James W. Amrine Jr., West Virginia University, Bugwood.org
Rose-seed Megastigmus is a seed-infesting wasp that is common in homes and establishments where dried flowers and potpourri containing rose hips are found. Generally, the wasps are noticed during the winter months when the adults fly to windows or lights. Although these tiny wasps are very easy to identify, their small size frequently contributes to misidentification.
Description of Adults
The adults vary greatly in size and coloration. Wasps that are raised from a wild or multiflora rose are considerably smaller than those raised from the larger cultivated rose. Typically, they are approximately 3 mm in length and are mostly yellow in color. The females carry an ovipositor that curves upward from the last abdominal segment and is about the same overall length as the wasp. Brown patches are on top of the head surrounding the ocelli, on the sides of the thorax, and on the dorsal side of the abdomen. The elbowed antennae are also brown and have eleven segments. The males are typically smaller, darker to nearly black in color, and do not have an ovipositor.
Life Cycle (In A Natural Setting)
The eggs are oviposited into rose hips during June and July. The larvae feed entirely within the rose seed and can grow to 2.8 mm or slightly more than 1/10 inch in length. The size, however, is greatly dependent on the size of the seed. The larvae become fully grown in the fall and remain within the seed casing until the following spring when they pupate. The adults emerge from the seed by chewing small openings in the seed in early summer. This timing is altered when rose seed is brought into the home over the winter months.
When owners notice the adult wasps in their home or establishment, they can assume that either potpourri, dried flower arrangements containing roses, or rose hips are somewhere in the structure. Although the wasps will not harm anything in the home, the homeowner is nevertheless alarmed. Removal of the infested commodity is the only way to prevent further emergence. Fortunately, the wasps will die within a few days of emergence and, therefore, do not require chemical controls.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Steve Jacobs, Sr. Extension Associate
February 2000 Reviewed January 2010