The Secret Superpower of Geraniums

The tiresome Japanese beetle has been busy doing its thing again, tormenting gardeners. The first time I encountered them in my garden I tried to look upon the Boston ivy as a greenish lace curtain draped over the fence. It didn’t work. There was no avoiding the realization that the leaves had been shredded by Japanese beetles.

Last year it was the hazel and by the time I noticed it was too late and the poor tree suffered the same treatment. There were just too many to deal with, and besides, they were out of reach. Other times these shiny brown and green beetles with locust-like appetites have appeared on my roses where at least they’re easy to spot and it’s easy to shake them off into a pail of soapy water. That’s about the most effective way to dispatch them. Sadly, for some gardeners, so many show up to feast they’d need to set up a dishwasher in the back yard and herd them in. So far this year I’ve only seen two and used the quick and simple squishing technique — ugh.

Maybe the winter was hard on the grubs, or enough lovely starlings fed well when on lawn patrol. That’s where the beetles begin their life cycle. After they’ve done eating, they mate, then the female lays eggs in a handy lawn, especially one where the where soil is moist. The following spring, eggs develop into grubs that feed on grass roots, pupate, then emerge as hungry adults in July and take off to the nearest food source, but only when conditions are right. They prefer to take flight when the air is clear, calm, humidity is high, and the temperature between 29 C and 35 C.

It’s at the grub stage in lawns where some control can be achieved by an application of nematodes, but to be effective, correct time of application, weather, and specific soil temperature must all coincide. Even then, it’s of little help if yours is the only lawn in the area to be treated.

You might be wondering why these voracious pests haven’t defoliated their homeland by now. In Japan they’re seen only as a minor pest, simply because of a difference in gardening culture. The country isn’t blanketed with lawns the way North America is, and that means far fewer places accommodate the lifecycle of the grubs.

There are traps available to limit damage, and they work extremely well, or sort of. They attract the beetles with two types of baits or pheromones, a sex one to attract the males and a flowery one to tempt both male and female. The drawback is they can attract thousands of beetles and as they pass over your garden on their incoming flight path, enough of them will stop off for a quick feed and anything else they might have in mind. No harm in trying with traps. Better still, convince all your neighbours to install a few.

The most effective solution, though not the easiest, is to gradually replace any plants and trees that the beetles are attracted to, and there are over 300 species. There are even more that they’ll never bother, and if that’s the route you choose to take, a quick search online will provide lists of both.

One
more possible way to limit the damage the beetles do, is to plant geraniums, lots of them. Research by a couple of scientists confirmed anecdotal reports that geraniums (pelargoniums) are toxic to these pests. After feeding on the flowers, the study showed the beetles became paralysed for up to 16 hours, however, it didn’t finish them off. When they recovered, they went back for more. Made them easier to pick off, I suppose, but it might take a lawn full of geraniums to provide any relief. Other anecdotal reports suggested the same effect occurred when beetles fed on flowers of bottlebrush buckeye (Aesculus parviflora), but when tested, no adverse effects were observed.

Nature does have a way of challenging us, so for now it looks like I’ll continue picking and squishing, at least until balance is restored.