First Year Using Bacillus Thuringiensis var. Galleriae Against the Japanese Beetle
Bacillus thuringiensis galleriae (Btg) is a bacteria that produces variations of a protein, such as Cry8Da, that demonstrate different levels of toxicity to certain species of scarab, and strain SDS-502 was recently released as a biological form of control against the Japanese beetle. It was something I decided to try this year, but since it was very expensive in comparison to many other pesticides and I was not entirely sure what to expect, I was encouraged to do two things. I wanted to use a lower dose than what the instructions recommended, and I wanted to wait about a week after they began to emerge before I sprayed the first application, so a higher density of them would make it easier for me to see what kind of effect it would have.
The lowest recommended dose of Btg you will find within the instructions for leaf cover is 4 ounces per gallon, which means that you would only get four gallons worth for every 1 pound bag that you purchase. That is approximately $16.5 per gallon, based on the time this was written, but even half of that amount is quite expensive for an insecticide. However, a few studies have demonstrated that 2 ounces per gallon is quite effective and occasionally provided similar results as 4 ounces per gallon, so I figured that it would be best to test a narrower range. That way, I would have just enough to cover a well-planted area roughly one-fifth of an acre for the season.
For the first gallon I prepared, I mistakenly used 1 ounce rather than 2, and since I did not have much to begin with, I decided to prioritize the sweet cherry. Year after year, that is what they clearly preferred over everything else, but they are also quite fond of the grapevines. The problem is, some of these vines produce an abundance of leaves that are quite large and I tend to spray when there isn’t much light, so it can be difficult to see what I am doing in the more densely grown areas. Regardless, I reserved what I could when spraying the plants they tend to feed on, but do not desire as much, to give the grape leaves the best possible application that I could.
At this time, there were at least a half-dozen Japanese beetles on the sweet cherry for two or three days in a row and about 3 to 6 on each grapevine. Most other plants had nothing on them, so the same thing I observed the other years was playing out once again. The next day (after I sprayed my first application), I only noticed one on the sweet cherry and 2 to 3 on each grapevine, with the exception of the one I did not spray. That vine had over a half-dozen on it. Clearly, something was happening, in-spite of only using 1 ounce per gallon, but I was wondering if it was acting more like a repellent rather than something that was supposed to kill them.
As time went on, their numbers would increase, but it was generally slow until rain diluted the application. This would cause their numbers to jump significantly, but less than 24 hours after Btg was re-applied, their numbers would take a fairly large hit. This observation would occur after every re-application other than the third. In that case, their numbers quickly increased for a few days before crashing down and, for the most part, stagnating once again within a range I would soon expect, and since this was the only time this occurred, I assumed a large number of them had recently emerged, thus overwhelming me to some degree and dealing a significant amount of damage to a couple of grapevines (where the additional Japanese beetles largely restricted themselves to).
In-spite of this event, it was clear that Btg was having a positive impact on the garden, even though I decided to use 1.5 ounces per gallon most of the time. However, many Japanese beetles were still present day after day, which included the time I used 3 ounces per gallon. This may imply that it was acting somewhat like a repellent, but as time went on, it became apparent that the effect this product had on them may have helped create this illusion. For example, Btg does not kill them immediately. They can be flight capable for some time after consuming the protein, but it supposedly stops them from feeding rather quickly. Considering this, as well as the distance they are capable of traveling, it seemed that the only explanation as to why their numbers were fairly stagnant much of the time was that the fallen were essentially being replaced one for one, so maybe that was exactly what was going on – in this high pressure area.
The more Japanese beetles you have on your property, the more you are going to attract, which, in turn, insures that leaf damage becomes more prominent. This ultimately causes the plant to release more of the kairomones that initially lured them in, and since the grapevines were the most difficult to protect, I believe that is why they were able to overpower me on a few of them for much of the season. The sweet cherry itself had so little damage from being prioritized by me and easier to protect than the grapes that, whenever rainfall encouraged a large number of Japanese beetles to accumulate on it, a re-application would cause them to quickly disperse.
There are a few plants in the area, such as the goumi, that Japanese beetles consistently do a moderate amount of damage to, but it can take some time to accumulate. I did not spray some of these plants during the first few applications to see what would happen, and of course, they congregated on them much quicker than what I would have expected. In fact, there were much more Japanese beetles on these plants than the sprayed ones they clearly preferred, but once the application had enough time to weather, or if rain diluted the application and the forecast prevented me from re-applying another, I could see their numbers decrease on them every day, as if they were slowly migrating over to the grapevines. Once I started to spray the less desirable, the Japanese beetles would, for the most part, cease to congregate on them, but they were less willing to abandon their favorites, even when a relatively fresh coat of Btg was present.
After about 3 weeks of using this product and a few days of rain in the forecast, I figured I had seen enough, but there was still something that wasn’t entirely clear to me. I wanted to confirm that Btg was not acting like a repellent in any meaningful way, so I finally decided to manually remove them by using a stick to shake them down into a container of soapy water. I did this for over a week, but my question was basically answered immediately. I would get well over 100 every day before it became difficult to find more, but the day after, there would be just as many to collect, provided that the temperature was over 70F. Cooler days would inhibit their movement, but when it was quite warm, there would occasionally be more than the day prior. New arrivals were clearly replacing the fallen practically one for one very quickly, and as strange as this may sound, this apparently occurred for over a month.
At this time, I can’t exactly say which method of control was more effective, because I don’t know how concentrated their numbers were in the area throughout the process, but if their numbers weren’t dramatically different between the week I manually removed them and the week I used Btg before and after this time frame (which seemed to have been the case based on what I counted within a designated area on a daily basis), I would definitely say that Btg was more effective. It certainly required less effort to obtain similar results. However, rain was extremely disruptive at times, so a combination of applying Btg when the weather is expected to be clear for a while and manually removing them right before and after it precipitates (or as long as rain is expected to occur within a short period of time) should be the most efficient form of control – if we were restricted to these two methods alone.
In a garden that exists for more ornamental purposes that must consist of a few plants they are quite fond of, you would, of course, want to use both methods as often as possible. One to two ounces of Btg per gallon will leave a fairly light residue, but this is somewhat temporary and you only really notice it up close. It definitely beats having a number of leaves with a substantial amount of damage throughout the rest of the year, and the few they do manage to ruin can always be plucked off, but I think this product might be beneficial in another way other than simply just thinning them out. It looked like Btg caused them to congregate more than usual. Oftentimes, it would seem like this occurred on leaves that I may not have coated very well, but other times, this was probably not the case. Either way, if this was something I was not imagining, the shift in behavior this bacteria seemingly caused would make them easier to spot and remove.
Alternatively, I believe that using a combination of Btg with one or more other forms of control that are relatively non-toxic, but often lack the efficacy to be reliable on their own (such as traps or a repelling oil), would potentially reduce the amount of effort required to control them and help obtain results more desirable than what I observed this year. If I still have access to a property that is well suited for such an experiment, this is something I plan to test out next year.
1. Strengths and limitations of Bacillus thuringiensis galleriae for managing Japanese beetle (Popillia japonica) adults and grubs with caveats for cross‐order activity to monarch butterfly (Danaus plexippus) larvae