By Guy K. Ames, NCAT Horticulture Specialist
Ever since Columbus sailed the ocean blue, travel and trade between the eastern and western hemispheres have been responsible for releasing stowaway pests and diseases into ecosystems unprepared for their devastations. Some of these are quite famous, like Dutch elm disease and chestnut blight, both brought from the “Old World” to the “New World.”
Lest you think it has been a one-way street, it has not! Accidental introductions of western hemisphere pests and diseases have seriously threatened the French wine grape industry and European apples and pears. Two relatively simple mechanisms explain why this occurs: 1) the hosts/victims had not been forced to evolve defenses since they had not before been exposed to these pests or diseases, and 2) the natural controls (usually other insects) that would otherwise limit the population growth of a pest were not introduced with the pest. Any number of pests would overrun us if this natural cat-eat-mouse dynamic was not normally in play.
Thus the success of the Japanese beetle in North America! It was apparently accidentally introduced to America in a cargo of iris bulbs from Japan in 1916. In Japan, a parasitoid fly and parasitoid wasp (and probably other organisms) keep the beetle in check, but those “beneficial insects” missed the boat when it left Japan. Released into our country without its natural enemies, the beetle has run rampant over various plants and landscapes, and we’ve struggled to keep up ever since. The USDA has released several of the beetles’ natural enemies into the United States over the years, but it will take some time before these natural enemies are able to exert a modifying influence on beetle populations.
Japanese Beetles Stage a Grape Invasion
Thankfully, the Japanese beetle does not find every plant equally attractive or nutritious. So, mostly by chance, the Japanese beetles will devastate roses and not touch nearby irises. And sometimes, even within species, the beetle will show preferences. Witness the photos below. The first shows our pest chewing on an American bunch grape, ‘Cynthiana’. The second shows the kind of damage it can inflict, but the third is of an immediately adjacent bunch grape ‘Champanel’, which has a thick, leathery leaf that the beetle apparently does not find appetizing.
It is hard to know under natural conditions if the beetle would eventually get around to eating the ‘Champanel’ foliage if no other grape foliage was present. A lab test where the beetle had nothing else to eat could answer the question, but I run a farm, not a laboratory. And because I love ‘Cynthiana’ grapes and the exceptional wine they can make, I’m not going to yank all my ‘Cynthiana’ vines and plant only ‘Champanel’!
With many so-called “secondary pests” of fruit (“primary pests” feed directly on the fruit; secondary pests feed on other parts of the plant), I often advise growers to be patient. I know that beneficial insects will almost always show up and take care of the pests. A common example is aphids on apples. Rarely will the aphids become so numerous that they significantly compromise the health or performance of the apple tree. Lady beetles, green lacewings, and parasitoid wasps eventually exert control over the aphid population. Not so with the Japanese beetles, however, at least on most of my bunch grapes! I had to do something.
Beetle Battle Plan
I’m a Certified Natural Grower—essentially the same rules as Certified Organic when it comes to pest, disease, and weed control—so my options are somewhat limited. Beneficial nematodes and milky spore disease can be effective, but they are aimed at the subterranean larval stage of the pest. I have adult beetles chewing on my grapes now! And traps that attract the beetles using sex pheromones can be effective but also have the potential to bring in more beetles to your area without trapping the necessary number.
I settled on a spray mix of Pyganic™ and Venerate™. Pyganic™ is relatively expensive, and it has virtually no residual effect. That is, it will kill the pests it hits, but any residue on leaves is ineffective at controlling the target pest. But from past experience I know I can rely on Pyganic™ for a quick knock-down of the Japanese beetles. Venerate™ is a new, OMRI-approved, microbial insecticide with knock-down and residual properties, and it was registered for “beetle control.” This was my first year to have Venerate™ in my last-resort arsenal. Two days after spraying, we are seeing very few beetles. Those we do see are torpid and slow, presumably because they have been feeding on the leaves sprayed with Venerate™.
This exotic pest has virtually no natural enemies in North America. So Japanese beetles can be a serious problem to farmers, gardeners, and landscapers, especially if they’re organic. Some species and varieties have a serendipitous resistance to beetle feeding. But in my case, there’s not enough resistance throughout my grape vineyard for me to rely on. It’s one of those pest management situations where I had to call out the big guns—pesticides (albeit organic ones)—to get the job done.
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