Beetles make up forty percent of insects, and while we need them in our gardens, some species leave a wake of destruction. The rose chafer beetle, also known as rose beetles and rose bugs, is a species of scarab with a vast number of food sources and few predators. The voracious feeders eat the tender parts of various flowers, trees, fruits, and shrubs.
Fortunately, chafer rose beetles do only minor damage as pesky invaders unless they are in large numbers. High populations are a challenge with the potential to cause severe plant stress and dieback.
What Are Rose Chafer Beetles?
Rose chafer beetles (Macrodactylus subspinosus) occur throughout southeastern Canada, the eastern United States, and as far west as Montana and Colorado. They feed on a wide array of host plants and are especially fond of roses, peonies, hollyhocks, poppies, dahlia, and chrysanthemums. They feed on fruits like grapes, blueberries, peaches, and strawberries. Evergreens like spruce, Frasier fir, and Scots pine suffer their damage. Some vegetable crops like corn, peppers, and beans are a food source.
Adults also eat the tender foliage of trees and shrubs. The grubs (larvae) eat the roots of weedy grasses. Grubs don’t damage lawns or landscape plants, but adults in high numbers harm fruits and ornamentals.
The beetles are prone to sandy soils as they suit their egg-laying and hatching, but they fly readily to other habitats for food. Most birds and small mammals don’t eat them – they contain a distasteful chemical toxin. Be sure not to share them with your chickens.
Signs of Damage
The nibbling insects leave large, irregular holes in blossoms, buds, fruits, and leaves as they “chafe” away upper leaf and petal surfaces. They skeletonize leaves by feeding on soft tissues between the veins.
You’ll likely see the adult insect if you notice its damage. They all emerge at roughly the same time in late spring and feed during the day. The insects are strong fliers capable of traveling great distances to find food sources. They may fly in from other areas throughout their active weeks.
Identification
Adult rose chafers are slender, medium-sized, one-half-inch-long beetles. Their bodies are greenish-tan with orange legs and dark spiny nodes. Their short antennae have segmented points on the tips.
Like other beetles, their straight wings form a line that looks like shields of armor. In flight, they resemble wasps.
The larvae (grubs) are rarely visible; they burrow into sandy soils to feed and overwinter. They have curved white bodies, prominent brown heads, and six visible legs. They reach up to three-quarters of an inch long.
Life Cycle
A single generation hatches in a year. Adults emerge from the soil in late spring and feed for about three weeks.
Females lay eggs (between six and forty) as deep as six inches in sandy soils, each in a separate cavity. Eggs hatch two to three weeks later.
Grubs feed on the roots of grasses and weeds, mostly in open sandy areas. They don’t feed on or damage landscape plants or turfgrass at the grub stage.
When the temperatures drop in the fall, they burrow deep into the soil below the frost line. The larvae overwinter in the soil, pupate, and emerge as adults with warming soils the following spring.
Treatment
Rose chafer beetles emerge in late May. Start scouting in late spring, especially if you’ve experienced an infestation before.
Fortunately, their presence is usually small in number, with mild to moderate damage in their short period of activity. Mature trees and shrubs typically recover without intervention.
During the Growing Season
Chafers are active for only two to four weeks. Hand-pick or brush small populations off of leaves and stems and place them in a bucket of soapy water. Higher numbers of busy beetles may warrant hand-picking more than once a day.
For large shrubs or tree branches, place a tarp beneath the plant and give the stems a good shake. Into the bucket, the dropped insects go.
In extreme cases of large numbers and multiple affected plants, BT, specifically Bacillus thuringiensis subsp. galleriae (strain SDS-502) is sometimes effective. This particular strain of bacteria targets beetles, but there is a connection to negative impacts on monarch butterfly larvae. Skip this strain of BT if your garden houses monarch host plants or food plants.
BT is a bacteria that disrupts the digestion of beetles (including Japanese beetles) and other pests. It’s a biological control that stops them from feeding. BT is effective if consumed by the insect, and needs frequent application during their active weeks.
Pesticides like a pyrethrum and isopropyl alcohol mixture are possible treatments. Use one tablespoon of alcohol per pint of diluted pyrethrum mixture. Apply it every three to five days for two weeks during the active period.
Use caution with pesticides to reduce damage to pollinators and beneficial insects. Remove blossoms before treatment. Apply in the evening or early morning, according to label directions, when pollinators are least active.
Prevention
In areas prone to infestation, a physical barrier covering plants helps. A floating row cover, fine mesh netting, or cheesecloth are good defenses in late May and June. Place the covers as the pests become active and remove them after the feeding cycle.
General prevention for beetles involves lightly cultivating the soil in spring and fall. The idea is to expose larvae to birds, other predators, and cold temperatures. Uncovering the rose beetle grub, though, is challenging, as they opt for sandy, open areas that may be away from the garden.
An additional beetle preventative includes milky spore powder (Bacillus popilliae), though it takes a few years to become effective, and rose bugs travel. They can fly in at any time whether or not they emerge in a given area.
Frequently Asked Questions
Macrodactylus subspinosus, or rose bugs, are a nuisance because they can quickly consume large amounts of plant material in swarms. In average seasons, their numbers are low, and their damage is minor.
In times of high population, they significantly damage various plants with a widespread range of food sources and a voracious appetite. Their feeding on soft tissues impacts leaf surfaces and the plant’s ability to photosynthesize. Their flower-feeding hinders pollination.
The two rose-feeding scarabs are relatives but different species. They seek out roses and other tender blooms and leaves as food sources. Rose chafers emerge a few weeks earlier than Japanese beetles and have a shorter active period, feeding only for three to four weeks. Japanese beetles persist for about six weeks, beginning in early summer.
“What don’t they eat?” may be a better question. These indiscriminate feeders rely on soft, tender vegetation for development. They’ll go for buds, blooms, fruits, and foliage, from roses to apples to pine species.
