It's a tough job, but native bees can do it. (Biology).

Conservationists fretting about the dwindling of North American bee species have new evidence of the insects' importance.

North American farmers typically rely on a single European honeybee species to pollinate crops, explains Claire Kremen of Princeton University. This focus on one species for such a vital service raised alarms in the mid-1990s as diseases and other menaces attacked honeybees. Adding to the concern, biologists noted that native bee species, indeed, were disappearing.

Kremen devised a study to assess the pollinating capabilities of wild bee communities, which can include dozens of native species.

As a test crop, she and her colleagues chose watermelon, which is known to be particularly challenging for pollinators. Each female flower needs to receive between 500 to 1,000 pollen grains to yield a marketable melon. The researchers monitored bee activity at several watermelon farms in California. They found that the European honeybees cruised all the farms but that the farms nearest to a habitat of wild bees had the largest workforce of native bees.

To measure pollen loads delivered by each species in the field, the researchers fastened virgin female watermelon flowers to a stick and counted the pollen grains delivered to it by an individual bee. "We call it interviewing the bees," Kremen says.

The researchers calculated that if the imported honeybees would vanish, the native bees could save the crop on the farms near these bees' habitat, but not on the farms far from such habitats. The message, Kremen says, is that farmers could do themselves a favor by restoring bee habitats around their farms.

The research appears in the Dec. 24, 2002 Proceedings of the National Academy of Sciences.--S.M.