by Laura Rodley
The winter moth defoliates leaves of oaks, maples, apple, and other deciduous trees and can be responsible for failure of blueberry and apple crops.
A green inchworm-like caterpillar was discovered in the 1990’s to be defoliating such host plants in communities in eastern Massachusetts. Initially, it was thought to be the activity of a native insect such as the Bruce spanworm or cankerworms. By 2003, these caterpillars were determined to be those of a non-native insect, the winter moth (Operophtera brumata). Like gypsy moths, the females of this species don’t fly. This insect used to be a pest in Nova Scotia, but has since been detected in eastern Massachusetts, Rhode Island and coastal points north in New Hampshire and Maine.
“The rate of their spread is about 8.3 kilometers a year,” said Dr. Elkinton of UMass speaking at the Invasive Insect Certification Program for Landscape, Nursery, and Urban Forest Pests provided by UMass Extension. “Once an outbreak starts, [winter moth] stays in an outbreak; there are currently no pathogens to control it.”
However, there are two important insects that use winter moths as hosts that have been successfully implemented as biocontrol agents in areas such as Nova Scotia. The tiny parasitoid wasp, Agrypon flaveolatum, and a parasitic fly, Cyzenis albicans (C. albicans) also parasitizes winter moth. This fly is incredibly host-specific, only killing winter moths, and has been introduced into Massachusetts to help decrease the destructive caterpillar population.
To control winter moths with the parasitic fly, C. albicans, Dr. Elkinton and his crew travelled to Vancouver, British Columbia where it keeps the winter moths in check and “spent all day beating caterpillars off trees” to collect them and return to his lab.
The caterpillars with the parasitic fly inside them were brought back to his UMass lab. When the resulting flies emerged and were confirmed to be C. albicans, Elkinton and his lab released them at different sites in Massachusetts to manage winter moth. “To get the flies established, it takes time and tenacity,” he said, and a period of years to determine if any have been established.
So far, the parasitoid has been released in 41 sites from coastal Maine to southeastern Connecticut, and successfully established in 21 locations.
“In Wellesley, Massachusetts, Cyzensis albicans has established and winter moths’ density has declined about 90 percent,” said Elkinton in evidence of the success of this biological control agent in managing winter moth.
In Wellesley, C.albicans is spreading at a rate of 10 km in all directions, and it is expected that it will soon be spread throughout the areas in Massachusetts that are troubled by large winter moth populations.
Chemical management of winter moth on ornamental plants includes application of B.t. Kurstaki (Bacillus thuringiensis Kurstaki) to target young, actively feeding larvae when foliage of the host plant is completely expanded. Spinosad is effective against older larvae but cannot be applied directly to flowering plants as it is hazardous to pollinators when it is still wet. Once it has dried, however, spinosad is no longer hazardous to pollinators. It is important to manage winter moth early, when the caterpillars are still young. Homeowners are encouraged to contact a licensed pesticide applicator if considering chemical or biorational management options.
Another pest insect that has been problematic in eastern Massachusetts is the black oak gall wasp, now known to be Zapatella davisae. It is a stem-galling insect that lays its eggs on twigs, subsequently causing swelling and leaving tiny exit holes as the adult wasps emerge. Symptoms beyond stem swelling include leaf dieback in clumps, also known as flagging.
Black oak is the favored host. “DNA analysis reveals that the gall wasp on Cape Cod and the islands are identical to those on Long Island, NY which have previously caused problems in that location,” said Elkinton. This wasp was named in honor of Monica Davis, one of Elkinton’s graduate students at the University of Massachusetts.
In a study conducted by Davis on Cape Cod, trees were found to be more severely infested with the gall wasp, while on Long Island there is currently a low infestation. Davis’ research has identified natural enemies of the gall wasp, present in their populations in Massachusetts, that are in some cases previously unknown to science. Further research regarding the black oak gall wasp is necessary.
Winter moth and black oak gall wasp in eastern Massachusetts
by Laura Rodley