Last year I started noticing egg masses from the moth, Lymantria dispar (as part of the Better Common Names Project, the Entomological Society of America removed the moth’s common name, which was offensive to the Romani people, and is actively seeking suggestions for a new name), but I hadn’t seen the wholesale destruction that they’re capable of until this summer.
The solar eclipse. The Halloween storm. The 1998 Ice Storm. Every now and then there’s some event in the natural world so grand, so all encompassing, and often so destructive that it demands our attention. And this summer it seems to be the outbreak of the invasive moth, Lymantria dispar. It’s been about 3 decades since the last plague-like outbreak of the moth here in New England and in places, the results have been jaw-dropping. In this and the next couple newsletter’s we’ll look at the life cycle of the moth and why this year was saw an explosion in their population.
At maturity the hairy caterpillars are dark gray/brown with six pairs of red dots and five pairs of blue dots on their back. Many people have contact reactions to the long hairs and can develop rashes if they touch the caterpillars. The caterpillars have a voracious appetite for the leaves of trees. I’ve noticed egg masses or the caterpillars feeding most commonly on oaks, maples (red, silver, Norway, boxelder, and sugar), aspens, cottonwoods, and elms, but they’ll forage on 300+ different species, even extending their diet to include conifers and herbaceous plants in the absence of their preferred species.
And the diet does have an affect on the vitality of the caterpillars. Caterpillars fed a tannin-rich diet of their favored oak leaves were 4x less susceptible to a viral infection than those feed a sugar-rich diet (source). This plays out in forests, where L. dispar caterpillars fed for 50 days on oaks compared to 62 days on maples before pupating (source).
Nitrogen concentrations in leaves are highest in June after leaf out but before nutrients are redistributed throughout the plant (source). Another study looking at nitrogen concentration in the diet found that higher nitrogen concentration increased growth rate (source), so it’s not surprising that the caterpillars forage in that May/June Goldilocks period.
The study all found that caterpillars develop faster and eat less frequently when raised in warmer conditions. Since caterpillars in colder conditions have to eat more frequently to make up the energy deficit, this makes them more susceptible to predation, parasites, and disease. As summer temps continue to warm, caterpillar development will continue to speed up, providing a narrower window of time to effectively control populations.
Effect on trees
As seen on that image at the top, the effect can be shocking, with the caterpillars entirely defoliating stands of trees. Site health and soil composition of a forest seem to be less important that stand composition for determining the severity of the outbreak (source). Stands with low-diversity and high concentrations of oaks seem to be hit the hardest, while mix-diversity stands were more resilient. An initial outbreak tends to see the highest mortality rates amongst trees, while subsequent outbreaks were less fatal. We’ll look more at the response of trees in the next newsletter. Until then, keep hope!