Last week I wrote about the remarkable burying beetles, which typically inter and prepare the carcasses of mice and small birds on which the beetle larvae feed. The individual beetle that prompted my interest was just crawling across the ground, but it looked very strange. It was covered, top and bottom, with at least two layers of tiny, squirming mites, hundreds of them.
So many mites could not, realistically, be feeding on the beetle, so what was going on? A cursory search of the literature revealed that certain kinds of mites regularly ride around on burying beetles, using the beetles for transportation to another carcass. In effect, the mites are hitch-hikers. There are several kinds of mites that hitch-hike on burying beetles, and at least some of them have strong preferences about which beetle species they ride on. The beetle we found had probably left its nest very recently and had not yet dumped off many of its riders on other carcasses.
Like their beetle transporters, the hitch-hiking mites reproduce on carcasses. Their effect on reproduction of the beetles varies from positive to neutral to negative: In some circumstances they benefit the beetle by killing the eggs and larvae of flies or other carrion-users such as nematode worms that colonize the same carcass and compete with the beetles for use of this food supply. Thus, they can enhance the reproductive success of the beetle. However, at very high densities, the mites may start to eat beetle eggs too, which sounds like a simple negative effect from the beetles’ perspective. But reducing the number of beetle eggs can leave more food for the remaining larvae, which then grow faster and bigger. And because large larvae turn into large adults that can lay more eggs (if female) and dominate other males (if male), a limited amount of egg-eating can ultimately benefit the beetles (provided that not too many of the beetles’ eggs get eaten).
Other kinds of organisms are also carried to new carcasses by burying beetles. For example nematode worms can be common in beetle nests, and they are competitors for the food supply in the carcass. They ride in the guts and genitalia of the beetles when the adults move to a new carcass. But this relationship is not as well-studied as the one with mites.
The official name for these hitch-hiking relationships is ‘phoresy’ or ‘phoresis’. Some phoretic mites latch onto bumblebees or honeybees and get carried from one flower to another, where they eat pollen and nectar. Similarly, tropical flower mites hop onto the bills and into the nostrils of nectar-seeking hummingbirds, and catch a ride to another flower. Because the flower mites eat nectar, they compete with the hummers for food. By reducing the amount of nectar in a flower, the mites may cause the hummers to move to more and more flowers, and thus become both better pollinators (from the plant’s perspective) and better transporters (from the mites’ perspective). The mites apparently do not harm their hummingbird carriers directly but may cost them a little energy if the hummers have to visit more flowers to get a full meal.
Insects that feed on wood often have phoretic fungi, whose spores are carried on the wood-boring or bark-eating insects. The fungi help weaken the tree and may also be food for the insects’ larvae. Cactus-loving fruit flies carry a yeast that detoxifies certain plant chemicals, allowing the fly larvae to grow faster and bigger as they eat the cactus.
One of the best-developed phoretic relationships involves blister beetles that lay their eggs in flowers. The larvae of certain beetle species mimic the appearance and the chemical sex-attractants of bees. Male bees visiting the flower, to feed on nectar, try to copulate with the mimics, which grab onto the bee’s legs for a free ride to the bee’s nest. There the beetle larvae act as parasites, eating bee eggs, larvae, and honey stores.
This is just a small sample of the diversity of phoretic relationships in nature, most of which have not been studied in detail.