Winter Phenology: HWA Lays Eggs

For an introduction on phenology and why studying the phenology of HWA is important, see our previous blog post about phenology by clicking here.

A plump adelgid (left) with eggs (right) on a hemlock twig with the waxy, cottony wool ovisac removed.

Winter is upon us, and so is the egg-laying season for hemlock woolly adelgid. It’s also the season for our Citizen Science volunteers to begin looking for signs of HWA laying eggs and hatching into crawlers. At this point, HWA have been growing and developing since autumn and have accumulated wool which forms an ovisac, where eggs will be laid. Yesterday we took samples from here on Cornell’s campus, observing that egg-laying is already beginning much earlier than expected after looking at last year’s egg-laying data. Our observations are helping us understand the life cycle of HWA and are helping us prepare for and schedule releases of our biocontrol insects.



The eggs being laid in the winter are the eggs of the first generation, or sistens generation, of HWA. The crawlers of this first generation hatch into the second generation, or progrediens generation, which, after another around of egg-laying in the spring, will result in the next round of sistens eggs. Our phenological studies are helping us see the subtle and often overlapping changes HWA goes through throughout its life. Here’s a glimpse of HWA’s life cycle over a year:

HWA life cycle throughout the year, showing the sistens and progridiens generations


With the sistens generation laying its eggs earlier than last year, we may also find a similar trend for the progrediens generation. As we collect more data, we will learn even more and be able to add a few more pieces to the phenology puzzle.

NOTE:¬†Since this is the time of year that HWA is reproducing, it is easy to move HWA from one tree to another and potentially infest healthy hemlocks. If you are out and about near hemlocks during this time, wash clothes and gear well after spending time outdoors to make sure you’re not spreading HWA!