The alkali bee is an important pollinator of alfalfa. The bees build nests in the ground and stock them with pollen for their brood. The Pavement Ant, Tetramorium caespitum, sometimes builds nests near the “bee beds” of alkali bees. Pavement ant foragers will enter unattended nests of alkali bees and consume the pollen. The alkali bees often occupy and guard the tunnel, attacking ant invaders with their mandibles. A pavement ant entering an inhabited nest elicits a loud buzzing struggle that sends the ant scurrying to the surface for safety. Inside its tunnel the bee has the advantage against the ant.
George Schultz* observed pavement ants engaging in an interesting behavior with the alkali bees. On finding a bee nest, the ant would fetch soil and drop it down the hole onto the head of the bee. Nest mates often joined the ant in this activity. Soil was tossed into the nest until it was level with the surface or until the bee emerged, pushing the soil from its nest. When a bee emerged ants surrounding would grasp the bee by the antenna. As the bee struggled to dislodge the attacker, more ants joined the struggle, grabbed and killed the bee. The ants butchered the bee and returned to the nest with the food.
The number of nests in the bee bed was about 50,000 and fewer than 30 ant attacks were observed each day. Not all attacks were successful. The effect of the ants on the bee population is unnoticeable. For the ants, a successful attack results in a significant amount of food.
*George Schultz. 2000. Soil-dropping Behavior of the Pavement Ant, Tetramorium
caespitum (L.) (Hymenoptera: Formicidae) Against the Alkali Bee (Hymenoptera: Halictidae). Journal of the Kansas Entomological Society. 55:277-282.