Winter Moth Numbers at a Record Low: Promising Data from the Elkinton Lab
It’s that time of year again, following the emergence of winter moth (Operophtera brumata) adult moths, where anyone with vested interest in the health of oak, maple, apple, and many other deciduous plants such as cherry, basswood, ash, white elm, crabapple, and blueberry, begin to wonder what is currently known about the winter moth population in Massachusetts.
Dr. Joseph Elkinton, Professor of Environmental Conservation at the University of Massachusetts, Amherst, MA, has excellent news for us all: data from his lab’s research locations in eastern Massachusetts suggest that this invasive pest’s population size is at an all-time low. In fact, the 2017 winter moth population was the lowest they have seen since studying and working toward the biological control of this insect for the past 13 years. Hopefully, this will mean low winter moth caterpillar numbers for the 2018 season, in most areas where they are present in eastern Massachusetts.
Winter moth outbreaks and subsequent defoliation of the above-mentioned host plants were first noticed in Massachusetts on Cape Ann on the North Shore and near Cohasset, Hingham, and Rockland on the South Shore in the late 1990’s. At the time, this defoliation was thought to be caused by a native insect. However, the persistence and severity of the outbreak in Plymouth County caused Deborah Swanson, UMass Extension Horticulturist for Plymouth County, Retired, to send caterpillars to UMass Extension Entomologist Robert Childs and Dr. Elkinton to further investigate what species could be responsible.
In 2003, following a collaborative effort with UMass Extension and Plymouth County Extension, Dr. David Wagner of the University of Connecticut and Richard Hoebeke of Cornell University confirmed that winter moth (Operophtera brumata) was responsible. Over the next decade, winter moth caused defoliation spread over all of the North Shore into the western suburbs of Boston and across the South Shore onto Cape Cod and Martha’s Vineyard and into Rhode Island and coastal Maine.
Winter moths overwinter in the egg stage, hidden in the cracks and crevices of bark or under lichen on host plants. Eggs are tiny and difficult to see without magnification. Each female moth may lay 150-350 of these eggs, which are initially green but quickly turn a red-orange color. In March, prior to hatching, eggs will turn bright blue and then dark blue-black. It is believed that egg hatch in Massachusetts occurs when 20–50 Growing Degree Days1 (base 50º F) have accumulated, which can be anywhere from late March into early-mid April, depending on the year and the location. (The Elkinton lab at UMass uses a base 40º F when completing Growing Degree Day calculations for this insect and suggests that hatch occurs between 177 and 243 GDD (base 40°F).2)
In most years, egg hatch coincides with bud break of the host plants. Tiny winter moth caterpillars will crawl up tree trunks and branches toward these buds and if the bud has expanded enough, they can wriggle their way inside and begin feeding, even prior to leaf expansion. These caterpillars cannot chew their way into closed buds, but once buds open and they enter, they are capable of significant damage to the host plant leaves, which will later expand and reveal shot hole-like damage. Like gypsy moth caterpillars, winter moth caterpillars are capable of dispersing using a technique referred to as “ballooning”. A strand of silk is spun and the lightweight larvae use the wind to be blow haphazardly toward host plants. Once the inchworm-like caterpillars are full-grown (approximately one inch in length), they drop to the soil to pupate, usually in late May, where the pupae remain until late fall.
Adult winter moths usually begin to emerge around Thanksgiving, or earlier, as evidenced last fall on November 16, 2017, as reported by Deborah Swanson (UMass Extension Horticulturist for Plymouth County, Retired) in Hanson, MA. Adult emergence continues through December and even into January, whenever air temperatures are mild (above freezing) and the ground remains unfrozen. Small, light brown male moths can be seen flying around, often attracted to porch lights or other outdoor light sources. Females are wingless and cannot fly; they crawl up tree trunks and release a pheromone to attract males for mating.
Since 2005, Dr. Elkinton and his laboratory, with the help of dedicated volunteers, have managed the large task of collecting, releasing, and monitoring the efficacy of Cyzenis albicans, a tachinid fly parasitoid of the winter moth as a biological control agent attacking winter moth (C. albicans can only successfully develop in winter moth). This fly was originally from Europe and had previously been established, in other North American locations such as Nova Scotia in the 1950’s, with much success in reducing outbreak populations of winter moth to a non-pest status. The female fly (Cyzenis albicans) lays its eggs (up to 1,000 of them) on host plant foliage of the winter moth, and while feeding, winter moth caterpillars will eat the eggs of the flies. The egg will hatch and a tiny fly maggot will lie in wait, inside the immature winter moth, until the caterpillar is done feeding and drops to the ground to pupate. The immature fly maggot will then complete its development inside of the winter moth pupa, where it will remain until the following spring.
In the spring, at approximately the same time winter moth eggs hatch, the flies (Cyzenis albicans) emerge from the deceased caterpillar pupae in the soil, mate, and lay more eggs for more caterpillars to ingest. There is one generation of flies per year.
To collect the fly, Elkinton’s lab manager George Boettner traveled each April-May for seven years to Vancouver Island in British Columbia where both winter moth and the fly were established in the 1970’s. At that location, he managed a crew of helpers who collected many thousand winter moth caterpillars over about a one-month period. About half of the winter moth caterpillars they collected had the fly larva inside them. The flies were then reared to the pupal stage in the laboratory in Massachusetts and the adult flies released at various locations the following spring.
To date, the Elkinton lab has released Cyzenis albicans at 43 sites in MA, CT, RI, and ME. The fly has been established at 32 of those locations and the Elkinton lab has evidence of its ability to spread throughout the local winter moth population at those sites from the initial point of release. They now have data for sites located in Falmouth, Hanson, Hingham, Wellesley, Wenham, and Yarmouth, MA indicating that parasitism of winter moth pupae by C. albicans ranges from 15-48%. Typically, this might not be high enough of a percentage to reduce a pest population, but in the case of winter moth – that is what the data shows at these locations. As parasitism rates for the tachinid fly increase, the density of healthy winter moth pupae decreases.
So how are these tachinid flies able to have such an impact on the winter moth population at this time? Data collected by the Elkinton lab shows that native predators in Massachusetts prey heavily on winter moth pupae in the soil where they reside between May and November. At their research sites, Hannah Broadly, one of Dr. Elkinton’s PhD students, has trapped and identified 29 species of predatory ground beetles (Carabidae) feeding on winter moth pupae. She has also identified what may be a previously undescribed (unknown to science) parasitoid wasp in the genus Pimpla (Family: Ichneumonidae). With the help of taxonomic collaborators, she is currently working to determine if this is a new species or whether it corresponds to one that has been previously described.
It would seem at this time, the decline currently seen in the winter moth population may be a combination of the impact of Cyzenis albicans, predators present in the environment, and parasitoids such as this Pimpla spp. coupled with abiotic factors. For example, recent warm springs have led to early egg hatch of the winter moth, and in some cases were followed with a cold snap lasting a week or so. In situations such as those, bud expansion may be delayed in which case young caterpillars cannot wriggle their way in to feed, and subsequently starve. Overall, the decrease in winter moth larvae, pupae, and adults seen in 2017 is a welcome sight for many in eastern Massachusetts and we certainly hope this trend continues!
For more information about winter moth, please visit the UMass Extension Winter Moth Fact Sheet or the History and Biological Control of Winter Moth Fact Sheet.
1Growing degree day information is available in the Landscape Message for areas throughout Massachusetts, using a base 50ºF. For more information on how to calculate growing degree days, please visit this fact sheet.
2The Elkinton lab at UMass uses a base 40º F when completing Growing Degree Day calculations for this insect and suggests that hatch occurs between 177 and 243 GDD (base 40°F, January 1 start date, double sine method). Resources such as uspest.org will allow you to manipulate how GDD’s are calculated, including using a base 40ºF and different mathematical models. The reason for using Base 40 is that 40ºF is very close to the minimum temperature for winter moth development and considerable GDD accumulation occurs between 40-50ºF. Predicting hatch with GDD’s is imperfect for this insect, however. Research in the Elkinton lab and in Europe has shown that later springs with older eggs require fewer GDD to hatch.
Dr. Joseph Elkinton, Professor of Environmental Conservation at UMass Amherst and Tawny Simisky, Entomology Specialist at UMass Extension
