Gypsy moth management is complicated, possible
The gypsy moth is a non-native (exotic), invasive insect from Europe.
The gypsy moth, like all moths and butterflies, undergoes “complete metamorphosis.” Within one year, the insects change from an egg (found in 1-inch beige, furry, masses on trees), to a larva (caterpillar), to a pupa, to a reproducing adult.
Eggs begin to hatch mid-April and the quarter-inch black caterpillars eat and grow and undergo a change in appearance over the next 40 days.
Gypsy moth caterpillars are voracious eaters. In high numbers, they can cause defoliations of deciduous trees. Because trees produce their energy through photosynthesis in green leaves, gypsy moth caterpillars stress many forests, especially those dominated by oaks and aspens.
However, trees store energy in their roots and have other evolutionary responses to defoliation by native insects. Thus, the caterpillars of gypsy moth do not often cause widespread direct tree mortality, as does the emerald ash borer (another exotic/invasive forest insect). Gypsy moth caterpillars do cause trees to look unsightly and are a pest in many other ways, but trees that die during gypsy moth outbreaks are often of reduced vigor already.
According to a paper in the journal “The Great Lake Entomologist (1983)”, gypsy moths were first documented in Michigan in 1952. Early management attempts aimed for eradication using DDT, the same chemical that prompted Rachel Carson’s “Silent Spring”. Subsequently, a naturally occurring bacterium (Bt) has been aerially sprayed to suppress populations. Eradication is rarely discussed by professionals now.
Gypsy moth management is complicated.
First, as discussed in “Pest Management and Sampling (1991)”, a monitoring program must be devised and systemically applied so that treatment efficacy can be quantified. The need for monitoring is essential, because populations of gypsy moth and many other forests insects fluctuate (cycle) over time. Moreover, population fluctuations can occur in a discontinuous, patchy fashion. Some areas may experience more moths than other areas in the same year.
It is during times when the gypsy moth population is high that tree defoliation is most easily observed and most management actions occur. In effect, however, gypsy moths are always around, but the moth population is low.
As discussed in an article in the journal “Trends in Ecology and Evolution (1996)”, gypsy moth populations may be held in check in most years by parasitic organisms that prey on the moth’s eggs, larvae, and pupae. When the populations of the parasites are low, the gypsy moth populations erupt for a couple years on approximately 10-year cycles.
Another issue that must be considered is the non-target impacts of Bt, the commonly applied bacterium used to kill gypsy moths.
In another study, in “The Great Lakes Entomologist (1997)”, researchers demonstrated that Bt also kills the endangered Karner blue butterfly. Researchers surmise that other members of our native moth and butterfly community in Michigan are also susceptible.
Thus, like many forest management actions, there are collateral issues that must be considered.
So, what can landowners do to manage or mitigate for gypsy moths without spraying?
Techniques suitable for backyard trees are dissimilar from what a landowner with 40 acres should do.
For backyard situations, keep trees well-watered, but do not fertilize. Reduce the number of egg masses by scraping them off trees in late winter or early spring. Place the egg masses in a soap water solution for a couple of hours or burn them. Later in the spring, emergent caterpillars can be impeded and collected by a combination of tree “barrier and collection bands,” as discussed by the Wisconsin Department of Natural Resources (Google search).
Forest landowners should take a step back and consider their property from the perspective of its capabilities based on soils, climate, seed sources, and past management.
Gypsy moths impact oak and aspen forests more so than deciduous-coniferous mixed forests. As demonstrated by a number of studies, past forest management actions have generally increased the dominance of oak and aspen relative to historic conditions in northern Michigan.
Thus, as stated before in this column, landowners are well-served by promoting forest tree diversity within the context of known successional pathways on a given soil type. On many northern Michigan sites now dominated by oak and aspen, irregular planting of conifers — such as white spruce or eastern white pine on better sites with partial sun, or red pine or jack pine on drier sites with full sun — may be a simple and cost-effective way of promoting diversity.
Gypsy moths are here to stay for the foreseeable future. Making our forests less susceptible may be the most time- and cost-effective way of mitigating the impacts of gypsy moth.
Managing for resistance and resilience in our forests is key.
Greg Corace is the forester for the Alpena-Montmorency Conservation District. Greg can be contacted at firstname.lastname@example.org or 989-356-3596, ext. 102.