Cool wet weather affects insect populations

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The cool, wet weather this spring and summer is responsible for a rapid growth of some pest insect populations and the decrease of others, explained Mary Roduner, SDSU Extension consumer horticulture field specialist. He talked about pests found in horticulture but ones that are also problems in field crops.

"The extra moisture, with cooler temperatures is encouraging lush green plant growth later in the season than is normal for many areas of the state. This lush growth is encouraging the rapid growth of some pest insects. While at the same time, the wet weather is decreasing other insect populations," Roduner said.

Due to their reproduction habits, the saturated soils and excessive moisture has been devastating to populations of pests like, grasshoppers, cutworms and spider mites.  However, because of their feeding habits, for other pests like, aphids and plant bugs who are attracted to lush foliage, weather conditions have resulted in a population boom.

More about the pests & insects

Grasshoppers, family Orthoptera, are a pest that becomes a serious problem in hot, dry weather. However, because female hoppers lay their eggs in soil mainly during the fall months, Roduner said this year, when the soil is cool and water saturated coming out of the winter, eggs will develop fungal infections and rot before they hatch. The result is greatly reduced grasshopper populations.

"Young grasshopper nymphs are also very susceptible to fungal infections and will avoid areas that remain wet or have high humidity. There will always be a few but the numbers are not high enough to do serious damage," she said.

Cut worms, family Noctuidae. Cut worms are the caterpillars that cut tomato and pepper seedlings off at ground level during the night hours. "This year's weather will cause many overwintering larvae or pupa to drown. This can reduce the population for several years," Roduner said.

The adult moths are often referred to as "miller moths." In warm, dry years the numbers can be very large, causing incredible amounts of damage to field and garden crops. Control is difficult, Roduner explained, 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.

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," Roduner said.

Spider mites 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," Roduner said.

Aphids, family Aphididae are very small soft-bodied insects that suck plant sap.

"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," she said.

The best control for aphids, Roduner shared, is to apply lightweight horticultural oil. "This oil will clog their breathing spiracles or horticultural soap that breaks down the waxy coating on the exoskeleton," she said. "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," Roduner said. "Monitor plants closely to start control methods before the population is too large."

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," Roduner said.

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 best control method is to use a pyrethrum spray according to label directions.  Be sure the plant is listed on the label. 

Weather Creates Pollination Challenges

Cool wet weather will deter honeybees from flying and pollinating flowers, explained Roduner. "They are unable to fly when temperatures are below approximately 57 degrees Fahrenheit or if 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 the weather we've experienced this growing season," she said.

Honeybees, family Apidae, are our main pollinators and we depend on them for pollination of apples, pears, squash, cucumbers, some field crops, flowers and many other plants; however 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," she said.

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