Including strawberry sap beetle (Stelidota geminata) (Glischrochilus quadrisignatus)
Sap beetles are a common group of insects in home gardens. Two species that are particularly attracted to the ripe strawberries in Minnesota are the strawberry sap beetle and picnic beetle.
Sap beetles are generally small insects, usually less than 1/2 inches long with oval-shaped bodies. They are generally dark colored, sometimes with orange or yellow spots.
The adult strawberry sap beetle is dark brown, oval, less than 1/8-inch long, and has no prominent markings on the wings. The adult picnic beetle is somewhat longer at 1/5-inch long, is thinner, and has four orange blotches on the back. Look for the "knobbed" antennae when identifying sap beetles.
Sap beetles overwinter as adults in organic matter in protected sites and become active early in spring. They will mate and begin laying eggs in fermenting material in May and June. Adults emerge in late June and July.
When the strawberries begin to ripen, sap beetles are attracted into gardens. Strawberries are the primary host for the strawberry sap beetle which prefers over-ripe fruit but will also readily attack ripening fruit. The strawberry adult sap beetle tends to feed on the underside of berries creating holes. The picnic beetle is attracted to all types of over-ripe and damaged fruit. Beetles usually overwinter in sites outside gardens. There is usually only one generation per year.
Sap beetles feed on ripe and overripe strawberries, sometimes congregating in large numbers. They can leave deep cavities in the berries, an injury similar to slug injury. At the same time they introduce fungal spores of organisms that can further decay the fruit
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.
Keep the field clear of ripe and over-ripe fruit, as reducing the attractant is the best management tactic. Place "trap buckets" baited with whole wheat bread dough and over-ripe fruit outside the patch to intercept immigrating beetles and reduce numbers. The beetles are highly attracted to anything that has the ability to ferment.
Chemical control is not effective for several reasons. First, the beetles are moving into the fields from other areas and killing the ones in the field is no guarantee that others will not enter the field following the spray. Second, populations do not build up until harvest so nothing preventative can be done.