By Jordan Haber

What’s the Buzz?

20 March 2020

Since the Hemlock Wooly Adelgid or HWA (Adelges tsugae) first arrived from Japan in the 1950s the invasive insect has decimated Eastern Hemlock (Tsuga canadensis)  populations across Appalachia. Treating Adelgid infestations is both time consuming and costly, and even untreatable in cases of severe infestation. The best chances of treating hemlocks for Adelgids is to catch the insects before an outbreak occurs. According to recent findings by entomologist published in the Journal of Economic Entomology, sticky trap mechanisms are a cheap and easy way to determine areas most at risk of an outbreak.

The entomologists concluded that by placing sticky traps below the crown or canopy of Hemlock trees, scientists can monitor low density Adelgid populations and thus identify areas most at risk of an outbreak. The researchers concluded forest services can save both time and money by focusing on specific areas most at risk of an outbreak rather than targeting larger regions which may or may not be at risk.

The Adelgids are immobile for most of their life, but for a brief period of time before adulthood, the adelgids go through a mobile phase. During this phase, the Adelgids are nicknamed ‘crawler’s’. It’s during the ‘crawler’ phase that the adelgids will scale a hemlock to reach the crown or canopy where they lay eggs.

Eastern Hemlocks can grow too over 150ft tall and can live for more them 500 years which is why some refer to them as the ‘redwoods of the east’, regardless the adelgids are capable of scaling such heights. Although growing only slightly bigger than a pin head, in large numbers the adelgids can bring down even the biggest of hemlocks. According to Carol Loeffler, a professor of Biology at Dickinson College, “the adelgids feed on the Hemlocks sap causing needle drop-offs, limb die back, and in many cases tree mortality”.

The small cotton balls above are actually HWA egg sacs. Hatching generally occurs twice a year in Spring and Winter.

As Hemlock numbers continue to decline, ecologists worry that one of the most common tree species on the east coast will be gone by the end of the century.

“This has huge implications for wildlife” says Loeffler.

The Hemlock is a major source of food and shelter for small mammals and birds. If the Hemlock were to disappear from the forest, a lot more species than just the Hemlock could vanish.

By placing sticky traps beneath the canopy, the researchers were able to trap adelgids during the ‘crawler’ phase, and get an idea of which trees were most at risk of an outbreak. Of the 180 traps used, ‘crawlers’ were found in 115 of them. In Hemlocks where HWAs were caught in the sticky traps, adelgid egg cases were also found in the canopy. These findings suggest that sticky traps positive for ‘crawlers’ correlate to the presence of adelgid egg cases in the hemlock canopy. Thus, the more ‘crawlers’ observed in the traps, the more adelgid egg cases are likely to be found within a Hemlock’s canopy and the greater the risk of an outbreak.

Adelgid ‘crawlers’ are small, but what they lack in size they make up for in numbers. In North America all HWAs are female and they reproduce asexually. A single egg sac can contain hundreds of Adelgid nymphs.

To determine the impact of the sticky traps on low density adelgid populations, a comparison of canopy residing egg cases and trapped ‘crawlers’ was conducted. The results suggest that in adelgid infected sites where an array of three traps were used, around 85% of adelgids were removed.

These results suggest that sticky trap mechanisms may be an effective tool in controlling HWA outbreaks, especially if used as a preventative measure.

Going forward more research will have to be conducted regarding trap placements, as colonizing adelgid populations may be transported from the canopy of neighboring vegetation. It’s also important to keep in mind that these traps only prove effective in cases of low adelgid densities, and higher adelgid populations may call for more costly control measures such as insecticides use or controlled burnings. If the Eastern Hemlock is to have a fighting chance to continue to thrive in Appalachia’s forests it will be important for future research to focus on controlling high density HWA populations.



Fidgen, G. J., Whitmore, C. M., Studens, D. K., MacQuarrie, J.K.C., and Turgeon, J. J. 2020. Sticky traps as an early detection tool for crawlers of Adelges tsugae (Hemiptera: Adelgidae). Journal of Economic Entomology 1-8.