Darryl P. Sanders
Department of Entomology
Several species of beetles may invade homes. Because they rarely damage anything in the home, they generally are considered to be nuisance pests. These insects vary widely in size and habits. This publication describes some of the more common household-invading beetles found in Missouri.
As the name suggests, longhorned beetles (family Cerambycidae) have long antennae, along with relatively long and slender bodies varying in length from 0.4 to 1.5 inches, depending on the species. In their growing stages, longhorned beetles are grublike larvae that develop inside solid wood and are known as roundheaded wood borers. Eggs are laid by the adult female beetles on the bark of an appropriate tree or log.
These beetles do not enter homes on their own, but are carried into the house as larvae inside wood, usually firewood. Larvae in unused firewood will develop into adults in late winter to early summer. These new adult longhorned beetles then emerge inside the home and typically fly to the light of windows trying to get outside. Their presence may cause consternation, but longhorned beetles will not damage anything in the house, nor will they lay eggs on any wood in the house.
The best control method for longhorned beetles is to pick them up and return them to the outdoors. Removing unused firewood from the home in early spring also prevents their presence. Spraying firewood before bringing it into the house is not advisable. No pesticide labels are approved for this practice, and the chemical probably would not penetrate deeply enough into the wood to reach the larvae.
Longhorned beetles are unique and fascinating insects, and they pose no threat to humans or wood in the home.
Foreign grain beetle
The foreign grain beetle, Ahasverus advena, is a tiny reddish-brown beetle about 2 millimeters long. It is strongly attracted to lights at night and is so small that it can readily pass through window screening. These beetles are prevalent outdoors during summer and fall and are most likely to enter homes during this time of year.
Foreign grain beetles show a strong attraction to houses, mobile homes, recreational vehicles, and even trailers when these structures are new. Their attraction is believed to be due to minute mold that is often present in new buildings. The adult beetle apparently feeds on this mold. However, once inside, they may be found feeding on a variety of products such as grain and cereal products, ground nuts and seeds, dried fruits, herbs and spices. In most cases, they are found in large numbers only when the feeding material is damp and moldy.
Because of their mobility and small size, no effective or practical means exists for controlling foreign grain beetles as they enter buildings. In new structures, they tend to disappear in late fall and early winter because of indoor heating and corresponding drying. This probably eliminates the mold they feed upon.
Flying beetles may be killed with a household aerosol spray containing pyrethrins or pyrethroid-type insecticides. Stored dry food products should be kept in insect-proof containers and safeguarded from mold. Aside from the nuisance caused by their presence, the foreign grain beetle is generally harmless.
Ground beetles belong to the family Carabidae, one of the largest beetle families. The adults vary in length from about 0.1 inch to more than 1 inch. They are usually dark colored, often shiny and somewhat flattened. They tend to be most active at night, with some species highly attracted to lights. During the day they are found under objects on the ground. They are fast runners and it is common to see them moving quickly along the ground.
Indoors, they tend to seek hiding places under boxes or other objects on the floor, particularly in moist places such as basements. They invade houses more often in the fall when nighttime temperatures become cooler. The larval stages occur outdoors in the same surroundings as adults, but adults and larvae are not necessarily found together. Both adults and larvae feed primarily on other insects. Because they are predators, they should be considered beneficial.
When ground beetles enter homes, it is an accidental invasion. They cannot survive and reproduce there.
Control measures indoors usually are not necessary except to hand-pick or vacuum the beetles and return them outdoors. Removing rocks, boards or other objects that ground beetles may hide under from around outside walls discourages beetle accumulation near the home. Caulking cracks and making sure that doors and basement windows fit snugly also helps prevent entry.
To help prevent ground beetles from entering homes, spray 2 to 3 feet of the foundation from the ground up, and treat a band around the house from the foundation out about 5 to 10 feet. For these purposes, use a spray of 0.5 percent diazinon or 0.5 percent chlorpyrifos (Dursban®).
Weevils are beetles of the family Curculionidae. They are characterized by a snout and a pair of elbowed antennae attached to the sides of the snout. They often "play dead" when disturbed. Some species of weevils may migrate into homes in the summer and fall.
The grublike larvae feed on a wide variety of outdoor plants, particularly ornamentals. After the grubs pupate and become adults, they are often attracted indoors. Cool nighttime temperatures may be the reason for the invasion. Weevils enter by crawling through cracks or openings around foundations, doors and windows. They cannot fly. Indoors, they do no damage and are pests only by their presence.
Four species of house-invading weevils are common in Missouri.
- The imported longhorned weevil, Calomycterus setarius, is about 0.2 inch long and gray colored due to a covering of tiny gray scales. Its larval stage develops in grass roots and other plants in grassy areas. The adults may feed upon the foliage of many plants. Even though they cannot fly, the adult weevil is attracted to lights at night and then may invade the house.
- The strawberry root weevil, Otiorhynchus ovatus, is brown to black and about 0.25 inch long. Its larval stage develops in the roots of strawberries, raspberries, blueberries, grapes, many ornamental plants, clover, and even grasses. The adults may do slight damage to leaves of the same plants.
- The black vine weevil, Otiorhynchus sulcatus, is very similar but slightly larger (about 0.4 inch long) and has small patches of yellowish setae (bristles) in the black wing cover. It has approximately the same range of host plants as the strawberry root weevil.
- The Asiatic oak weevil, Cyrtepistomus castaneus, is about 0.25 to 0.3 inch long and greenish-gray in color. The larval stage is a root feeder, while the adults attack the leaves. They attack many species of woody plants but seem to prefer oak and chestnut. They are attracted to houses during the fall, presumably in search of hibernation quarters.
Caulked cracks and snug-fitting doors and windows prevent weevils from entering homes. Weevils, especially the strawberry root weevil, are strongly attracted to water and can be trapped in shallow pans of water placed around building foundations. Physical removal is best with a vacuum sweeper. Chemical control usually is not necessary. If populations are very large, a barrier spray around the home with either chlorpyrifos or diazinon, as described above, will help.
Multicolored Asian lady beetle
The multicolored Asian lady beetle, Harmonia axyridis, like several other species of lady beetles, is considered beneficial because both the growing stage larvae and the adult beetles eat aphids and other soft-bodied insects. The multicolored Asian lady beetle feeds on its prey in forests, orchards, alfalfa and other field crops, home gardens and urban environments. In other words, wherever there is a food source, this beetle will probably be found.
In the fall, these lady beetles converge to hibernate. In Japan they do this in crevices on the sides of cliffs, huddled together by the thousands. In Missouri, they often invade houses in great numbers to hibernate.
The adult multicolored Asian lady beetle is about one-quarter inch long and oval shaped, about two-thirds as wide as it is long. The general color is yellow to yellowish orange. The wing covers that form a domelike shell over most of the body may have as many as 19 black spots or none, or any number in between. In addition, markings on the white pronotum, the small area between the head and the wing covers, resemble a large, black capital "M" when viewed from the rear of the insect. The larval stage looks like a little alligator and is found only on or near the plants where the egg from which it hatched was deposited. Only the adults hibernate; they do not reproduce indoors during their hibernation cycle.
The lady beetles enter houses probably through poorly fitting window screens and doors, cracks and crevices, and vents. The beetles will congregate in wall voids, attics and other dark, undisturbed spaces. On warmer sunny days in the winter, they get the urge to move and may enter the human living space and fly about, particularly near windows. Lady beetles do not harm humans or pets but are pests only by their presence inside the home. During the spring, they will leave their hibernation sites. This may be the time when the greatest numbers will be seen in houses.
Since lady beetles are beneficial, it is best to try to keep them out of the house so they do not become pests. To prevent home invasion, all cracks and crevices should be caulked. Make sure that window screens are in good shape and fit snugly. Louvered vents should have a screen backing and a tight fit. If these beetles are a problem indoors, they can be vacuumed up and released outdoors, preferably in a sheltered spot. If this is not practical, then as a last priority, one can spray those beetles in the human living space of the house with a household aerosol containing pyrethrins or one of the pyrethroids (compounds whose common names end in the letters "thrin"). A combination of the vacuum cleaner and an aerosol may be used. Even through one may remove all of the beetles one day, others may come out of their hibernation quarters the next day to replace them.