The annual late-spring invasion of mid-sized, metallic tan beetles with orange legs has begun in our area. Seemingly overnight, they will appear and begin heavy feeding on roses, raspberries, grapes, apple trees, and many other trees and shrubs. The beetle that pulls this magic appearing act is the rose chafer, a native species that does best in sandy soil regions.
It is a very serious pest of the plants listed above and can nearly defoliate shrubs and trees in very short time periods, due to their habit of group feeding. Further, they can decimate flowers and small fruits of many other plants as they will feed heavily on them. They are particularly notorious for feeding in flowers and chewing the bases of flower petals, causing them to drop off. This is most likely to occur on roses, lighter colored peonies, and a few others.
This beetle does not completely eat leaves. Instead, it feeds on leaf layers and generally leaves the net-like veins in a symptom called leaf skeletonization. On the developing fruits of apples, pears, juneberries, and others, their feeding is very serious. They will sometimes eat the entire little fruits, but their feeding almost always causes the young fruits to drop off, leaving you with a very small crop left on the tree.
Biology: The rose chafer is a leggy beetle about 1/2 inch long with orange legs and tannish-green body coloration. The overwintering stage is the nearly-mature larvae, which pupate in early spring and usually emerge as adults in June. The adults are short-lived, only about 2-3 weeks, but can inflict heavy damage because they are usually found in groups and eat a lot of plant material each day. Eggs are laid in sandy, well-drained soils. They may lay eggs in lawns, but they prefer to lay them in taller plant growth areas such as road ditches, pastures, hay fields, meadows, and similar settings that have grasses or other similar height plants. The larvae feed on plant roots, but generally do little or no damage.
Control: Rose chafers are an annual phenomenon in most areas of Oconto, Marinette, and Menominee Counties because of our sandier soil types. Thus, you should plan on having to deal with them and be prepared to do so. Damage to smaller, higher value plants can be averted by placing an insect-proof netting, such as cheese cloth or the like, on your plants during the weeks they are present. Diligently watching preferred plant species and noticing when the insects first appear is key to making this technique work, however.
Physically removing the insects from smaller plants is also a viable option, but needs to be a daily routine. Actually, it probably needs to be done every 3-4 hours on certain flower species during their peak flight. Hand control is especially useful for ornamental plants that you don’t want to have covered up or flowering fruits that need to be pollinated. One method is to place a large cloth under larger shrubs and trees and shake the plant hard, then quickly wrap the cloth up and dump the insects into a water/soap solution. This works better when it is cooler, as they are very likely to just drop down to the ground, then. The warmer it is, the more likely they are to fly away. Remember that the beetles are strong fliers and large numbers of them can literally appear overnight. (Don’t sic your chickens after them, they are fairly toxic to chickens.)
Another management option for some types of plants is to use a chemical/physical barrier. Surround (kaolin clay) can be sprayed onto plants and this acts as a barrier to rose chafer damage. To the beetles, it is like us having to eat really sandy spinach, so they go elsewhere for better eating. This works pretty well on fruit crops, but is akin to white-washing, so many people don’t like this option on ornamentals. In addition, it needs to be reapplied after a rain event. Surround is tricky to apply, as it is a powder that you need to keep in suspension while you are spraying it, but it can work very well.
Chemical control is also a possibility, and can take two forms, grub control and/or adult control. Grub control is not the recommended approach. Although the ground around preferred plant species can be treated in late summer with an insecticide to kill the grubs in the immediate area, this does not eliminate next year’s adults. There are two reasons for this: Strong flying ability and preferential egg laying in sandier, less densely plant populated areas. Adults can be controlled with insecticides such as carbaryl and permethrin, but may require multiple applications. If you decide to use an insecticide, read and follow the label directions carefully.
Also, remember that insecticide applications onto blooming plants are likely to kill pollinator insects. You can attempt to minimize this very negative effect by spraying in the late afternoon, as most pollinators are more active earlier in the day. The insecticide will hopefully form a repellent to them by the next day and minimize their exposure.
You can find this article with pictures on the Marinette County page of UW-Madison, Division of Extension, at https://marinette.extension.wisc.edu/files/2010/06/rose-chafer.pdf There are other Extension publications related to rose chafers, including the Wisconsin version at https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3122.pdf or the Minnesota version at: https://extension.umn.edu/yard-and-garden-insects/rose-chafers
If you have any horticultural or agricultural questions, contact Scott Reuss, Agriculture & Horticulture Agent, at 715-732-7510 or e-mail to firstname.lastname@example.org