Some insects are downright scary. Not because they can strip your bones in just a few minutes like a school of piranha, but because they can strip a tree of all its foliage relatively quickly.
The red-headed European saw fly, an insect we occasionally see in the spring stripping needles off long-needled pines is an example of such an insect. Feeding in groups of hundreds they can quickly devastate the tree they select for their attack. Another example is the cottonwood leaf beetle, Chrysomela scripta. The larvae, looking like large potato beetle larvae, skeletonize the leaves. What is even more scary is the massing of the ugly pupae suspended from a twig or dead leaf. They look like the pupae of some of the alien creatures seen in horror movies hanging from the rafters of the homes the aliens invade.
The larvae of this leaf-feeding beetle ooze droplets of a stinking, white liquid when disturbed. The smell is revolting and might even be disgusting to taste. I haven't taken the opportunity to find out if the latter is true. This latex-like ooze is reabsorbed by the larva after the danger has passed.
Up to five generations of this insect occur each year stripping off the new leaves that develop to replace those lost to the previous generation of the insect. Each time this occurs, energy is expended which should be used to increase root length and maintain the health of the tree. The energy loss can reduce the diameter of the tree's trunk by as much as 70 percent.
If leaves develop too late in the summer, the tree may not have time to set a new crop of buds for the following year's leaves. Such trees may not even have sufficient starch in their system to push leaves the following spring. Starch is the overwintering energy source produced through photosynthesis of green leaves. No leaves means no energy. In addition to reduced growth, such trees are also more susceptible to other insects and diseases due to the lack of energy necessary to ward off these problems.
The adult yellow and black beetles, with a body similar to the Mexican bean beetle, emerge from the suspended pupae and either mate and lay eggs to start a new generation of leaf-feeding larvae or move to the debris around the base of the tree or they squeeze under loose bark on the trunk to overwinter. The overwintering adults move back into the tree as soon as spring arrives and feed on buds, new leaves and developing twigs. This feeding injury opens the tree to infection by Cytospora canker and other diseases. The weakened state of the tree permits increased infection and the more rapid spread of the disease through the tree.
Some cottonwoods are more attractive to this insect than others. Populus deltoides, the eastern cottonwood, appears to be the desired species while aspen is seldom attacked by this insect. Aspen has its own leaf beetle, Chrysomela crotchi. Even though P. deltoides is desired, genetic differences in this species of tree make some more desirable than others. Hybrids of the eastern cottonwood also have varying levels of desirability for this beetle.
Recommendations to ignore the damage caused by this pest can result in the loss of trees, especially when susceptible trees are grown in a plantation setting. The eggs of the cottonwood leaf beetle are laid in clusters on the underside of the leaves like the eggs of the squash bug.
Crushing these will help reduce the overall population but this is not possible in mature trees. Control of this insect in large trees is best accomplished by the use of insecticides. This includes the use of insecticidal soaps or products such as SucraShield applied when the larvae are still small. These disrupt the integrity of the insects protective covering allowing the cell contents to leak causing the insect's death. These products must cover the insect; they have no residual effect. SucraShield for those of you who are unfamiliar with this product is a registered organic-approved, sugar-based product.
Products containing imidacloprid or dinotefuran, both synthetic systemic products, are also effective control materials. The former is sold by Bayer and other formulators; the latter is available as Safari and Xylam.
Applied this fall, these two products will provide control of leaf-feeding insects into next year. Dinotefuran is absorbed into plant tissue very quickly and thus used as a trunk spray to control this and similar insect pests. Other products such as the pyrethroids, Sevin, etc. are old standbys for control of this and many other insect pests. Keep in mind that Sevin can increase spider mite problems as it can kill beneficial insects that help control mites.
Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curt.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.