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7/26/99
Contacts:
Donald Lewis, Entomology Extension, (515) 294-1101,
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Yard and Garden Column for the Week Beginning July 30
Picnic Beetles Love (What Else) a Picnic
By Donald R. Lewis
Extension Entomologist
Iowa State University Extension
Insects aren't out to spoil all your fun. It just seems that way. Take your average picnic, for example. It seems like every insect under the sun wants to join the party. Ants, mosquitoes, chiggers, yellowjackets, and one special unwanted guest appropriately named for the occasion, the picnic beetle.
The picnic beetles go by several names. You may know them as the picnicbugs. Quite frequently they are simply called "those (expletive deleted) little-black-bugs!"
Picnic beetles are scavengers with a fondness for sweet or fermenting plant juices. Put out the fruit salad, open the pickles, or simply pop the top on your favorite fermented grape or malt beverage and the picnic beetles fly in for the festivities. And they are not content to stand at the edges and nibble like a well-mannered guest. They wade or swim right into the middle of things with all six legs.
The picnic beetle is one member of a diverse family called the sap beetles. The 180 species in this family are small insects only 1/8 to 1/4 inch long. They are dark or black in color; flattened and broadly oval in shape. The knobs on the ends of their antennae make identification a dead giveaway.
The best known sap beetle is the fourspotted sap beetle, the species most commonly also called the picnic beetle. Each beetle is 1/4 inch long and as shiny black as new patent leather shoes. There are four, irregular yellow-orange spots on the back.
A Garden Variety Pest
If you've not seen sap beetles at a picnic, you've probably seen them in the garden or at the Farmer's Market. Sap beetles are a common pest in strawberries, raspberries, tomatoes, sweet corn and other garden fruits and vegetables. Some of their more annoying habits are feeding at the tips of sweet corn ears, inhabiting the cracks in tomatoes, eating into the end of a ripe muskmelon and literally destroying entire strawberries and raspberries. Sap beetles are attracted to all ripe, damaged or cracked produce.
Sap beetles hibernate through the winter and become active by May. Damage is low early in the season but increases as populations increase through the summer. Sap beetles are most troublesome from midsummer to mid-autumn.
Where Do They Come From?
Those early spring sap beetles spend April, May and June laying eggs near decomposing plant material. The breeding site may be something obvious such as a pile of spoiled grain, but more often the eggs are in rather inconspicuous places such as rancid sap in tree wounds or under loose bark, and under leaf litter and other accumulations of rotting vegetation.
The eggs hatch into tiny white larvae that feed on the watery, fermented juices. After 30 to 35 days the larvae are full-grown at approximately 1/4 inch long and they wander out of the food source. After a brief period as a pupa in the soil new beetles emerge just in time for your picnic and fresh garden produce.
What to Do
There is lots of advice for managing sap beetles in the garden. Some of it is even useful. Unfortunately, the management options at the picnic are usually much less satisfactory.
Sanitation is the best management strategy for the home garden. Keep the garden or fruit production area as clean as possible through frequent picking and removal of damaged, diseased and overripe fruits. Bury or discard all damaged or decomposing fruits and vegetables to eliminate beetle food sources.
Spraying insecticide for sap beetles is impractical and difficult because the crop is ready for harvest or harvest already is underway. If you do spray, use an insecticide with a short harvest-waiting interval and follow label directions carefully. Malathion, rotenone or permethrin are suggested if you choose to spray in the home garden.
Trapping sap beetles remains a popular pastime even though it is doubtful that it actually "controls" the pest. The theory behind trapping is to place traps outside the garden or several feet away from the picnic table to lure them away from you and your food. Trap early and trap often.
Any container of fermenting plant juices will attract sap beetles. No special design is needed as the beetles will fall in and drown. Some research suggests that the best baits would be bananas or muskmelon. Other baits include stale beer, molasses and water with yeast, vinegar and any overripe fruit. Sprinkling insecticide on the bait is not necessary.
Trapping might cause a noticeable reduction of sap beetle populations. However, it is more likely you will just end up with a bucketful of dead beetles to discard.
Enjoy the picnic and be of good cheer, for soon after the sap beetles are gone, the fruit flies will appear.
ml: isugarden
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