Homeowners in the central U.S. may have missed the window for taking a two-step approach to controlling 2009's bagworms, but they still have an opportunity to rid their landscape of the pests.
They still can achieve good results with a single spray, if they wait until all of this year's month-long hatch is complete. In Kansas, for example, that typically means applying insecticide some time in late June or early July, according to Bob Bauernfeind, entomologist with Kansas State University Research and Extension.
"The bagworms that hatched earliest will have attained some size during the time that they had to feed on spring's lush foliage. But, they'll still be small enough that their damage will be fairly insignificant, too," he said.
The two-spray approach differs in that it adds an insecticide application in the middle of the year's hatch — usually the last week of May or first week of June. So, none of the "worm crop" has long to grow or eat. The approach is particularly prudent, Bauernfeind said, when a tree or shrub has been so hurt by previous bagworm attacks that it needs the best protection possible.
"Since you now can spray only once, though, you must achieve thorough coverage. That's more important than your choice among the bounty of insecticides labeled for bagworm control. Misting all over the outside of an infected tree or shrub simply won't get the job done," the entomologist warned.
Thorough coverage means getting insecticide deep into the central parts of infested trees and shrubs, as well as on their outer needles or leaves, he said. Penetrating dense foliage to achieve that inner coverage typically requires using a hand sprayer at its highest pressure setting. Homeowners with an infested windbreak or other tall, mature tree may have to hire a professional with a high-pressure ground sprayer that's capable of delivering high volumes of the recommended insecticide-water mixture.
"If you really want to achieve control, don't stop after treating the heavily infested plants. Especially if they're nearby, treat the trees or shrubs with light infestations, too," Bauernfeind said. "Then, as a follow-up, pick off and destroy any bags you find on your plants next winter, when they're easier to see."
Twospotted spider mite
The warm weather we are experiencing throughout Kansas means it is time to be aware of potential damage caused by the twospotted spider mite, Tetranychus urticae. Twospotted spider mite is considered a warm weather mite because populations are active from late spring through early fall.
Summer temperatures allow twospotted spider mites to reproduce rapidly, so that they overwhelm natural enemy populations, which under moderate temperatures are able to regulate them.
Twospotted spider mite has a broad host range, feeding on a diversity of ornamental trees and shrubs including ash, azalea, black locust, elm, euonymus, maple, oak, poplar, redbud, and rose. Twospotted spider mite also will feed on many herbaceous annuals and perennials such as marigold, pansy, aquilegia, buddleia, clematis, daylily, delphinium, phlox, rudbeckia, salvia, Shasta daisy, and verbena.
Twospotted spider mite adults are oval and about 1/16 inch long, requiring a magnifying glass. Color varies from green-yellow to red-orange. Adults possess two lateral dark spots visible when the spider mite is viewed from above. Both adults and nymphs may be present on plants, but they are often more numerous on older leaves. Twospotted spider mites produce fine silk, which may be seen between leaves.
Webbing produced by twospotted spider mites protects them from predators. Heavy rainfall may disrupt and remove the webbing. Twospotted spider mites feed on leaf undersides, removing chlorophyll (the green pigment) from individual plant cells and feed near the leaf midrib and veins. Leaves are stippled with silvery gray to yellow speckles. Heavily infested leaves appear bronzed, turn brown, and eventually fall off. The warm, dry conditions of summer favor rapid development of twospotted spider mite populations, in addition to enhancing feeding and reproduction. The life cycle from egg to adult occurs within five days at temperatures less than 75 degrees. Twospotted spider mite females don't have to mate to reproduce, laying up to 300 eggs during their two- to four-week lifespan.
Managing twospotted spider mite populations involves sustaining plant health, implementing sanitation practices, and/or using miticides. Maintain proper watering, fertility, and mulching to reduce plant stress and potential problems associated with twospotted spider mite populations. It is best to monitor for twospotted spider mite populations by knocking spider mites off plant parts such as branches or twigs onto a white sheet of paper.
An effective and rapid method to deal with twospotted spider mite populations is to apply a hard water spray throughout the plant canopy. This will dislodge eggs and motile life stages (larvae, nymphs, and adults) and preserve natural enemies. Removal of plant debris and weeds eliminates overwintering sites. In addition, many broadleaf and grassy weeds are hosts for twospotted spider mites. Miticides recommended for regulation of twospotted spider mite populations outdoors are numerous so read the product labels carefully. Read the label and apply before twospotted spider mite populations are extensive and causing aesthetic injury.
Many pest control materials used to control plant-feeding beetles, caterpillars, and other insects may be harmful to natural enemies of twospotted spider mite, and may inadvertently increase populations. Two repeat applications is recommended as many of the products will not kill the eggs. Repeat sprays will greatly reduce the populations.