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Cool, Wet Weather Affects Insect Populations

Written by Mary Roduner, former SDSU Extension Consumer Horticulture Field Specialist.

Every year our insect problems are different. Warm, dry weather has an entirely different set of problems than a year that is cool and wet. Spring and early summer can been cool and wet in South Dakota. The extra moisture with cooler temperatures encourages lush green plant growth later in the season than normal for many areas of the state. This lush growth encourages the rapid growth of some pest insects and the wet weather decreases some insect populations.


grasshopper sitting on plant leaf
Adult grasshopper. Courtesy: Mary Roduner

Grasshoppers, Family Orthoptera, are a pest that becomes a serious problem in hot, dry weather. Large numbers of grasshoppers can reduce a field to nothing in a matter of days, so their numbers are a great concern to farmers and gardeners.

Female hoppers lay their eggs in soil mainly during the fall months. In years like 2014, when the soil is cool and saturated coming out of the winter, eggs will develop fungal infections and rot before they hatch. Young grasshopper nymphs are also very susceptible to fungal infections and will avoid areas that remain wet or have high humidity. This reduces the grasshopper population to very low levels. There will always be a few but the numbers are not high enough to do serious damage.


Several small green insects on a plant leaf.
Aphids. Courtesy: Whitney Cranshaw, Colorado State University,

Aphids, Family Aphididae and others are very small soft bodied insects that suck plant sap and pass many diseases. In 2014, aphids are being drawn to the very lush foliage of plants. Alfalfa, sweet clover and ornamentals have especially large populations. Aphid females give birth to live females who are already pregnant. Each newborn aphid is pregnant with multiple generations causing populations to explode rapidly. Some females will develop wings and are able to fly or be blown by strong winds to new locations.

The best control for aphids is lightweight horticultural oil which will clog their breathing spiracles or horticultural soap that breaks down the waxy coating on the exoskeleton. Insects are not able to heal themselves so when the wax coating is removed or damaged, soft bodied insects like aphids will quickly dehydrate. Using an insecticide spray is not recommended. Sprays will kill the lady beetle larvae that feed on aphids and the aphids themselves develop resistance very quickly. Monitor plants closely to start control methods before the population is too large.

Plant Bugs

Brown insect on green plant leaf.
Tarnished plant bug. Courtesy: Louis Tedders, USDA,

Plant bugs, Family Miridae, are a very diverse group of insects that feed on plant sap. Some species will pass diseases, but the main damage done to many plants is the removal of large amounts of sap, reducing a plant’s ability to produce and store nutrients. Tarnished plant bugs feed on strawberries, raspberries, blackberries, peaches, tomatoes and other fruits. Their damage is called “catfacing”. As the liquid in the developing fruit is removed, it dries and becomes hard and bitter.

The large amount of very lush growth is causing a surge in the populations of the many species of plant bugs damaging fruit crops as well as forage crops like alfalfa and sweet clover. The best control method for homeowners is to use a pyrethrum spray according to label directions. Be sure your plant is listed on the label.

Cut Worms

Grety to brown colored worm curled up next to plant stem.
Cutworm and damage to corn seedling. Courtesy: Clemson University - USDA Cooperative Extension Slide Series,

Cut worms, Family Noctuidae, are the caterpillars that cut tomato and pepper seedlings off at ground level during the night hours. Many gardeners have planted their plants only to find the remains lying on the ground the next morning. The wet, and in some areas, saturated soils of 2014, will cause many overwintering larvae or pupa to drown. This can reduce the population for several years. The adult moths are what people call the “miller moths” that are drawn to lights at night. In warm, dry years the numbers can be very large, causing incredible amounts of damage to field and garden crops.

Control is difficult because the larvae spend the day below the ground and targeting sprays are rarely successful. For the home gardener, a paper collar around the new transplant will prevent the larvae from reaching the plant stem.


yellow and black striped bee resting on a white flower.
Honeybee on garlic chive flowers. Courtesy: Mary Roduner

Honeybees, Family Apidae, are our main pollinators. We depend on them for pollination of apples, pears, squash, cucumbers, some field crops, flowers and many other plants.

Cool wet weather will deter honeybees from flying and pollinating flowers. They are unable to fly when temperatures are below approximately 57° F or there is mist and rain in the air. This means that early spring flowers and fruit trees will not have pollinators when temperatures are below average like in 2014.

Fuzzy, black and yellow bee on purple flower.
Bumblebee on fan flower blooms. Courtesy: Mary Roduner

Bumblebees are able to fly and pollinate many crops during cool wet weather. Encouraging bumblebees will increase pollination and fruit crops.

Avoid spraying insecticides when either bee is active to prevent sprays and residues from killing them.

Our bee populations are in danger and cool, wet weather is making their lives harder.

Spider Mites

small, green to yellow colored mites on green plant leaf.
Two-spotted spider mites. Courtesy: David Cappaert, Michigan State University,

Spider mites, Order Acari, are a hot dry weather pest. While not an insect, but a spider relative, spider mites cause so much damage that most people lump them with insect pests.

Two main varieties are found in gardens. Red spider mites make dense, fine webbing on plants and will cover plants completely. Two-spotted spider mites do not make much webbing but are far more destructive to plants.

During cool and wet seasons like spring 2014 or during times of high humidity, spider mites do not show up in large numbers.


Green plant leaf with brown specks and white discoloring around veins.
Two-spotted spider mite damage on green bean leaves. Courtesy: Mary Roduner

They generally stay on the undersides of leaves and suck large amounts of sap. Leaves take on a spotted appearance and dry out. Mites are able to also pass some diseases.

Spider mites can be sprayed with Neem oil weekly. It is organic and will not damage other insects. Because it is an oil formulation, be sure to spray on cloudy days or at dusk to prevent damage to the leaves.

Close observation will help you identify insect problems early before they are a serious problem, or let you know when an insect is not a serious problem.