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New Mexico State University

New Mexico State University

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Dog Days of Summer Busy for Bugs

LAS CRUCES -- Many bugs that can harm trees and shrubs are on the move during the last months of summer, said a New Mexico State University entomologist.


"Borers are the most destructive pests, because they infect the vascular system of the tree, affecting the movement of water and food throughout the plant," said Bob Cain with NMSU's Cooperative Extension Service. "Leaf-feeding insects cause less severe damage. If trees are healthy, they have enough stored energy to withstand damage caused by defoliation."

The bark of stone fruit trees, such as peach, plum and cherry, is where peach tree borers lay their eggs. This is a good time of year to apply an appropriate insecticide to the trunk to prevent problems, Cain advised. Before treatment, remove mulch from the base of the tree. Once larvae tunnel beneath the bark, they can't be controlled easily.

Bark beetles, wood borers and twig beetles look for weakened trees to infect. Trees can be weakened by crowding, drought, transplanting or mechanical injury. To reduce moisture stress on trees, water during hot, dry periods, Cain advised.

Many leaf-feeding insects attack both conifers and deciduous trees. "Piņon sawflies in the larval stage are actively feeding on piņon foliage right now," Cain said. "This is a few weeks later than usual."

Mature larvae are pale green with dark green stripes down their sides and a white stripe down the center. Small outbreaks usually defoliate only a few branches and cause insignificant damage, he said. Sawflies can be controlled with insecticidal soap or chemical insecticides.

Now is the peak time for piņon tip moths to emerge and lay eggs, Cain said. Control is not usually needed, because unlike other pine tip moths they do not cause serious damage.

Fall webworms are hairy, tent-making caterpillars that have just started hatching and forming their web tents. "Look for and remove newly formed tents in cottonwood, poplar, elm, mulberry and other deciduous trees," Cain said. These insects feed in groups, creating large tents in the upper branches of trees. They cause minimal damage, but strip the leaves from the branches where the tents are located.

Bagworms are common in Albuquerque and some eastern New Mexico communities, he said. The larvae become more noticeable as the bag of silk and the clipped foliage that surrounds each larva increases in size. Both conifer and deciduous trees can be affected by these. Bagworms can be controlled by chemicals or Bacillus thuriengensis (B.t.), a natural insecticide. On small trees and shrubs, Cain recommends simply picking off and destroying the bags.

Circular holes in leaves may be caused by leafcutter bees. Adult females cut pieces from leaves to build nesting cells. Leafcutter bees affect rose, ash, lilac, redbud, fruit trees and other deciduous plants.

"In most situations, established trees and shrubs can withstand leafcutter bee feeding," he said. Newly transplanted plants should be covered with mesh until established. The insects prefer tender leaves, so planting conifers, or plants with heavier, waxier leaves can help avoid problems in areas where this insect is abundant.

"If it looks like mutant purple tomato hornworms have invaded your grapevines, you've probably discovered the larvae of the achemon sphinx moth," Cain said. "During its month of feeding, this moth can damage grapes or Virginia creeper, but usually is just a curiosity."

They can easily be controlled with B.t., he said. If not, they will emerge as large, heavy-bodied moths with marbled, brownish-gray forewings, well-defined dark spots and rosy-pink hind wings.

Plants with dense foliage including juniper, cypress, arborvitae and spruce are the most susceptible to spider mites. Heavy rains will reduce spider mite numbers, he said. If summer storms are not hitting an area, spray foliage with a garden hose periodically to reduce numbers.

"This also is the time of year to examine leaflets for honey locust spider mites," Cain said. "They usually occur near leaf veins, particularly at the base."

Trees also may be damaged by elm leaf beetles and poplar blackmine beetles, and these problems are treatable in the spring, he said.