Sap Beetles

Description of sap beetles

Sap beetles are a common pest problem in strawberries, raspberries, tomatoes, sweet corn and other garden fruits and vegetables.  These scavenger beetles feed on developing, ripe or overripe produce as well as plant sap exuding from wounds and fungi.  The large irregular holes and decay spread from fruit to fruit and can cause a large amount of produce to be unusable.

Most of the over 180 different species of sap beetles are small insects, black or dark in color and flattened and broadly oval in shape.  The best known sap beetle is the fourspotted sap beetle, also known as the picnic beetle or “picnicbug.”  This very common beetle is 1/4 inch long, shiny black and has four yellowish dots on the wing covers.  It will feed on over-ripe or damaged fruit or vegetable and outdoor picnic foods. 

Four-spotted sap beetle, AKA "picnicbug"
Four-spotted sap beetle, AKA "picnicbug"

Life cycle of sap beetles

Sap beetles spend the winter as adults and become active in April and May.  Damage frequently is low early in the season, but increases as populations increase through the summer.  Sap beetles are attracted to ripe, damaged or cracked fruits.  Any injury that exposes plant sap that has a chance to ferment will also attract sap beetles. 

Management of sap beetles

Sanitation — Sanitation is the best management strategy for sap beetles.  Keep the garden or fruit production area as clean as possible through timely picking and removal of damaged, diseased and overripe fruits.  Problems often occur during rainy spells at harvest when there is a buildup of overripe fruit.  Excessive weed and grass growth and other obstacles that interfere with clean picking may contribute to sap beetle problems.

Traps — Traps for sap beetles are sometimes used in home plantings even though there is no convincing evidence of their effectiveness beyond indicating if sap beetles are present.  The theory of trapping sap beetles is to place traps outside the garden or patch.  Any container of fermenting plant juices will attract sap beetles.  No special design is needed as the beetles will fall in and drown.  Common baits include stale beer, molasses and water with yeast, vinegar and overripe fruit from the planting.  No specific spacing or distance to place the traps from the produce is recommended though traps should be well outside the garden.

Insecticides — Sprays are available for sap beetles but they are difficult to use because they are applied to a crop that is ready for harvest or while harvest is underway.  If you do spray, use an insecticide with a short harvest waiting interval and follow label directions carefully.  Sevin, rotenone, permethrin or malathion are suggested for home garden use.

Prepared by Donald R. Lewis

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Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 13, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.