The Year After: Using Bacillus Thuringiensis var. Galleriae Against the Japanese Beetle

After eliminating thousands of Japanese beetles last year while primarily using Bacillus thuringiensis var. galleriae (Btg), they ended up causing very little damage this year, even though I used a much less suitable method of control. Instead, I placed 5 traps on the property (about a fifth of an acre) upwind from their favorite plants to see how well it would lure them away, and while this is hardly relevant, I used neem oil on some of the grapevines as well.

I probably wouldn't do this on a small city property under normal circumstances or without hiding the traps in some way, since using that many isn't exactly a good look and using only a few is said to lure in more than they catch, but "mass trapping" has had a positive effect for some people. Neem oil, on the other hand, supposedly acts as a repellent, and while I haven't seen any data on this, I don't entirely doubt it, because there is data on the repelling effect of a few other oils against this species. However, I hear that you need to start using it as soon as they emerge and re-apply every week or so, because once they become established, they will just ignore it. The reason why I consider this part unimportant is that I only sprayed one application. There just weren't enough of them around to justify another when I have been trying to phase out my activity within this city, but I figured I should mention it anyways.

While the traps may have helped, they were clearly not the reason for such a difference. It does, however, make sense that Btg would have a noticeable effect on their population the year after, but to this degree? Plants that consistently lose 30-50% of the surface area of their leaves, as well as the others they feed on but show less interest in, only lost 2-4% this time. That is still enough to ruin ornamentals, since their entire purpose is to look pretty and the damage is often spread out, but the grapes looked much happier this year than they have in a long time, and healthier leaves ultimately translates to improved fruit quality, cold hardiness, and productivity for next year.

What I don't understand is that there are plenty of other food sources for them in the area, so why did this property not get that many from nearby locations, especially since I had so many traps out trying to lure them in? Maybe the weather had an effect on their population too, but it was definitely not from excess cold or a lack of moisture. The only thing that makes sense right now is that they are more mobile than I realize and may not remain in a small area once they find a good location, but I haven't read anything about this yet. Maybe this experiment knocked their numbers down throughout much of the neighborhood, which I suppose is possible since I did not see many when I looked at a property about 200 feet away, but who knows what really happened. As I said, I am sure Btg played a role, but I would have to do this at a few different locations that experience high pressure to confirm that it really can have such a dramatic impact.

The post about my experience last year.