A. Several kinds of wasps can be problematic because nooks and crannies around barns and sheds provide good nesting sites. Paper wasps and mud daubers commonly build their nests under eaves and overhangs. Generally, these insects are not aggressive until late summer. Colonies are at peak numbers then, and caterpillars that are the bulk of their diet are becoming harder to find. The wasps seem more likely to go after people and horses that invade “their space.”
Large European hornets might build nests in wall voids. They, too, are more aggressive late in the season, attacking without obvious provocation. Dry weather also spurs conflicts with wasps and hornets. They visit water tanks more frequently when puddles and other normally wet areas are scarce because of increased demand to cool and hydrate larvae developing in the nest.
As with many pest problems, preventing wasps in horse barns is the key. Watch overhangs and favored nesting areas in early summer, when overwintering queens start building their nests. Check nesting areas weekly during this establishment period. It’s much easier to deal with a single queen or a few workers when the nest is small than later when numbers are higher. Swatting with a broom or using a wasp and hornet spray can be very effective. Remember that good nesting sites will always be attractive, but preventing establishment early will prevent problems later.