Phenology: The Key for an Effective Biocontrol Program

“Know your enemy” is key advice for anyone going into battle. For those of us at the New York State Hemlock Initiative (NYSHI) that means getting to know the hemlock woolly adelgid (HWA) inside and out. One of the ways that we are getting to know HWA is by tracking its phenology, or life cycle, in different locations throughout the state. New York’s climate varies greatly over terrain, with large changes in everything from topography to temperature between the state’s northernmost and southernmost reaches, as well as east to west. These climatic gradients result in different conditions throughout the state, prompting varied responses in HWA. As a result, HWA often reaches certain critical points in its life cycle at different times throughout New York. One of the phenological changes that we are tracking is “aestivation break”, the time at which HWA wakes up from its summer resting period. During the summer aestivation period, HWA doesn’t grow or develop much. It has reached the particular hemlock needle that it will be sucking at throughout the winter, but is still waiting for mid-autumn to begin its period of growth. This year we experienced the earliest aestivation break in recent memory, with HWA here on Cornell University’s Ithaca campus showing signs of aestivation break just this week.

This photo of HWA was taken on September 25, 2017. This individual shows signs of aestivation break, the most obvious being the clear body segmentation, indicating growth taking place. Photo from Cornell University campus in Ithaca, NY, taken by NYSHI field technician Nick Dietschler.

Understanding when aestivation break occurs throughout the state is important for our biological control program because it tells us when it is time to release our HWA predators. In order for successful biocontrol establishment, there must be enough HWA for our predators to feed on. When HWA breaks its aestivation, this gives us an idea of when they will be plump and ready to be a filling snack for the biocontrol bugs. Since the aestivation break varies so widely across the state, NYSHI relies on citizen science volunteers to monitor HWA-infested trees and tell us when aestivation break is occurring in different locations throughout New York. Our phenology volunteers are responsible for monitoring the same site each week from mid-September into November, looking for the evidence that HWA is beginning to grow and develop. Clues like body segmentation, lightening in color, and waxy wool accumulation are great tip-offs, and help us determine the best times to release bugs to manage HWA populations.


In order to fine-tune our biocontrol program, the NYSHI is studying how HWA’s life cycle changes from one year to the next while looking for overarching patterns. With help from dedicated volunteers, we are establishing a network that will help us improve our understanding of HWA in New York for a more effective biocontrol program, hopefully creating long-term management solutions in the fight against an invasive pest. With a greater understanding of HWA and its phenology, we will be more prepared to fight HWA this year and in the future.