365 Urban species. #348: Hemlock Woolly Adelgid

[urbpan]

Photo by urbpan. Location: Forest Park, Springfield, Mass.

Urban species #348: Hemlock Woolly Adelgid Adelges tsugae

This insect spends most of its life hidden in a fluffy white shelter. It sucks the fluid from the needles of hemlock trees, and it is believed that as it feeds it introduces a toxic substance into the tree. Every plant has sap-sucking pests that feed on them, and most are not seriously damaging. An infestation of these hemlock woolly adelgid can cut the lifespan of an eastern hemlock down from hundreds of years to less than ten. The Asian hemlocks that the insect feeds on in its native range developed resistances that allow the trees and adelgids to coexist.

Adelgids are close relatives of aphids. Like some aphids, hemlock woolly adelgids are all female, and several generations are born in a single year. In the short time that the insects are mobile, they are carried from tree to tree by animals or by the wind. Adelgids have fully infested the hemlocks in all counties of southern New England, as well as the areas around New York CIty, Philadelphia, Washington D.C. and New Jersey. It's possible that the adelgid affects urban trees more severely because they are already weakened by pollution and other stresses. Methods to control the infestation include saturation of individual trees with pesticide, and the introduction of predatory beetles.

Comments

[hai-kah-uhk.livejournal.com]

Do you know whether the predatory beetles have been a success?

[urbpan.livejournal.com]

No, I don't. I wasn't aware of the predatory beetles until I researched this tonight. I'm hoping one of the ecologists who reads this can shed a little more light on that part. From what I read it looked like the beetles were promising: they don't feed on other species, and when the adelgids aren't available, they stop breeding.

[phlogiston-5.livejournal.com]

The predatory beetles have not been a success yet. In NJ we released one main Coccinellid species, Sasajiscymnus tsugae, as well as a couple of Scymnus species. None of them really took off in population size. Over two seasons of sampling ~60 release stands with beat sheets, sticky traps, and 30 foot sweep nets we only recovered four beetles. There is a new beetle that is being worked on, a Derodontid called Laricobius nigrinus. We released some into a test plot near the water gap this past summer.

The good news is we have a pretty effective control for the elongate hemlock scale, which is another pretty serious pest in NJ and probably other neighboring states. If a tree has both scale and adelgid infestation it dies much faster than just with adelgid. There is a Coccinellid beetle called Cybocephalus nipponicus that finds the scale quite tasty and has already established with good sized populations in many hemlock sites (and is dispersing to non-release sites). So hopefully that will help slow the general hemlock decline. It would be really troublesome to lose the hemlock stands in NJ because it would screw up the natural water filtration processes (in addition to eliminating key climax communities).

[urbpan.livejournal.com]

Thanks, as always, for your highly informative comments. (!!)

[hai-kah-uhk.livejournal.com]

Ohh. That explains something I've been wondering about for years. My botany professor, who was also a NH legislator, regaled the class with horror stories of predator introduction attempts gone horribly horribly wrong. Bugs that decided they preferred to munch on vulnerable native species instead of the target invader; that kind of thing. It left me wondering why anyone would ever try it again.

But the raging disasters must be in the minority. More often, either the new species introduction helps more than hurts, or doesn't even take hold. Am I right?

[phlogiston-5.livejournal.com]

The raging disasters were the result of poor science in the early days of biological control. Things were introduced willy nilly without much regard for non-target effects. These days there is a barrage of testing that goes on for several years in the lab, then in field enclosures, to make sure that potential bio controls are very specific. As a result, a lot of the programs have been successful, particularly in agricultural settings. In nature, one of the biggest success stories is control of purple loosestrife (at least in NJ). We had a release program using Galerucella calmariensis, a Chrysomelid beetle that is doing a great job of controlling (but not eradicating) purple loosestrife density in wetlands.

[hai-kah-uhk.livejournal.com]

Purple loosestrife, the bane of my existence! (Actually it's Oriental bittersweet. I haven't even gotten around to touching the loosestrife yet. Worse, I do it all by hand. I know firsthand why hungry beetles are an appealing solution.)

It's reassuring that we're approaching it with more responsibility now. Thanks for letting me know that. I respected my botany professor, but I'm glad to know his angry tirades missed some information. And he was, after all, a politician. :)

[ndozo.livejournal.com]

I need some of the predatory beetles. Those woolly *%#$ adelgids are eating my trees and I hate them. When I'm feeling cranky I go and squish them on the branches but there are so many. Where do you get the beetles and what problems will they cause when the adelgids are gone?

[phlogiston-5.livejournal.com]

The beetles are not commercially available. As far as I know, NJ is the only state with a formal rearing program for them.

If you have only a few trees, it is probably more effective to spray insecticidal soap or horticultural oil, which are not toxic. You can also use soil/trunk injections of imidacloprid, which works, but involves somewhat not-so-nice chemicals. This method is used to conserve small stands of very large trees in some key recreation areas.

[stephanietberry.livejournal.com]

University of Tennessee (Knoxville) was raising the predatory beetles several years ago; not sure if they are still raising them.

[phlogiston-5.livejournal.com]

Yeah, I know prior to the introduction of the rearing program in NJ there were a few universities rearing them for research purposes. The NJ rearing program was government funded and on a pretty large scale to cover most of the state parks with adelgid infestations.

[antarcticlust.livejournal.com]

A paleo perspective- I'm writing a term paper on the Eastern North American hemlock decline of the mid-Holocene, ~4600 years ago. IT's belived they're caused by the same pest- the adelgid- possibly exacerbated by drought. There was a severe, region-wide drop in hemlock that had an impact on local stream and lake ecology and forest composition. Even after the hemlocks returned, in some places the forests had restructured from their pre-decline composition.

[urbpan.livejournal.com]

Very interesting! Thanks for that.

[antarcticlust.livejournal.com]

I'll probably post my paper on lj when it's done. I've just friended you- nice to meet you! :)

[stephanietberry.livejournal.com]

I would love to read your paper, if that's a possibility.

[antarcticlust.livejournal.com]

I could e-mail it, or maybe I'll post it on LJ (I'm doing some data mining in the North American Pollen database to assess the before and after effects)...it won't be done until next week, I don't think, but I'll let you know when it is. :)

Thanks for the interest!

[antarcticlust.livejournal.com]

Oh- I'm in grad school, so my recent posts haven't been nearly as entertaining or creative as I'd like. It comes in cycles, and I'm unfortunately in a downturn right now. I just checked out your journal and it's very nicely written and full of interesting musings, so I thought I'd warn you. :)

[stephanietberry.livejournal.com]

I know I've probably driven you crazy with these comments---my browser was doing weird things, and I only meant to send you one comment! Oooops! I promise, this will be the last one today. unless.....

[antarcticlust.livejournal.com]

It's fine. :)

[urbpan.livejournal.com]

lj isn't sending my comments until after a long delay, so it's all kindsa messed up. Comment away! I've been outed as a comment whore lately anyway :)

[urbpan.livejournal.com]

Oh you weren't talking to me anyway *blushes*

[aplomada.livejournal.com]

I am puzzled. What could possibly be the reason for a parasitic insect to poison its plant food source? If it was a toxin that was already a part of the aphid's biochemistry when it began using hemlock, wouldn't there at least be strong selection for this to change as it began to depend on the plant?

[urbpan.livejournal.com]

I don't know what to say about this except that the North American hemlocks aren't it's normal food source, and whatever kept it in check in Asia isn't in N.A. The selective pressure you refer to would have happened a continent away, but there hasn't been time for it to happen in N.A. yet.

I agree--it seems like the adelgid is eating itself out of a food source, like a virus that kills it's host before the host can pass it on.

[(Anonymous)]

My sense here in Mass. is that the adelgid has been at least temporarily slowed, possibly because of a few really cold nights in the last few winters or...? Some trees, which had adelgids before, now don't , or only traces and they weren't sprayed.

The hemlocks certainly would leave a big hole in the North-central New England ecosystem if they were to disappear.

Dwight

[urbpan.livejournal.com]

That's good anyway. In researching this, I found that there are places where the adelgids are at the bottom of some trees but not at the top (presumably) because it gets colder in the "higher elevation" at the top of the tree. I knew New England's horrible winters were good for something! Now I feel guilty for enjoying our t-shirt weather December.

fighting fire with fire

[(Anonymous)]

Personally, I think we should all be a little wary about the use of an introduced predator to fight HWA. To say that the days of biocontrol agents-run amuck are behind us is presumptuous and dangerous. We have spent millions attempting to rear and releace biocontrols (includind Larcobius) which have yet to have any effect on the HWA. If anything, we need to look for resistant trees (and not by breeding them with Tsuga chinensis: by find a native canadensis or caroliniana that has developed resistance). Thats the only way to ensure these forests stick around.