If you garden most places east of the Prairies, you know that Japanese Beetles are everywhere. Last summer was a terrible year for them, but somehow they didn’t find my gardens. This year, I wasn’t so lucky.
I don’t love Japanese Beetles, or any pesky unwelcome insect, but seeing them in my garden gives me an opportunity to observe them and their surroundings, which included the plant and soil conditions. I first found the beetles on my bush beans and started checking daily for damage and signs on other plants. Occasionally I knocked them off, but I was curious to see what would happen to the plant. The bush beans were only there because I had seeds and they’re good for the soil. When the beetles found them, I was more interested in what would happen than I was concerned about the plant’s demise.
Interestingly, the beans fought back. Day after day the beetles were snacking on a new leaf, but not destroying any. There was new plant growth regularly and plenty of leaves untouched. More beetles showed up (at peak there were probably 10), but only about 10% of the leaves were partially eaten by the beetles. This is very different from what I’ve observed on other plants.
What’s really going on with the plant
Obviously this got me excited about what’s going on below ground. Beans require a minimum fungal biomass of 101 microg/g and bacterial biomass of 135 microg/g. They also prefer an F:B of 0.75. My beans were growing in soil with fungal biomass of 59 microg/g, bacterial biomass of 206 microg/g and an F:B of 0.29. Additionally, protozoa was within the desired range and there weren’t any detrimental microorganisms. There weren’t any nematodes present, which means less nutrient cycling and the possibility of plant stress.
While the bacterial biomass was in the desired range, the fungal biomass and the F:B were a bit low for the plant’s ideal conditions. But, there was fungi present. Last summer I looked at soil samples from 12 plants that were attacked by Japanese Beetles. Of the 12 samples, 11 had fungal biomass ranging from 7 to 62 microg/g (the last sample had a fungal biomass of 181 microg/g). Additionally, all of these plants were larger than beans with higher fungal needs, leading to a greater fungal deficit than my beans.
It looks like my beans were stronger at fighting back. They had more resources available to them in the fungi feeding the roots, even though it was low. This is by no means a solution to the Japanese Beetle problem, but some insight into how we can give plants the tools they need to fight back. Feed the soil, feed the plant. I’m feeding that section of the garden mulch this fall. By increasing the fungal biomass, next year’s plants will have more resources to fight back.
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