaces stay extremely comfortable throughout the winter, even on the coldest of days. Additionally, insects use a variety of methods for surviving the extreme cold. They include migration, hibernating and overwintering as larvae, nymphs, eggs, pupae or adults. Other pest, like mice for instance thrive in the winter. Some insects produce a natural type of antifreeze called glycerol protecting them against below temperatures. Even after the harshness of winter, when temperatures got just a little warmer the very next week, insects are back on the move. Not surprisingly, insects are extremely resourceful and have evolved a number of strategies to deal with the rigors of lower temperatures.
It is one method to get rid from the killing temperatures. For example; the Monarch Butterfly is the best example of this scheme. It migrates to warmer climates. Crop pests are the most obvious of these migrants.
Overwintering as Larvae:-
Many insects successfully pass the winter as immature larvae. The protection of heavy covers of leaf litter or similar shelters protect the caterpillar, while other insects replace the water in their bodies with glycerol, a type of antifreeze. Some grubs simply burrow deeper into the soil to escape the cold.
Overwintering as Nymphs:-
Insects are generally inactive in winter but the nymphs of dragonflies, mayflies and stoneflies live in waters of ponds and streams, often beneath ice. They feed actively and grow all winter to emerge as adults in early spring.
Overwintering as Eggs:-
Few insects lay eggs which survive in winter. The most prominent insects in this category are Praying Mantids. Corn Rootworms are also examples of this strategy.
Overwintering as Pupae:-
Some insects overwinter in the pupal stage, then emerge as adults in the spring. Moths in the Silkworm Family, Saturniidae, may be found attached to food plant branches as pupae in the winter.
Hibernation as Adults:-
Many insects hibernate as adults. Lady bird beetles are a well-known example, and are sometimes seen in great numbers in the autumn. Many large wasps seek shelter in the concealed places of houses or barns. Tree holes, leaf litter, and under logs and rocks are common shelters for overwintering adult insects. The Mourning Cloak Butterfly is usually the first butterfly that is noticed in spring, and this is because it hibernates in tree holes or other shelters during the winter as adult. Honey bees stay in hives during the winter, and form clusters when temperatures fall. They also are able to raise the temperature by vibrating wing muscles.
In general, insects are able to survive cold temperatures easiest when the temperatures are stable, not fluctuating through alternate thaws and freezes. Many insects can gain shelter and nourishment through the winter in a variety of micro-habitats. Among these niches are under the soil, inside the wood of logs and trees, and even in plant galls. One kind of fly is known by fishermen to be present in certain galls in winter, and the fly larvae are consequently used as fish-bait. Blankets of snow benefit insects by insulating the ground and keeping the temperature surprisingly constant. Honeybees have been studied during the winter and are found to remain semi-active in hollow trees through the generation of body heat. The consumption of up to 30 pounds of stored honey during the winter months makes this possible. Heat energy is produced by the oxidation of the honey, and circulated throughout the hive by the wing-fanning of worker bees. Insects that are inactive during the winter months undergo a state in which their growth, development, and activities are suspended temporarily, with a metabolic rate that is high enough to keep them alive. This dormant condition is termed diapauses. In comparison, vertebrates undergo hibernation, during which they have minor activity and add tissues to their bodies.
There’s warmth in numbers for some insects. Honey bees cluster together as the temperatures drop, and use their collective body heat to keep themselves and the brood warm. Ants and termites head below the frost line, where their large numbers and stored food keep them comfortable until spring arrives.
Certain insects, particularly ones that live in higher altitudes or near the Earth’s poles, use a state of torpor to survive drops in temperature. Torpor is a temporary state of suspension or sleep, during which the insect is completely immobile. The New Zealand weta, for example, is a flightless cricket that lives in high altitudes. When temperatures drop in the evening, the cricket freezes solid. As daylight warms the weta, it comes out of the torpid state and resumes activity.
Unlike torpor, diapauses is a long-term state of suspension. Diapauses synchronize the insect’s life cycle with seasonal changes in its environment, including winter conditions. Put simply, if it’s too cold to fly and there’s nothing to eat, you might as well take a break (or pause). Insect diapause may occur in any stage of development:
· Eggs – Praying HYPERLINK “http://insects.about.com/od/roachesandmantids/p/mantodea.htm”mantids survive the winter as eggs, which emerge in spring.
· Larvae – Woolly bear caterpillars curl up in thick layers of leaf litter for winter. In spring, they spin their cocoons.
· Pupa – Black swallowtails spend winter as chrysalids, emerging as butterflies when warm weather returns.
· Adults – Mourning cloak butterflies hibernate as adults for the winter, tucking themselves behind loose bark or in tree cavities.
Many insects prepare for the cold by making their own antifreeze. During the fall, insects produce glycerol, which increases in the hemolymph. Glycerol gives the insect body “supercooling” ability, allowing body fluids to drop below freezing points without causing ice damage. Glycerol also lowers the freezing point, making insects more cold-tolerant, and protects tissues and cells from damage during icy conditions in the environment. In spring, glycerol levels drop again.
Dr. Muhammad Anjum Aqueel1,3, Dr. Rashad Mukhtar Balal1,2, Dr. Muhammad Adnan Shahid1,2, Dr. Muhammad Mansoor Javaid1
1 University College of Agriculture, University of Sargodha, Pakistan;
2 Cornell University, NY, USA
3 Imperial College London, Silwood Park Campus, Ascot, SL5 7PY, U.K.