Gypsy moth finding a niche in evergreens

Spruce trees provide haven for low populations of gypsy moth. Inspection of plants and sites where eggs could be laid is the key to preventing damage next year.

Gypsy moth
Photo credit: William M. Ciesla,
Forest Health Management International, Bugwood.org.

Over 15 years ago, much of Michigan was caught up in a struggle to prevent defoliation of trees by gypsy moth caterpillars. Populations of caterpillars were so large that it impacted recreational areas, forests, cemeteries, and did not spare home landscapes. Populations built up in central areas of the state and quickly advanced over the Lower Peninsula. Across Michigan, aerial spray efforts (regulated by the Michigan Department of Agriculture) utilized a biological control, Bt (Bacillus thuringiensis), to target young gypsy moth caterpillars within days of hatching. Bt was selected for use to reduce the caterpillar population, not eliminate it. Maintaining the insect’s population at low levels allowed the development of predators, pathogens and parasites needed for long term management.

Robins and cuckoos began to make gypsy moth larvae part of their staple. Ooencyrtis kuvanne, a tiny wasp, parasitizes the outer eggs of the buff colored egg masses. Calasoma beetles, Calasoma sycophanta, were introduced into Michigan to eat the larvae. A virus called nucleopolyhedrosis, also known as NPV, kills large number of caterpillars leaving them hanging in an upside down V on tree trunks. Unfortunately, NPV only works well when populations of the insect are very high. Despite all these pests, gypsy moth populations continued to be a problem in many communities. A real break came when a soil borne fungus, Entomophaga maimaiga was found along the East Coast killing off large numbers of caterpillars. The fungus attacks the caterpillars in midsummer, leaving them hanging straight down on tree trunks. The fungus is effective even in sites where as few as five larvae per acre are present, according to Amhurst research from the University of Massachusetts.

The fungus was brought to three sites in Michigan in 1991 and has now spread over much of the state. Entomophaga develops and infects caterpillars during periods of consistent rainfall in late spring and early summer. It proved to be an important biological control keeping gypsy moth populations at very low levels in Michigan over the last 8 to 10 years. The weak link in this pathogen is moisture and humidity. If rainfall is low, it will reduce the effectiveness of the fungus, allowing small pockets of the moth to grow.

A recent trend seen in many counties in southeast Michigan is for gypsy moth populations to live at low levels in blue spruces. Some years we have seen serious damage to the spruces. The close growing branches and sharp needles protect caterpillars from many predators. The pest seems to have found a niche surviving in the spruces. These low level populations can surge if dry conditions prevent development of the fungus. Severely dry conditions this past June and early July may have reduced development of the Entomophaga fungus. The spring of 2012 may see a surge in the population of gypsy moth. Inspection of plants and sites where eggs could be laid is the key to preventing damage next year.

Late summer on into autumn is a great times to go hunting for the buff-colored egg masses in trees. If you have spruces, check under the branches for the egg masses. If you find egg masses on structures, outdoor furniture or trees, knock them off into soapy water. This sanitation practice greatly reduces the number of caterpillars the following year. Some of their favorite trees include oak, birch, apple, willow, hawthorn, white pine, blue spruce, serviceberry and poplars, but they will eat leaves from maples and other trees that are not their most preferred species. The greatest risk is to evergreens like blue spruces and white pines that do not come back from defoliation as well as deciduous trees. Inspection now, especially in the spruces, will give you insight for possible control measures needed next spring and help prevent gypsy moth from finding refuge in these trees.

For more information related to gypsy moths, contact your local MSU Extension office.

Detailed information on Entomophaga can be found by reading MSU Extension Bulletin E-2604, Entomophaga maimaiga – A Natural Enemy of Gypsy Moth.

Related Source:
Gardening in Michigan

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