Hemlock Woolly Adelgids & Their Predator Beetle, Laricobius nigrinus

Beetle reduces the winter HWA generation, but spring population rebounds

a tiny black beetle in between leaves of hemlock tree
An adult Laricobius nigrinus beetle on a hemlock twig. Photo by Bryan Mudder, USFS.

Laricobius nigrinus is a small beetle that eats an even smaller bug – the hemlock woolly adelgid, or HWA.

Since 2003, Laricobius has been used to help control HWA. But the beetle, which is native to western North America, is only active during the fall, winter and early spring.

Recently, USDA Forest Service research entomologist Bud Mayfield and his colleagues, including students and prominent researchers, published the results of a six-year collaboration on Laricobius as biocontrol.

Laricobius beetles have a significant impact on the winter, or sistens, generation of hemlock woolly adelgids,” says Mayfield. “However, the spring, or progrediens, adelgid generation rebounds, counteracting the positive effects of the Laricobius predation.”

Carrie Jubb, at the time a Master’s student at Virginia Tech, led the study on the sistens generation, which was published in the journal Biological Control. Jubb and other coauthors worked closely with Virginia Tech professor Scott Salom. “Jubb’s study is part of a long history of biocontrol work done in Doctor Salom’s lab,” says Mayfield, who helped design and implement the study and served on the students’ steering committees.

The study on the progrediens generation was led by Ryan Crandall, a Master’s student at the University of Massachusetts. That study was also published in the journal Biological Control and uses some data from the study Jubb led.

The researchers used cages to keep the beetles away from some hemlock branches. Then, after counting adelgids and beetles in the spring, they removed the cages and put fine-mesh bags over the branches. This prevented newly hatched adelgids from crawling to another branch.

a twig held between forefingers
Taken through a 10x hand lens, this photo shows large cottony egg sacs of the HWA sistens generation plus the small, numerous, and recently-hatched nymphs of the progrediens generation. Photo by Bryan Mudder, USFS.

Jubb’s study took place at nine field sites in six states: Georgia, Maryland, New Jersey, North Carolina, Virginia, and Tennessee. Crandall’s study used five of those same nine sites. Most of the sites were on state forests, national parks, or other public lands. The Georgia sites were on the Chattahoochee-Oconee National Forest.

Over 400,000 Laricobius nigrinus beetles have been released across the eastern U.S., from Georgia to Maine.

Every fall and winter, the Laricobius beetles chow down on the HWA egg clusters, or ovisacs. They also eat adult adelgids. And the beetles lay their eggs in the adelgid egg sacks, so the beetle larvae also eat the adelgids. The beetles can destroy or disturb 80 percent of egg clusters, drastically diminishing the size of the winter sistens generation.

However, hemlock woolly adelgids in the US reproduce asexually. A single adelgid can produce hundreds of others, almost all of them hungry for only one food: the sap of a hemlock tree. Once the eggs hatch, the nymphs crawl to young branch tips to feed on the sap.

“Of the eggs that survive, those crawlers that make it, there are so many of them that the spring progrediens generation is still high,” says Mayfield. Although both insects go dormant during the summer, the Laricobius beetle larvae drop to the soil to pupate in early spring, while the second generation of adelgids keep feeding. From April to June, a generation of hemlock woolly adelgids sucks hemlock sap without any significant predation.

Predators that are active during the late spring and summer would help fill this gap. There is hope that silverflies in the Leucopis genus, another potential biocontrol species, can be the spring and summer predators.

Mayfield and colleagues have released silverflies at several locations in the eastern U.S., including the insectary at Bent Creek Experimental Forest, but have yet to determine whether silverfly populations are surviving through the winter. Collaborative research on Leucopis between the Forest Service and several universities is ongoing.

The HWA life cycle in North America. The sistens generation develops from summer through winter, and the progrediens generation develops in the spring. Image by Vince D’Amico and Nathan Havill, USFS.

Similarly, Japanese ladybeetles (Sasajiscymnus tsugae) were an early focus in the biocontrol program. Millions of them could easily be reared in a lab. In the field, however, they have proven elusive. “You can sometimes recover them in the field,” says Mayfield. “But recoveries have been inconsistent and low enough that this species isn’t a focus of the HWA biocontrol program right now.”

In addition to insect predators, silvicultural treatments could give hemlock the summer boost they need. Bringing more sunlight to the hemlocks with silviculture (such as creating gaps in the forest canopy) is a promising addition to the arsenal of adelgid control methods.

Mayfield has been involved in several recent studies that point towards the value of silvicultural treatments. One of his recent papers, written with Robert Jetton of Camcore, found that adelgid nymphs crawl away from sunlight. The study was published in Agricultural and Forest Entomology.

A study on hemlock physiology led by research ecologist Chelcy Miniat also reports that increased sunlight can help eastern hemlock survive HWA infestation. That paper was published in Forest Ecology and Management. The results are in line with the team’s previous studies using potted hemlock seedlings.

Silvicultural recommendations for managing HWA are still in development. But when they are available, Mayfield and his colleagues plan to update their resource manager’s guide. They wrote the guide for managers who want to use an integrated pest management strategy to control hemlock woolly adelgids.

Read the paper on Laricobius impacts to the winter generation.

Read about the rebound of the HWA spring generation.

For more information, email Bud Mayfield at albert.e.mayfield@usda.gov.

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