Mites and burying beetles (2 of 2)

passengers on the beetle

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.

beetle-2-with-mites-by-bob-armstrong
Photo by Bob Armstrong

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.

Burying beetles and mites (1 of 2)

a beetle with many stories to tell

On a recent junket to watch bears catch salmon, we encountered a beast with a different story to tell. This was a beetle about an inch long, and its entire body, top and bottom, was covered with layers of tiny, squirming mites. Together, the beetle and the mites represent a remarkable and fascinating story—in two parts.

nicrophorus-with-mites-2-bergeson
Photo by Pam Bergeson

First, the beetle. This kind of beetle is called, variously, a carrion beetle, a sexton beetle, or a burying beetle. It belongs to a genus of beetles that is widespread in the world, especially the northern hemisphere, with about 68 species known so far, and three species in Southeast Alaska. The name of the genus is Nicrophorus, but formerly this was spelled Necrophorus, which give a clue to its behavior. ‘Necro’ (as in necropsy and necrophilia) refers to the dead, and ‘phor’ refers to carrying or bearing something –thus, a bearer of the dead. The beetles are typically black, with orange blotches on their wing covers.

These beetles are scavengers, eaters of vertebrate carrion (and one species has recently been found to use snake eggs). Adults can feed on any large or small carrion, but perhaps the most interesting relationship is generally with small mammals or birds.

The entire life history of burying beetles typically centers on finding a dead shrew, mouse, or small bird. There usually aren’t lots of these just lying around, up for grabs, and what few there are, are very attractive to other scavengers, be they ravens, blowflies, worms, or microbes. So it pays to find a carcass quickly; competition for dead mice or birds is intense. One study showed that the size of the population of burying beetles depended on the availability of suitable carcasses, which depended on the size of the populations of small mammals.

In addition to competition with other kinds of organisms, there is competition among the beetles themselves. When a male beetle locates a suitable carcass, he advertises for females, using airborne chemical scents or, in some cases, courtship songs produced by rubbing a rough part of the abdomen against the underside of the wing covers. Large males can outfight smaller males for possession of the carcass, but the subordinate males may hang around and sneak some copulations with arriving females. In addition, because female beetles can store sperm from a previous mating elsewhere, a male parent on a carcass may not be the father of all of her eggs. In at least one species of beetle, females also fight each other for rights to use the carcass, but subordinate females may add some eggs to the brood.

The beetles use the carcass to rear their young. When a burying beetle finds a relatively fresh carcass, it buries its find on the spot, by scratching out a hole under the body, or it moves the prize to a usable spot, by lying on its back under the body and pushing with its legs, and then buries it. These beetles are strong, and can move a carcass over and around obstacles.

Once the body is buried, the beetles chew off the fur or feathers, roll the body into a ball, and anoint it with chemicals that deter the growth of microbes that would decompose the carcass, competing for the flesh and reducing its nutritional value. Adult beetles carve a hole in the top of the ball, chewing some of the flesh into a soup that fills the hole.

Female beetles lay their eggs in a tunnel near the carcass. The eggs hatch in just a few days, and the larvae move to the carcass. There they may be fed by the parents from the soup of chewed up flesh, or they may feed for themselves. The larvae beg from the parent beetles, and larvae that hatch a day or so ahead of others are more successful at begging.

Usually both male and female parents tend the brood of larvae, although either one can also do it alone. The primary role of the male appears to be in defense of the carcass against flies and other beetles but, in addition, the female benefits, because she doesn’t have to work so hard and can save energy for a second brood on a different carcass.

The number of eggs laid by a female beetle depends largely on her body size, her body condition (which depends on how well fed she is), and the size of the carcass. However, if too many eggs are laid (for example, if a second and subordinate female has deposited her eggs in the same area), females may destroy some of the eggs or larvae.

The larvae go through several molts as they grow. In some species, they turn into adults in summer, overwintering as adults that come out the next spring. In other species, the larvae spend the winter as inactive pupae, and adults emerge from the pupae the next summer.

Some of the beetle species, including one of those in our area, may nest communally. That is, several pairs of beetles may share a large carcass (if they can get there before another scavenger gets it!). On the north Pacific coast, salmon carcasses offer a relatively predictable source of carcasses in late summer and fall. Bears and other foragers leave partially eaten carcasses on the floor of the forest, where they become available to burying beetles. A salmon carcass may harbor three or more pairs of beetles, all rearing their broods on different portions of the carcass. In this case, the carcass is not buried; instead, the beetles just move in underneath the body. The larvae are then reared on salmon flesh, instead of the more usual mammal or bird.

The story of the mites riding on the beetles will have to wait until next time.