Those Exotic Digger Bees

FOCUS NATURE ARIZONA'S EXOTIC DIGGER BEES

TEXT BY JOHN ALCOCK Most people give bees a wide berth, but the Sonoran Desert has many native bee species that deserve admiration rather than fearful avoidance. Two of these special bees come with the scientific labels Centris pallida and Centris caesalpiniae, but I prefer the simple title "digger bees" for both of them. The two exhibit very similar behavior, especially in the skill with which females and males of both species dig into the desert ground, tunneling downward like professional earthmovers.

A female digger bee does its excavating when constructing an underground nest, a downward-sloping tunnel with a bee-sized container at the end. It fills this amphora-shaped container with pollen and nectar from paloverdes and ironwoods, and then it lays an egg on the surface of the molasses-like mixture. The egg hatches into a small grub that feasts on the larder left by its mother. The grub grows underground and eventually metamorphoses into an adult, a process that takes about a year to complete. The adult finally digs its way up through about six inches of earth around the time the paloverdes and ironwoods will be blooming again.

Although female digger bees do a beautiful job of nest construction, they are not unusual in this respect as females of many other bee species fashion burrows in a similar way. However, it is extraordinary for male bees to tunnel into the soil to meet the surfacing females - a practice male digger bees do with exuberance. A male can really send the sand flying on a spring morning as it bites the ground and then kicks the loosened soil beneath its raised abdomen.

Amazingly, males easily detect potential mates under a blanket of hardpacked desert soil. A male removes the final inch or so of dirt, creating a vertical shaft that meets the one the female is creating as it gnaws its way up to the open air. When the tunnels meet, the female struggles to the surface to be grasped and mated by the waiting male. But before the two meet, the digger often must fight furiously with interlopers for possession of the spot. A large and powerful male can steal digging sites from rival males, winning the right to mate with the female it finds. After mating, the female goes on its way to start the nesting cycle while the male returns to search for more buried females.

So of dirt, creating a vertical shaft that meets the one the female is creating as it gnaws its way up to the open air. When the tunnels meet, the female struggles to the surface to be grasped and mated by the waiting male. But before the two meet, the digger often must fight furiously with interlopers for possession of the spot. A large and powerful male can steal digging sites from rival males, winning the right to mate with the female it finds. After mating, the female goes on its way to start the nesting cycle while the male returns to search for more buried females.

How do males manage to find their hidden partners? These bees have a highly refined sense of smell. If a devious human experimenter buries a dead female bee in a shallow grave, males skimming over the surface of the ground will land at the spot and dig out the dead insect. They clearly do not need to hear any sound signals from an emerging female to find it. The nearly imperceptible scent emitted by a female below ground is sufficient to alert males to its presence, thanks to what must be hypersensitive odor detectors on the males' antennae. With their special sensors, male digger bees turn what would be an impossible task for a human into an everyday achievement as they participate in the scramble to reproduce.

Author John Alcock is a professor of zoology at Arizona State University. His most recent book is Sonoran Desert Summer, published by University of Arizona Press.