They loiter around a home, slip through an unguarded entrance, grab the treasures, and they're off.

It's "home invasion" in the insect world: robber bees.

As resources dwindle, honey bees searching for a free source of sugar invade neighboring colonies, said bee breeder-geneticist Susan Cobey, the manager of the Harry H. Laidlaw Jr. Honey Bee Research in the Department of Entomology at UC Davis.

"It's the stronger hitting on the weak," she said.

The robber bees are after one thing: honey, the UC Davis researcher said. "And they are not neat; they leave the combs messy with wax bits everywhere. This invites other pests, such as ants and yellowjackets, to grab what is left — the pollen and brood."

The robber bee thievery occurs more often in the late summer and early fall, when flowers dry up and preparations for winter stores are necessary for survival.

"Robber bees are opportunists," Cobey said. "As resources become scarce, they scout and look for a sugar source or hives that have few guards, hives with too many holes, or hives with large entrances too big to guard. They also look for colonies under attack by ants or other pests, such as yellowjackets."

Beekeepers can exacerbate the thievery when they open a hive and expose the honey. For the robber bees, smelling and tasting all that honey "is like kids in a candy store," she said. Deadly fights often burst out at the entrance to the hive, as the guard bees gang up on and kill robber bees, pushing their carcasses onto the ground.

"Small hive beetles will join the party, too," Cobey said. "They are very opportunistic. The small hive beetle is another new pest, originally from South Africa that's now in California."

Cobey, who maintains some 50 to 60 hives at the research facility, takes specific precautions in the late summer and early fall. Like beekeepers around the world, she screens or reduces the size of an entrance to a hive. A smaller opening makes it easier for the bee guards to protect their turf from predators.

Robber bee behavior is rare "if colonies are equal in strength and healthy and undisturbed (by pests or beekeepers)," she said. "If colonies are stressed, crowded, diseased or fighting pests, the other bees — as well as other pests — sense this 'weakness' or 'opportunity' and go for it. It's survival of the strongest. Mother Nature isn't always nice."

Colony collapse disorder, a mysterious phenomenon in which bees abandon their hives, also yields opportunities for robber bees. CCD is thought to be caused by multiple factors, including stress, malnutrition, pesticides, parasites, diseases and changing climate, according to UC Davis Apiculturist Eric Mussen.

Cobey, internationally known for her classes and research on queen bee rearing and queen bee insemination, said the honey bee hive is often referred to as a "superorganism."

"We admire the incredible cooperation of a honey bee hive because of its large and diverse workforce that performs a dynamic range of tasks without conflict," she said. The queen bee lays the eggs and the worker bees (sterile females) do all the work. Besides gathering nectar, pollen and water, the worker bees serve as air conditioners (fanning the queen and the hive with their wings), architects, construction workers, nurses, dancers, guards and undertakers. The only role of the drones is to mate with the queen.

As temperatures drop, fewer "home invasions" will take place, Cobey said. "I like to work in the morning now, as the bees are quiet. As it warms up, the scouts are out and the problem intensifies."

As fall turns to winter, bees cluster and go into a wintering mode, she said. "Foraging is over. This change is more gradual in California, with the milder climate, hence more robber bee activity."

Robber bee behavior does have its pitfalls to the thieves. "A consequence of this robbing is the robbers usually also take home diseases and pests," Cobey said. "This is one way they are spread."