Insidious Insect Fungi

The Walking Dead is a popular television show these days, but reality is perhaps even more sinister. Did you know that there are fungi that hijack the brains of insects causing them to behave in ways that benefit the fungus? It's true, and you have probably seen evidence of this macabre life cycle without knowing the answer to the mystery.

Fly infected with fungus attached to a leaf (Massachusetts)

Cheryl Harner, a naturalist and writer at Weedpicker's Journal, asked me about this bizarre phenomenon a couple of years ago, and I always meant to learn more about it myself. You owe it to yourself to check out her blog, and I'll try to complement her post rather than compete with it. She did an excellent job of researching and linking, though.

Dead grasshopper nymph that has climbed a grass stem due to funal infection (Massachusetts)

It is not just grasshoppers and ants that are victimized by what are called "entomopathic" fungi. Flies are among the most conspicuous of hosts. You have probably seen flies clinging to foliage or twigs in odd, contorted positions, with bloated abdomens. The spores of the Entomophthora muscae fungus have their hosts literally bursting at the seams, the membranes between the abdominal segments.

Meanwhile, grasshoppers and related orthopterans are plagued by Entomognatha grylli, and lady beetles are often afflicted with Laboulbeniales fungi.

The life cycle of fungi in the order Entomophthorales begins when a spore lands upon an insect. However, many of the fungi are very host-specific, so if a spore lands on the "wrong" bug it is likely done for. Should the spore contact the correct host, it soon germinates, penetrating the cuticle of the insect and growing internally, absorbing the host's nutrients, eventually killing it. Before the doomed creature perishes, the fungus does something remarkable. In many cases it stimulates the host to seek a high point, either by climbing or flying, whether or not the insect normally performs such behavior. This phenomenon is sometimes referred to as "summit disease."

Headless grasshopper corpse in wake of fungal infection (Colorado)

Once the host insect ascends a grass stem, for example, the fungus sends out special structures called rhizoids that are white, thread-like filaments. These fibers often emerge from the insect's tarsi ("feet") or mouthparts, anchoring the bug to the substrate such that it is not easily dislodged after death. Now the spores erupt, sometimes in the form of fruiting bodies like other fungi, or simply exuding from holes in the host's cuticle.

Carolina Leafroller ("cricket") showing fungal rhizoids penetrating its feet and anchoring it to the leaf (Ohio)

Grasshoppers are among the most conspicuous victims if only because of their size, but most types of insects, and spiders, too, are vulnerable to fungal infection. The typical appearance of fungus victims are stiff, dessicated specimens that are essentially mummified.

Harmonia axyridis with perithecia of Hesperomyces virescens at the tip of its wingcovers (elytra)

Fungi in the order Laboulbeniales affect beetles in particular, and are very different in their mode of dispersal and effect on the host. Hesperomyces virescens apparently afflicts only the Multicolored Asian Lady Beetle, Harmonia axyridis, at least here in North America. It is microscopic, and is essentially a sexually transmitted disease since the overwhelming means of infection occurs during mating of the host. Spores can also transfer when large groups of overwintering lady beetles force the insects to rub against each other.

Mating pair of Harmonia axyridis in Illinois exhibiting symtoms of Hesperomyces virescens

Most adult beetles exhibit symptoms in the form of tiny, yellow, scale- or flake-like protrusions from the cuticle, especially at the tip of an elytron (wing cover). These are the fruiting bodies, called perithecia, from which spores are issued, if I comprehend the literature and language of mycology correctly. Interestingly, the fungus has little or no effect on the host, but depends on it for reproduction.

When one thinks of mortality factors that affect insects, or most other animals, we tend to forget the impact of fungal organisms. Maybe I have seen one too many episodes of the television show Monsters Inside Me, but I have a whole new respect for fungi now. It helps to remember that fungi are often very host-specific, so handling a fungus-ridden bug is not going to pose a health threat to a human. As far as we know.

Fungus-infected fly corpse on leaf (Colorado)

Sources: Eiseman, Charley and Noah Charney. 2010. Tracks & Signs of Insects and other Invertebrates. Mechanicsburg, Pennsylvania: Stackpole Books. 582 pp.
Parker, Abigail M., et al. 2010. "with Labioulbeniales fungus," Bugguide.net.

Indoor Insects of Autumn (part 4 of 4)

This is the final installment of a four-part series addressing insects frequently seen indoors at this time of year when they seek shelter for hibernation during the colder months. This entry will introduce the “multicolored Asian lady beetle,” Harmonia axyridis.

Like the brown marmorated stink bug, the multicolored Asian lady beetle, also known as the “Halloween lady beetle” for its abundance at the end of October, is not native to North America. It was repeatedly introduced here by state, federal, and private interests to augment native lady beetles for control of aphid pests in orchards. As early as 1916 an effort was made to establish this species in California. Subsequent efforts there and elsewhere appeared to fail each time.

Finally, in 1988, viable populations were discovered near New Orleans. Whether this was the result of a planned introduction, or an accidental importation, it marked the start of something big. Today, Harmonia axyridis is found over most of the United States and adjacent southern Canada, save for the southwest U.S.

This species is not easily identifiable because it is so variable in color and pattern. Most individuals have bright red elytra (wing covers) with eighteen or nineteen black spots, but they may be orange and spotless, or even black with only a pair of red spots. There is every combination in between, too. They can thus be confused with many native species.

The overriding clue to their identification seems to be their sheer abundance. There is some circumstantial evidence that they might even be displacing native species, but probably not as much so as another non-native lady beetle, the seven-spotted (“C-7”) lady beetle, Coccinella septempunctata.

Nowhere is the abundance of Harmonia axyridis more obvious or obnoxious than when they congregate indoors while seeking snug spots in which to overwinter. Their sudden appearance usually means a flood of telephone calls from irate homeowners to the state department of agriculture and county extension agents. It is no one agency’s “fault,” though, and at most the beetles are a “nuisance pest” that can sometimes emit a foul odor or stain fabrics. They most definitely do not breed indoors or eat clothing or blankets. In fact, they subsist on fat reserves during the colder months, not feeding at all.

Larva of Multicolored Asian Lady Beetle

There are persistent reports of the insects biting people, but this does not represent an attack as much as curiosity. Insects in general investigate things through taste, smell, and touch. Nor does it mean the beetle is acting in self-defense. Lady beetles defend themselves by secreting a noxious yellow fluid from their “knee” joints in a behavior known as reflex bleeding. This liquid smells awful and can leave a stain.

Mating pair, male on top

Preventing multicolored Asian lady beetles from entering your home or office building is the easiest way to avoid problems. Replace worn weatherstripping on doors. Repair holes in widow screens, and seal other cracks and crevices. Consider vacuuming up any beetles that do make it indoors and releasing them outdoors near a woodpile or other sheltered situation away from your home.

You might also consider painting your home a different color. Evidence shows that the beetles are most attracted to pale hues such as white, gray, and yellow.

More information about these beetles can be found online through fact sheets from Ohio State University and the USDA Agricultural Research Service.