Home, Yard, & Garden Newsletter article from 6-22-15

Japanese beetle adults are present throughout Illinois. Robert Bellm, Extension Educator, reported their presence in southern Illinois, and Martha Smith, Extension Educator, reported them in northwestern Illinois. Extended deep soil freezing in northern Illinois during the winter of 2013-2014, and the statewide drought in the second half of the summer of 2013 caused reductions in their numbers during last summer. The combination of relatively normal rainfall last summer and shallow soil freezing last winter due to moderate temperatures and adequate snow cover should allow their numbers to increase significantly from last year.

Adult Japanese beetles feed on the upper leaves of crabapple, linden, serviceberry, flowering cherry, birch, willow, rose, and many other trees and shrubs from late June through mid-August. They can defoliate or window-feed at least the upper third of the tree. Window-feeding consists of the upper surface and center of the leaf being eaten, leaving the lower surface which is whitish and somewhat transparent. The cells of this exposed lower surface soon turn brown. The adult Japanese beetles are three-eighths to one-half inch long and stocky with metallic green bodies and coppery wing covers.

Japanese beetles and two types of damage on grape.

Japanese beetle adults on grape.

Imidacloprid, sold as Merit and other brand names, moves systemically through the tree into the leaves where it effectively controls the adult beetles. Imidacloprid takes just a couple of days to move into the leaves whether trunk injected or soil applied. Avoid application of imidacloprid into mulch, thatch, or other dead organic matter as the insecticide adsorbs onto it, making it unavailable for root uptake. Soil inject below the sod or mulch or pull back the sod or mulch to soil drench. Apply within two feet of the trunk.

Imidacloprid remains in the tree for at least a year after application, so application at any time during the growing season is effective as long as there are active leaves on the trees. The active leaves are needed to drive the transpiration which moves the imidacloprid throughout the tree.

There is concern about imidacloprid moving into the pollen and nectar of flowers and affecting honey bees and other pollinators. Imidacloprid is known to move into linden flowers, so its use on that tree should be avoided. Wind-pollinated trees, such as birches, are unlikely to be visited by pollinators. Roses with double blossoms are usually not visited by pollinators either, but watch for flower visits by bees or other pollinators before applying imidacloprid. Imidacloprid does not move into crabapple flowers. Realize that soil applications will move into flowers grown near the tree as well as nearby dandelions and other flowering weeds visited by pollinators.

Spraying the foliage with a labeled pyrethroid avoids most pollinator concerns. Although fresh residues are toxic to honey bees and other pollinators, pyrethroids will kill the visiting bee too quickly to carry anything back to the hive. Insecticide being carried back to the hive to affect larvae and other colony members is the biggest concern with imidacloprid and other neonicotinoid insecticides. Carbaryl, sold as Sevin, can be applied to non-flowering trees and shrubs. Although honey bees will actively gather the dried carbaryl residue and take it back to the hive, they are unlikely to discover it on a non-flowering plant. Pyrethroid or carbaryl sprays should be repeated every couple of weeks to maintain protection. With the Japanese beetle adults present in large numbers for about six weeks, three sprays are typically needed. (Phil Nixon)