Sap beetles, also referred to as picnic beetles, become a nuisance in gardens during late summer (June-September).
- Beetles appear at harvest and feed on damaged, overripe, or decomposing fruits and vegetables.
- They are common on corn, tomatoes, raspberries, strawberries and muskmelons that are wounded or overripe.
- There are over 180 species of sap beetles.
The most common species in Minnesota are the strawberry sap beetle (Stelidota geminata), picnic beetle (Glischrochilus quadrisignatus) and the dusky sap beetle (Carpophilus lugubris).
How to identify sap beetles
Adult sap beetles
- They are small, between 1/8 and 1/4 inch long, and oval in shape.
- They are generally dark colored, sometimes with orange or yellow spots.
- Strawberry sap beetle adults are the smallest (less than 1/8 inch long), oval-shaped, and mottled brown in color. They do not have any clear markings on the wings.
- Dusky sap beetle adults are 1/8-inch long with short wing covers and are uniform dull black in color.
- Picnic beetle 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. All sap beetles have this feature and is a useful tool when identifying sap beetles.
Eggs and Larvae
- Eggs are milky white, small, about 1/25 inch long and not easily seen because they are laid within plant matter.
- Larvae are small, (less than 1/4 inch long), white (pale yellow when mature) with a light brown head.
Life cycle of sap beetles
Sap beetles live through the winter as adults in sites outside gardens.
- They emerge in spring and lay eggs near fermenting and decaying plant material.
- Larvae feed for about three weeks and then transform into pupae.
- Adults emerge from pupae 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.
Damage caused by sap beetles
Sap beetles can injure fruits and vegetables.
They are more common on fruits and vegetables that have been damaged by another insect or infected with a disease.
Sap beetles may be seen on strawberries that are also infected with a disease.
They can leave deep cavities in the berries, similar to the damage caused by slugs.
They also introduce fungal spores of organisms that can further spoil the fruit.
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.
The larvae of sap beetles then feed on the undamaged kernels.
Sap beetles on strawberries
Strawberries are the primary host for the strawberry sap beetle. These beetles prefer over-ripe fruit but also readily attack ripening fruit.
When the strawberries begin to ripen, sap beetles are attracted into gardens. The strawberry adult sap beetle feeds on the underside of berries creating holes (less than ¼ inch).
Sap beetles on corn
Sap beetles can be found from silk to ear maturity.
Individual kernels are chewed, and some kernels look hollowed out.
Small yellowish or pinkish-white grubs may be found in ear tips along with adults.
The picnic beetle is also attracted to all types of over-ripe and damaged fruit.
How to protect your plants from sap beetles
Watch for sap beetles in gardens starting in early July when adults first start to emerge. Particularly check overripe strawberries, although they can also be found in ripening fruit.
Remove overripe fruits and vegetables
Remove any damaged, diseased and overripe fruits and vegetables from the garden 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.
Using baits for trapping
You may try bait trapping to reduce beetle populations.
- Place traps that are more attractive than ripening fruit. Trap buckets baited with whole wheat bread dough and over-ripe fruit outside the patch helps to reduce beetle numbers.
- A container of fermenting plant juices will also attract sap beetles.
- Common baits include stale beer, molasses-water-yeast mixture, vinegar or any overripe fruit.
- Traps should be placed a few feet outside of your garden.
- Discard trap contents frequently, every three or four days and rebait traps.
Use of pesticides is NOT very effective and is NOT recommended. Sap beetles are seen on ripe fruit, so pesticides should NOT be used on the crop.
Carbaryl and bifenthrin can be used to control severe infestations. These pesticides may kill existing beetles, but if fruit/vegetables are present, they cannot prevent additional sap beetles from moving into gardens.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the fruit/vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018