Crop & Pest Report


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Digger Bees in Lawns (5/19/11)

Several calls have come in concerning several hundred, small mounds of soil in the lawn (Fig. 1) and small (¼ to ½ inch), metallic bees swarming around these mounds. These bees are collectively called “digger bees” (Fig. 2) and represent several groups of bees – andrenid bees, halictid bees and colletid bees.

Digger bees prefer to infest areas of the lawn with sparse grass from drought conditions or tree shade. Although these bees are often blamed for causing the unhealthy lawn, digger bees do not damage a healthy lawn.

Digger bees have one generation a year. They overwinter as larvae in the underground cells. In the spring, digger bees pupate and emerge as adults. The female creates the nest in the ground and collects pollen, packing it into the chamber. Then, eggs are laid in the pollen-packed cell, and larvae feed and develop for the rest of the summer and over the winter. Males are not involved with nest construction or provisioning, and spend their time cruising over the nesting area for a chance to mate. Adult bees only live for about 4 weeks and then are not seen again until next spring. Digger bees are solitary bees and each hole is one bee’s nest. In contrast, honey bees are social bees with a queen bee and many worker bees in one nest.

 Hort1     Adult digger bee

Fortunately, digger bees are docile and not very aggressive. They typically do not sting unless trapped or handled. Still, large numbers of active bees swarming over an area of lawn can be distressing for homeowners. For management, homeowners can try watering, mulching or planting shade tolerant grass in the area to make it a less favorable habitat for digger bees. Insecticides are recommended as the last resort, since digger bees are valuable native pollinators which pollinate our trees, shrubs and flowers. Homeowners can use powder or liquid formulations of insecticide, and dust or spray into the entrance of the burrow. Some suggested insecticides include Sevin® (carbaryl), Tempo® (cyfluthrin), or any other insecticides registered for wasps and outside use in lawns. Please be certain that you are not dealing with other bees or wasps (hornets) that are very aggressive and harmful. They can quickly attack you in large numbers and cover you with stings! You may want to hire a professional pest control company for those aggressive insects.

 Janet J. Knodel - Extension Entomologist


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