July 22, 2011
Delaware County Master Gardener Volunteer
Do you have scarabs in your lawn? Are they eating up your garden? No, I haven’t been playing too much Halo or reading about ancient Egypt. Scarabs are alive and well and hitting our window screens and porch lights on hot, summer nights. You probably know them as June bugs, Japanese Beetles, or grubs.
Many of us are familiar with the Japanese Beetle, Popilla japonica, because it has a distinctive, metallic-green body. However, there are nine species of scarab beetles that frequent Ohio. The normal life cycle of these beetles is one year, although there are a few that have adapted to a two year cycle. This time of year, when it is hot, hot, hot is prime breeding and hatching time — and that means white grubs. Grubs in the lawn. Grubs in the garden.
White grubs are the larvae of these scarab beetles. On first glance, they all look very much alike. But each different type of beetle has slight differences in size, in feeding habits and in timing of the life cycle. Which type of beetle you are dealing with can tell you a lot about how, and when, to seek out some sort of control. You identify a white grub by the raster, a pattern of bristles and hairs on its abdomen.
All these beetles do share some common features. The adult females deposit eggs in small clusters buried in the ground. With adequate moisture, these eggs will swell and hatch into grubs. These grubs work their way to the surface and begin to eat organic material, especially the roots of plants. Over a period of about two to three months, they will grow and develop through three distinct stages, eating more and more voraciously. By September, when the soil begins to cool, they need to have stored up enough energy to dig down in the soil and wait out the winter. In the spring, they come up for a good feeding in late April or early May, and then dig back down to go through the final change into an adult.
Adults can decimate your ornamental and small fruit plantings. These beetles will send scouts ahead of the main population. If you see one or two beetles, you can sometimes remove them and prevent a problem. Whenever possible, avoid keeping bright lights on at night, or change the bulbs to a yellow bulb to reduce its attractiveness. You will often find the damage is worse near lighting features. You can also plant species of plants that are not attractive.
A few white grubs in your garden or lawn soil are not worrisome. However, in large numbers, you may begin to see signs of drought damage in the lawn or garden caused by excessive feeding on the roots. Grubs are most destructive if they are able to get to the third instar, which occurs in the late summer and early autumn. Turfgrass infested with a large population sometimes displays the “carpet” effect, where whole sections are no longer rooted to the ground. Any efforts at controlling large populations should be aimed at the period between when the egg is deposited and the second instar.
Moisture is critical to the ability of the eggs to survive. So, to reduce the likelihood of grubs developing, try to restrict watering in July and early August to the minimum needed for plant health. When you water, be efficient and focus on watering at the root area of the target plant. If you water your lawn, make certain that you are doing so as infrequently as possible during this time. Most lawns will go through a slight period of dormancy in the heat of the summer. Once the lawn has begun to brown and stop growing, watering it will only encourage weeds and aid the grubs in hatching.
Studies have shown that traps are not very effective for controlling these populations. While these traps can work in an isolated population of a specific beetle, in open areas with multiple breeding groups there is a risk that you may attract more beetles into the area. Insecticides are sometimes used, but the timing must be exact to the population you are controlling. Remember that controlling one beetle, like the Japanese beetle, opens up the habitat to other populations. The Northern Masked Chafer, Cyclocephala borealis, for example, is usually seen in higher populations in areas where Japanese beetle controls have been used.