06 March 2006

Applications of Evolution - More on the gall wasp

A while back I put up a couple of posts about an invasive gall wasp that's threatening a species of tree unique to Hawaii.

In the first of those posts, I made a few predictions, based on my understanding of ecology and evolution, on how researchers might be able to control the gall wasp infestation:
Ultimately, however, it may be impossible to save the Wiliwili without some sort of biological control. So let's assume that a decision is made to look for one. Normally, we would look to the species' native habitat to find a parasite or predator. Unfortunately, we don't actually know what the native range of this species actually is. It was only described for the first time last year, and it seems to be an invasive in all of the places that it has been found. The world is a really big place, and we have a limited amount of time until the Wiliwili follows countless other Hawaiian species into extinction. So where do we start to look?

Africa. Since this species hasn't been described from there at all yet, what makes us think that we should look there?


There are a number of related species of gall wasp in South Africa, including some that utilize Erythrina trees. There are also Erythrina trees that are native to this area. As I understand it from conversations with other grad students here, these trees do not appear to be experiencing the same sort of massive infestation that is being seen here in Hawaii. This indicates that one of two things is happening there. Either the South African Erythrina have an innate defence mechanism protecting them from the gall wasps (this is unlikely, as the gall wasps can and do breed using these trees) or there is something else, most likely a predator or parasite, keeping their population in check. This is the type of more stable ecological relationship that we expect to see in areas where the species evolved.

I don't know whether or not sufficient funding will be found to do this, but I predict that a thorough study of the gall wasps in South Africa will turn up a predator and/or parasite capable of controlling the gall wasp population. (Or that the Erythrina trees have a defence mechanism protecting them from gall wasps.) I also predict that, again pending sufficient funding, that molecular studies will show that the wasps invading Hawaii are relatively closely related to species from South Africa.
I'm a big believer in scientific integrity, and I believe that scientific integrity includes reporting negative results and incorrect or falsified hypotheses. With that in mind, I find that I must take this opportunity to admit that part of my hypothesis was not, in fact, entirely accurate. Researchers have not found a parisite or predator that targets these wasps in South Africa.

They found it in Tanzania.

An update in today's Honolulu Advertiser reports that a state entomologist spent two months in Tanzania earlier this year, and discovered a species of wasp that feeds on the larvae of the species of wasp that's killing the trees. He went to Tanzania, it appears, because he was a good bit more careful with his preliminary research than I was when I wrote the initial article:
Ramadan, a 54-year-old native of Egypt who has been working for the state agriculture department since 1997, studied the situation and recommended searching for a natural enemy in Tanzania because the country, which borders the Indian Ocean, has more than 15 Erythrina species.
Aside from that, the reasoning behind the approach was essentially what I outlined above. He went to look for wasps in an area where the wasps and trees were believed to co-exist. What he found was encouraging. He brought back a species of wasp that seems to successfully eliminate about 95% of the larvae of the tree-killing wasps.

The wasp has been tested against the invasive, and appears to be effective. They are now testing it for side effects (they want to make sure it won't kill things that it shouldn't). Those tests will probably take at least a year, but it looks like there may still be some hope for the Wiliwili.
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