I often return to Martin Prechtel’s idea of each species having its own eloquence that manifests at some peak moment during the year. It’s a pretty anthropocentric way of viewing the life histories of other organisms, but it captures quite nicely those incredible moments during the year when the staghorn sumacs turn a brilliant purple, the barred owls singing in the dark winter skies, sap staff flowing, amphibians migrate to their breeding grounds and so on. Well, for Lymantria, the time is August when the caterpillars pupate and the adults emerge in a torrent of fluttery reproductive frenzy. Here we look at adult Lymantria dispar (now called silky) moths.

More on the silky moth

Females vs Males

Lacking a digestive system, adult Lymantria dispar moths essentially the sex organs of the caterpillars. They only live about a week. But what a week (more on that below)! The specific epithet, dispar, is related to the word disparate, for separate or different, and refers to the sexual dimorphism between males and females. Females are about 33% larger than males, but it’s probably easier to differentiate the sexes by three key features: (1) color, (2) behavior, and (3) antennae. 

A brown male with large antennae - mounting a white female Lymantria dispar (St Marks Church, Burlington)
FEMALES

The females have bright white wings with small black speckles, and their shiny brown body is often visible near the head. Though they have large wings, the females are entirely flightless, remaining in one place on the trunk (often near clusters of pupal casings) waiting for the males to arrive. To assist the bumbling quests of males, females release a pheromone, disparlure (source), that attracts males to their white beacon of a body. Female antennae are thin, black, and poorly developed.

MALES

Males are brown, with the same shiny brown spot just behind the head. Contrasting with the sedentary females, males fly frantically near the base of trees trying to pinpoint the source of pheromones. Owls have offset ears to pinpoint the sound of voles under the snow (video), and this type of flight helps triangulate the source of the pheromone. They’re assisted in the hunt by well developed, feathery antennae.

Courtship

Again, the females secrete the disparlure pheromone over the course of about 3-4 days. Males will mate with multiple females, and as with males in most species, show no discrimination in mate choice (source). There’s definitely a courtship process the males run through before mating, but females don’t really seem to care much and rarely reject the advances of males (source). The males seem to walk around the female, fluttering the wings, making advances. Eventually the male will stand next to the female and curve his abdomen towards her, at which point the copulate. It’s not uncommon to see multiple males on top of or adjacent to females.

Multiple males attempting to mate a female Lymantria dispar (Centennial Woods, Burlington)

Egg masses

The female lays her eggs on the trunk of a tree (though not always, see image below) within a day or so of mating. After depositing the eggs, she then covers them in a thick, silky covering to protect the eggs through the winter. The egg masses are light brown and contain ~300-600 eggs (most of the ones I’ve counted were on the lower end of this range, though there are reports of up to 1,000 per egg mass). The silky covering reportedly causes rashes in some people (as do the caterpillars), but I didn’t react after touching the egg masses. Egg size and # is inversely correlated to population density. Not surprising, as higher competition for food means less energy available to adults for reproduction.

Breeding has all but ended by this point in the summer. The fertilized eggs will develop into larvae that will then go into a stage of dormancy (diapause) throughout the winter and hatch in the spring. 

Closeup of Lymantria dispar eggs, exposed after someone scraped away the silk covering (St Marks, Burlington)
A cluster of female Lymantria dispar moths on the trunk of a beech (Centennial Woods, Burlington)
Female Lymantria dispar egg cluster on a silver maple leaf (Colchester)

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