Gypsy Moth

The gypsy moth (Lymantria dispar) has been a serious pest of trees in the eastern United States.  It feeds on the leaves of many different species, but has a definite preference for oaks.  When present in large numbers, the caterpillars can completely defoliate mature trees.  The gypsy moth is not a native insect and originally had few natural enemies in this country.  It was first introduced into the United States in 1869, and outbreaks in the New England region occurred before 1900.  By the 1980s, the gypsy moth had established itself throughout the northeastern U.S., including Michigan.

The Rochester Hills Parks and Forestry Department has been monitoring the gypsy moth population within the city since 1983.  Gypsy moth numbers were low until the population began building rapidly in 1992-93.  In the fall of 1993, the Forestry Division identified specific areas of the city having a mature oak canopy that were most at risk to gypsy moth infestation.  A permanent network of egg mass survey sites was established in those areas.  The Division now visits survey sites each fall in order to estimate the potential gypsy moth population for the following year.  Areas with high egg mass counts are considered for possible treatment if it is decided that a suppression program (aerial spray) will be scheduled.

The first aerial spray program occurred in the spring of 1994.  The material used for aerial spraying is a biological insecticide known as "Bt."  Bt is derived from naturally occurring bacteria and only affects certain insects (caterpillars); it is not known to be harmful to other organisms (including people). From 1994 through 1996, gypsy moth numbers were relatively high, and aerial spraying was conducted each spring.  Populations generally declined from 1996-2002, but began to rebound in 2003.  An aerial spray was conducted again in May 2004.  This cycle of decline and increase is typical of gypsy moth populations, which normally alternate between high and low densities every 10-15 years. 

Due to lower population densities after 2004, no aerial spray has been conducted in subsequent years.  There is evidence that natural controls (diseases and parasites) have had a substantial impact on the gypsy moth population in recent years. The Forestry Division will continue to perform an annual egg mass survey and provide information on homeowner control tactics. 

If you suspect that gypsy moth egg masses are present in your neighborhood and want advice or assistance, please contact Forestry at 248-656-4673.