Frequently Asked Questions: Biological Control of HWA
1) What will the predator beetles eat when all the adelgids are gone?
Biological control of HWA by Sasi (or any other predator beetle) has not eliminated adelgids in Asia and is not expected to eliminate HWA in the US. While HWA is their favorite menu item, Sasi will also feed on two other introduced adelgids – the Balsam Woolly Adelgid and the Pine Bark Adelgid.
By feeding, beetles will reduce the HWA population to a level where it does not damage our hemlocks, establishing a balance between predator beetle and adelgid populations. When the adelgid food supply gets low, then beetle reproduction will decline to maintain a population balance. That is the goal of biological control!
2) Why does my local Forest Service or County extension agent not encourage private participation in biological control of HWA?
There appears to be a USDA Forest Service effort to discourage releases of predator beetles on private land, while continuing to raise hundreds of thousands of these beetles for release on public lands. The motivation for this effort is unclear, but it involves misinformation that seems to reflect a disconnect between USDA researchers and the bureaucrats who formulate public policy.
Over the last decade, USDA-supported laboratory and field researchers have provided valuable research results concerning the behavior and effectiveness of the HWA predator beetles. But rather than conducting scientifically adequate “intervention assessment field studies” at beetle release sites, Forest Service personnel have been content with limited “beetle recovery” efforts that ignore hemlock health measures and are inadequate by any reasonable scientific standard for environmental intervention assessment.
A scientifically adequate assessment for a biological control intervention for HWA should include both treatment and control sites, with careful measurements of all relevant components in the biological control process: including beetles, adelgids and new hemlock foliage (see Strategies for Assessing HWA Predator Beetle Releases). And to assess change, these measurements should be repeated over at least a 3-year period. See Cheah and colleagues for the best available approximation to a scientifically adequate field assessment effort for Sasi: (Assessments of Biological Control of HWA with Sasajiscymnus tsugae in Connecticut and New Jersey)
3) How can I tell if beetles will "work" on my property?
The major criteria for establishing a self-sustaining beetle population are access to an ample HWA food supply on site and "food trails" consisting of hemlocks or white pines that will allow the beetles to circulate around an area.
If you have only a few small hemlocks in a yard, with no other hemlocks in sight, then periodic chemical treatments - either spray or systemic - may be your best option for controlling HWA. But if you have larger hemlocks or hemlock hedges, with other hemlocks in the neighborhood, then biological control should prove an effective long-term strategy. If so, this will "work" for the entire area, not just the trees where you do your releases.
4) How do we know where our HWA "invader" came from?
Numerous USDA-supported research efforts are making important contributions to our scientific knowledge about HWA. Perhaps the most exciting of these are the efforts of Nathan Havil and associates to create DNA-based " biogeographic maps" for hemlock wooly adelgids, hemlocks, and even predator beetles (see DNA used to trace origin of the HWA in Eastern North America). The DNA research on HWA has identified numerous, genetically distinct adelgid populations in different geographic locations across the world. And our HWA import to the eastern US is a genetic match to an HWA population in the Osaka area of Japan. Fortuitously, this is the same area where researchers first collected the Sasi (Sasajiscymnus tsugae) predator beetles that are being used in biological control of HWA in the US. So Sasi is the native predator of this particular adelgid that is attacking our eastern hemlocks.
5) How do we know that the survival of HWA-infested hemlocks in Japan isn’t due to genetic resistance, rather than biological control?
There is at least one Asian hemlock (Tsuga chinensis) that exhibits high levels of resistance to our HWA "import" from Japan. However, researchers at the National Arboretum (see technical paper Resistance of Hemlock Species to HWA) have shown that our adelgid's native hemlock host in Japan (Tsuga seiboldii) has little more resistance to the adelgid than our two native eastern hemlocks, Tsuga canadensis and Tsuga caroliniana. The Sasi beetle is the major HWA predator on these nonresistant hemlocks in Japan. So biological control involving Sasi is the most likely explanation for the healthy coexistence of hemlocks and HWA in Japan.
6) Why are the beetles so expensive?
In general, a low-density beetle release to protect all the hemlocks in a residential area will cost less than systemic chemical treatments of selected large trees in the same area. And the biological treatment is one-time, while the chemical treatment will need to be repeated every 3-4 years. So chemical treatments will always be more expensive than biological control efforts.
For example, consider 8 hemlocks of 20” dbh (diameter) located on a 1-3 acre property. The first round of professional chemical treatments (say, Imidacloprid root injections) would cost ~$700 - $800 and would only affect those 8 hemlocks. A professional beetle release for those eight trees and all other hemlocks on the property would cost from $400 to $800 (depending on the physical distribution of the trees across the property). (Do-it-yourself applications would reduce the total costs of both treatments, without changing the cost comparisons.)
Lab-rearing of Sasi (Sasajiscymnus tsugae) beetles is a seasonal and very labor-intensive process. And staff at the USDA-supported labs have done a great job of developing a lab-rearing protocol for Sasi. But it still takes a lot of time, expertise and effort to “grow the beetles”. Nobody is getting rich from producing predator beetles, but the contribution of these lab-rearing efforts to our native hemlock environments is priceless!
7) What about other USDA-approved predator beetles?
The idea of releasing a “cocktail” of several different HWA predator beetles with different life cycles and feeding schedules is appealing. But at present Sasi (Sasjiscymnus tsugae) is the only USDA-approved beetle that is being lab-reared for use by private landowners. Sasi is also the only available native predator for our particular HWA invader from Japan. And it is the only predator candidate that is capable of exponential population growth to match HWA. So Sasi is the natural base ingredient or starting point for a biological control “cocktail” and it may even prove sufficient with the help of other resident predator beetles such as Laricobius rubidus and Harmonia axyridis.
8) If I release predator beetles on my hemlocks, how will I know if they are working?
If your hemlocks are already being defoliated by HWA, then the beetle predation on adelgids will stimulate new crown foliage growth on your larger trees, beginning in midsummer after your release. You can use a digital camera to compare before/after photos and measure the change in new crown foliage (see paper Deriving Hemlock Crown Density Measures from Digital Photographs). Hedges and smaller trees with sun exposure may show new growth bottom-to-top. But trees that are severely defoliated may take a second growing season to respond with new growth. And some severely defoliated trees may succumb to the defoliation.
If your trees are still relatively healthy and producing new growth, then the main sign of beetle activity will be a decline in adelgid densities. This will be most evident in crown areas, but you won't have to climb the tree to see. Just walk under your hemlocks after a mid-winter windstorm and examine the sample of new growth twigs that Mother Nature has provided. This will tell you what the adelgid densities in the crown areas are like.
9) Can I use chemicals and predator beetles at the same time?
Soap and oil sprays are compatible with biological control efforts, as neither should harm the adult beetles. Although once you have introduced predator beetles, there is no obvious advantage to destroying the "beetle food" that will support beetle reproduction and population growth.
If you apply systemic chemicals to trees and release beetles at the same time, you will destroy the beetle food and poison the beetles at the same time - not a smart move! But if you have applied chemicals in the past, you can release beetles in the same area after the chemicals have cleared the treated trees of adelgids – a minimum 6 month lag between chemical and predator beetle applications.
Chemical treatments will result in trees with healthier foliage. And so when the chemicals wear off and the adelgids return in force, such trees can provide “incubators” supporting beetle reproduction to help the surrounding, untreated and typically more defoliated hemlocks.
10) How do I know that the beetles I release will not fly away?
The Sasi beetles are very mobile, and they can move as much as ½ mile in a season – searching for new food supplies and reproductive opportunities. However, recent USDA-supported research at NC State indicates that the Sasi beetles released at a site do not disperse until the larvae produced from their egg-laying activities begin to appear and feed. And the beetle larvae are even more voracious HWA predators than the adults.
So when the beetles “fly away” to feed and lay eggs on other hemlocks in a release area, they will leave behind a successor generation of predators to carry on the biological control process. And often this next generation will be able to complete a second reproductive cycle that same season.