Sap beetles in home gardens
Sap beetles, also referred to as picnic beetles, often become a nuisance in gardens during late summer. They feed on damaged, overripe, or decomposing fruits and vegetables. There are over 180 species of sap beetles, yet the best known species in Minnesota are the strawberry sap beetle (Stelidota geminata), picnic beetle Glischrochilus quadrisignatus, and the dusky sap beetle (Carpophilus lugubris).
Most adult sap beetles are small, between 1/8 and 1/4 inch long, and oval in shape. Stelidota geminata adults are the smallest (< 1/8 inch long), oval-shaped, and mottled brown in color. Carpophilus lugubris adults are 1/8-inch long with short wing covers and are uniform dull black in color. Glischrochilus quadrisignatus adults are the largest (1/4-inch long), and are black with four orange-rust spots on the wing covers.
The antennae of sap beetles have a club (knob) at the end, which is an important identifying characteristic of all sap beetles.
Eggs are milky white, small (0.04”), and not usually seen because they are laid within plant matter. Larvae are small (< 0.8” long), white (pale yellow when mature) with a light brown head.
Sap beetles overwinter as adults. They emerge in spring and lay eggs near fermenting and decaying plant material. Larvae feed for about three weeks and then pupate, emerging as adults in late June or early July. Sap beetles take about 30-35 days to develop from egg to adult. There is one generation each year.
Sap beetles can directly injure fruits and vegetables, however they are more often found on fruit/vegetables that have been damaged by another insect, or infected with a disease. For example, in strawberries sap beetles may often be seen on berries that also infected with a disease (Figure 5.). They commonly infest corn, tomatoes, raspberries, strawberries, and muskmelons that are wounded or overripe.
If they are attracted to a garden by fermenting, overripe produce, they may also infest undamaged, developing fruits and vegetables, particularly berries or corn. In sweet corn, for example, an ear damaged by corn earworm will attract sap beetles, whose larvae then feed on the undamaged kernels (Figure 6).
Keeping your garden free of overripe fruit and vegetables is extremely important. Remove any damaged, diseased, and overripe fruits and vegetables from the area at regular intervals. Collect apples, peaches, melons, tomatoes, and other decomposing fruits and vegetables and bury them deep in the soil or destroy them to eliminate beetle food sources.
Bait trapping shows some promise in the reduction of beetle populations. The theory of trapping sap beetles is to place traps outside the garden on the assumption that the trap will be more attractive than the fruit. Any container of fermenting plant juices will attract sap beetles. Common baits include stale beer, molasses-water-yeast mixture, vinegar, or any overripe fruit. Traps should be place a few feet outside of your garden. Discard trap contents frequently, every three or four days, and rebait traps.
Since sap beetles do not appear until fruit is ripe, insecticide use on the crop is discouraged. However, some degree of control can be obtained in severe infestations by using carbaryl or bifenthrin. These insecticides may kill existing beetles, but as long as fruit/vegetables are present, they cannot prevent additional sap beetles from moving into gardens. It is also very important to observe the interval between insecticide application and when you can harvest fruits and vegetables (this information is found on the pesticide label).