Today’s episode of Spotlight on Research focuses on a paper published in the journal Horticultural Entomology in 2014. A pair of researchers from Eastern Illinois University conducted a study on how to manage Japanese beetle, a pest that can be problematic on several crops from grapes and raspberries to roses and zinnias.
You can read on, or, if you’d rather listen to or watch this presentation – check out this 8-minute video clip.
Before jumping into the study, let’s review a little information about Japanese beetle biology. Japanese beetles produce aggregation kairomones (chemicals like pheromones) that send a signal to other insects. This benefits the beetles that receive the signal because they then know where the food is, and they arrive and start to aggregate. In addition to being attracted by beetles, they are also attracted by volatiles produced by plants that are experiencing feeding damage. So this turns into a positive feedback loop, where beetles feeding on a plant beget more beetles feeding on the same plant.
The question addressed by today’s study is whether hand removal of beetles can reduce damage on plants in a meaningful way – and if so, whether time of day they are removed makes a difference. Note that this is most suited to management on a small-scale: a small farm or garden, rather than a very large farm. This doesn’t need to mean hand-picking, however – it’s easy to build a homemade insect collector to make collection easy and fast. An example: take two empty gallon milk cartons and tape the tops together, and slice the bottom off of one of them. My former colleague Alan Eaton told me that it’s possible to collect up to 1200 beetles per hour using a collector like this. Another simple collector is a shallow bowl filled with soapy water that the beetles can be knocked into.
The experimental setup in this study was straightforward. The researchers took 70 distinct 2-year old grapevines in large pots, and spaced them out 12 feet apart in all directions. They then managed the plants differently. For some, beetles were removed once every day at 8am, or at 2pm, or at 7pm – for others, beetles were removed all three times every day, and for the rest, beetles were never removed. After 8 days of doing this, the researchers counted the beetles and estimated leaf damage on the plants.
So, what did they find? Removal of the beetles, even once per day, kept beetle populations low. Also, removing the beetles resulted in less plant damage. Removing the beetles three times a day was the most effective, but it wasn’t significantly more effective than removing beetles once, in the evening. Further, when the researchers looked at how likely the plants were to be re-colonized by new beetles after removing beetles, the probability was WAY lower for the evening removal than for the morning or midday removal times – even though the duration of time that colonization could occur over night was much longer.
So, why would this be the case? The authors hypothesized that this could be because beetle activity is greatest in midday. Anyone who has tried to collect Japanese beetles knows that they are super active and lively midday – whereas they are much less active and more sluggish in the evening and early morning. Because beetles don’t move much in the evening, if beetles are removed prior to nightfall, no beetles are feeding on the plants at night – so all night long, the plants are not producing the volatiles that they would be if they were being actively fed upon. So this might cut down on even more of the signals that bring in new beetles.
An important piece of information you need if you want to use this practice: Japanese beetles have just one generation in NH. The adults emerge from the soil sometime in early-mid July most years, their numbers peak within a week or two, and then decline until mid-late August. While they are there, the adults are actively feeding and laying eggs, which hatch into grubs. Those grubs feed on the roots of many grasses, and are therefore an annoying lawn pest. The point here is that there is a limited time during which you’d have to concentrate your beetle harvesting activities, should you choose to take this approach. You’d want to be ready to start in early July.
Putting it all together: This is probably the most practical piece of research we’ve talked about yet – these results could be really useful for small-scale farmers or gardeners who want to take a no-spray approach. The take-home messages are simple: hand-picking Japanese beetles once a day can reduce feeding damage, and if you are going to do it once per day, consider doing it in the evening, both to reduce feeding injury and the chances of having the plants be re-colonized before night.
One final caution: Other studies have shown that hand-picking hasn’t worked particularly well. In particular, a recent study looking at roses, where Japanese beetles were collected three times per week, midday, showed that hand-picking was only marginally effective. From this study, we could guess that they might have had better luck in the evening, and by collecting more frequently – and it’s also possible rose flowers emit volatiles that are more (or differently) attractive to Japanese beetle than grape. Regardless, I think that hand-picking on a small scale is worth a try, and I have shifted my beetle collection activities to become an evening ritual.
Do you have questions about this topic, or suggestions for other topics to explore in Spotlight on Research? If so, please reach out and let me know. Thanks for reading, and stay tuned for the next edition.
Switzer and Cumming (2014) Effectiveness of hand removal for small-scale management of Japanese Beetles (Coleoptera: Scarabaeidae). Horticultural Entomology 107(1):293-298