Keeping the Legacy Alive

Phenology of Hemlock Woolly Adelgid


Phenology is the life cycle of an organism. Hemlock Woolly Adelgid has a complicated life cycle with multiple phases and changes in form. Since 2018, HWA phenology research in Nova Scotia has been conducted by NS Department of Natural Resources and Renewables (NSDNRR). Staff at NSDNRR have been studying HWA phenology in the western counties to gain a better understanding of its behaviour. Eventually, various types of natural predator controls (biocontrol) could be introduced into the population to help mitigate the spread of HWA. The phenology work will help to determine when the crawler stage begins and ends in the study areas. This will help reduce the spread by humans and provide up-to-date information for wood movement restrictions.

HWA reproduces asexually and has two generations per year, both completed on hemlocks. Both generations found here complete non-winged life stages. In its native range, HWA has a winged sexual generation that is completed on spruce trees, but this sexual stage is not completed in North America so this proportion of the generation is lost here.

HWA Life Cycle Diagram:

Simplified life cycle of hemlock woolly adelgid. The anholocycle part is all that occurs in eastern North America. The full holocycle occurs in Japan where this invasive insect originated. (Illustration by N. P. Havill and V. D’Amico)


1st Generation: Sistens

The first generation (sistens generation) of HWA hatches in the summer (June-July). HWA eggs hatch into nymphs known as “crawlers” — HWA’s only mobile phase. Crawlers settle on the tree’s new growth, insert their straw-like mouthparts (stylets) directly into the hemlock twig near the base of a needle, and immediately go into their dormant period (aestivation) for the remainder of the summer season. Once settled, HWA nymphs stay in the same place as they complete their life cycles.


In the fall, HWA break their aestivation and begin to feed and develop. During this time they begin to secrete their characteristic wool, which appears as white, cottony-waxy, woolly masses clumped on hemlock twigs near the base of needles.

HWA continues to grow and develop through the winter, completing four nymphal stages known as instars and growing from a length of ~0.2 mm to ~1 mm. HWA’s first generation reaches maturity at the end of winter, laying eggs in the spring that hatch into the second generation (progrediens generation), crawlers.

Second Generation: Progrediens

The second generation crawlers hatch in the spring (April-May). Once hatched, crawlers settle on a hemlock twig, insert their mouthparts, and immediately begin to feed, skipping the aestivation period. The second generation nymphs also go through 4 nymphal stages, reaching adulthood in the summer and laying the eggs that will become the next year’s first generation of HWA.



Phenology and Feeding Mechanism

As HWA progresses through its life stages, there are some developmental milestones that overlap. This is especially clear as the sistens generation lays eggs and the progrediens generation is hatching and beginning to feed. The progrediens that hatch from the sistens eggs settle amongst their mothers, completing their development (4 instar stages) on the same section of twig as the previous generation. The feeding of the progrediens individuals in this already-stressed section of branch causes additional damage to the twig and after several years, no new shoots will appear. 


HWA life cycle diagram
Illustration by Charlotte Malmbor