Pest-control strategies for gardens
Our Western Pennsylvania winter of 2016-2017 was mild and few of us are complaining.
You may be thinking that warmer winter temperatures will increase insect pest population this growing season; however, a moderate winter can also increase the survival rates of beneficial insects.
Even a less severe winter, with its variables of late frosts, cold snaps and prolonged, cold rains can be detrimental to early emerging insects and the plants that support their life cycles. The relationship between increased beneficial or pest insect survival and mild winter conditions is a very complicated one that is not easy to predict.
Insects are cold-blooded so cannot produce body heat to survive over the winter. To cope, they have several generalized strategies:
Migration
Diapause, a type of deep hibernation;
Production of antifreeze chemicals in their blood; and
Burrowing under plant debris or soil. Insects also overwinter as eggs, larvae or nymphs, pupae or adults which adds another dimension to survival strategies.
The monarch butterfly (Danaus plexippus) is a well-known migratory insect that avoids winter by migrating south to Mexico.
Successive generations move north in the spring eventually reaching our area where they reproduce a generation that once again migrates south as adult monarchs.
Some species may die during our winter, but return in spring on breezes from southern states where they survive the winter. They arrive and repopulate for the growing season. The Potato Leafhopper (Empoasca fabae) is an agricultural pest that uses this strategy.
Diapause occurs when metabolism slows and insects survive on stored body fat. It is generally triggered by shorter daylight hours in the fall. In spring, warming temperatures after a long period of cold signals insects in diapause to become active.
Insects may combine other strategies with diapause. The physiological manufacture of a chemical antifreeze in the blood prevents the creation of ice crystals which can kill the insect.
Adult insects mourning cloak butterflies (Nymphalis antiopa) or larvae (beetle grubs) may burrow under leaf litter or soil. The debris and soil layers above the insects act as insulation.Other insects create protective layers to allow eggs and larvae to survive, e.g. Eastern tent caterpillar (Thyridopteryx ephemeraeformis) eggs are surrounded by air-filled structures that insulate the eggs from winter temperatures. Gypsy moths (Lymantria dispar) have diapause larvae overwintering inside eggs covered with protective scales.In mild winters, the life cycle of early emerging insects may be out of sync with availability of their preferred food plant, leading to starvation.Moderate winter temperatures can also increase mortality of early emerging insects when their food source is killed during a spring cold snap or wet period.Multicolored Asian lady beetles (Harmonia axyridis) and brown marmorated stink bugs (Halyomorpha halys) both can overwinter in walls of homes; however, if they enter heated living spaces, they die as they use up fat reserves.We may see an increase in both beneficial and pest insects this summer.As gardeners, we can try a variety of strategies to enhance our chances for a successful growing season:Plant to attract beneficial insects;Plant a diversity of plants; andRotate planting locations so overwintering pest insects have a harder time finding their preferred food. Using pesticides in our garden kills both beneficial and pest insects.If you have additional questions about specific insects and how they survive the winter, call the Master Gardener's Greenline at 724-287-4761, Ext. 229. Watch for a future column on planting to attract beneficial insects to your garden.
