In recent years, I have noticed increasingly larger populations of Japanese Beetles in my garden. I first observed them in my garden a few years ago when I came upon them devouring my edamame soybean plants. Easy to spot with their metallic green abdomen and bronze wings, I picked off as many as I could, but it seemed impossible to keep up with their numbers. The following year, I found them devastating my sunflowers and zinnias. I have since learned that they will feed on over 300 species of plants, trees, shrubs, and grass, including common gardens plants such as sweet corn, sweet peppers, asparagus, soybeans, rhubarb, and more.
First reported in the United States in 1916 in Southern New Jersey, the Japanese Beetle, Popillia Japonica Newman, was accidentally imported here from Japan and quickly became a pest. Without its natural predators, a hospitable climate and ample food supply made it easy for the Japanese Beetle to spread it’s population through most of the Eastern United States. Efforts have been made to eradicate the beetle, but it has continued to thrive in the East, whereas Western and Southern states have prevented it from establishing through tough regulations and careful monitoring. While it may be extremely difficult to exterminate them all together, it is possible for you to keep their populations in check and at a manageable level with the following strategies.
Hand Picking or Trapping
This was my original first line of defense in dealing with my own Japanese Beetle problem. Hand picking is best done in the cool morning temperatures when they are more lethargic and dew on the beetle’s wings impairs their ability to fly. They will often fall from the plant to escape your grip, so first place a light colored sheet (for better visibility) at the base of the plant and then gently shake the plant so that the bugs fall onto the sheet. You can also pick them off the plant and dispose of them in a bucket of soapy water that you hold under the plant (to catch any strays as they fall). There are also mechanical traps that are designed to attract and trap adult beetles. These can be found at many lawn and garden centers, and should be used away from the main garden to lure the beetles away from your crops. While hand picking and trapping is a benign way of controlling the adult beetle, this will not stop their life cycle. Other measures must be taken to manage this beetle in its larval stage as well.
The adult Japanese Beetle is usually spotted in the garden from late spring through midsummer, while it is feeding and laying its eggs (which it does in burrowed holes in the soil). Its’ young larva, the white grub, begins to hatch during midsummer. This is a beneficial time to use biological controls to mange the beetle in its larval stage. There several parasites, nematodes, and fungi that will target and kill these grubs, while at the same time leaving other potentially beneficial organisms unharmed. Nematodes are microscopic parasitic roundworms that seek out the larva in the soil and inoculate the grub with a bacteria that feeds on the grub tissue. The nematode then eats these bacteria, therefore establishing its own population and sustainably keeping the grub population down. The most effective commercially available insect-eating nematode is called Heterorhabditis bacteriophora. Other biological controls are Bt, Bacillis thuringiensis which is an insect stomach poison and must be ingested by the grubs to kill them, and the bacterium Milky Spore, Bacillus popillae which infects the grubs with Milky Spore Disease and eventually perpetuates the disease in the soil to help keep the future of the population in check. Controlling Japanese Beetles with these biological methods takes longer than controlling with insecticides, but the results are much longer lasting.
Trap Cropping, interplanting repellant plants, keeping your lawn and garden as healthy as possible, and choosing landscaping plants and trees that are not susceptible to the Japanese Beetle are also great ways to help control the populations. Planting a trap crop of highly preferred varieties, such as pastel zinnias or marigolds, away from the main garden will help to lure the beetles to a tastier meal. Remember though, trap crops serve to concentrate your pest population and lure them away from other, more valued crops; but the pest population on the trap crop still needs to be addressed through one of the control methods mentioned above – otherwise you are just serving to increase the overall population!
Interplanting repellant plants like garlic or chives with their strong odor, among your susceptible plants like soybeans will further help to deter them. Remember also that insects are often attracted to diseased, undernourished, and injured plants. Therefore, keeping your garden healthy and removing diseased or damaged plants can be beneficial insect control.
Lastly, think outside the box…or outside the garden in this case. There are many trees, shrubs, and grasses that are highly susceptible to Japanese Beetles (and there are resistant ones as well). Therefore if you plan to do any landscaping in the near future, keep in mind that introducing certain species, such as Japanese or Norway Maple, crabapple, and plum trees to name a few, are sure to increase the Japanese Beetle population in your yard and therefore bringing them closer to your garden.
Photos from http://www.ag.purdue.edu/