Richard M. Houseman
Urban Entomology State Specialist, Division of Plant Sciences
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 (Figure 1). Their relatively long, slender bodies vary 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. The adult female beetles lay eggs 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. Longhorned beetles are unique and fascinating insects, and they pose no threat to humans or wood in the home.
Prevention and control
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. Do not spray firewood with a pesticide before bringing it into the house. No pesticides are approved for this practice, and the chemical probably would not penetrate deeply enough into the wood to reach the larvae anyway.
The foreign grain beetle, Ahasverus advena, is a tiny reddish-brown beetle less than 0.08 inch long (Figure 2). 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 in summer and fall and are most likely to enter homes during those months.
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 beetles apparently feed 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.
Although a nuisance, the foreign grain beetle is generally harmless.
Prevention and control
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. These conditions probably eliminate the mold they feed upon.
Stored dry food products should be kept in insect-proof containers and safeguarded from mold.
Ground beetles belong to the family Carabidae, one of the largest beetle families (Figure 3). The adults vary in length from about 0.1 inch to more than 1 inch. They are usually dark colored, and 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 are commonly seen 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.
Prevention and control
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. Entry can be prevented by caulking cracks and making sure that doors and basement windows fit snugly.
Weevils are beetles of the family Curculionidae (Figure 4). They are characterized by a snout and a pair of elbowed antennae attached to the sides of the snout. They often pretend to be dead when disturbed. Some species of weevils may enter 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 covered with tiny gray scales that give it a mottled appearance. Its larval stage develops in grass roots and other plants in grassy areas. The adults may feed on the foliage of many plants. Even though the adults cannot fly, they are 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 similar to the strawberry root weevil but is slightly larger, about 0.4 inch long, and has small patches of yellowish bristles, called setae, in the black wing cover. It has roughly 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. The larval stage is a root feeder, whereas the adults attack the leaves. They attack many species of woody plants but seem to prefer oak and chestnut. They are attracted to houses in the fall, presumably in search of hibernation sites.
Prevention and control
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 done with a vacuum cleaner.
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 (Figure 5). 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 0.25 inch long and oval-shaped, being 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 can have from zero to as many as 19 black spots. In addition, markings on the white pronotum, which is 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 is found only on or near the plants where the egg from which it hatched was deposited. Adult beetles do not reproduce indoors during their hibernation cycle.
These lady beetles enter houses probably through poorly fitting window screens and doors, cracks and crevices, and vents. They 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. In the spring, they will leave their hibernation sites. This may be the time when the greatest numbers will be seen indoors.
Prevention and control
Because lady beetles are beneficial insects, it is best to try to keep them out of the house so they do not become nuisance pests. To prevent home invasion, caulk all cracks and crevices. 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. A combination of the vacuum cleaner and an aerosol may be used. Be aware, however, that even if you remove all the lady beetles you see on one day, others might come out of hibernation the next day to replace them.