By Mark Shour
Iowa State University Extension
A new growing season means a new opportunity to learn about our environment. And being an entomologist, these lessons often involve “bugs.” Recently, when checking insects from monitoring stations, small brown bark beetles (about the size of a rice grain) were observed with red dots on them. Microscopic examination revealed these beetles had tiny, red mites clinging to various places on their body. This seemed odd. What was going on here?
After reviewing reference books and Internet sources, a new word popped into the vocabulary – phoresy. The Greek word “phorein” means “to keep carrying.” This phenomenon describes animal transport on another animal. It is considered a method of dispersal used by some animals to reach new suitable habitats or feeding grounds. The slang “hitching a ride” now has a broader meaning.
Apparently, one-third of southern pine bark beetles carry mites phoretically under natural conditions, with 20 percent of their total body weight consisting of these tiny hitch hikers. Researchers found that southern pine bark beetles heavily ladened with mites opt for the lower portion of a host tree, compared to those with fewer or no phoretic mites. A 2005 study in Austria reported 3,922 phoretic mites on 322 adult bark beetles (roughly 12 mites per beetle). The authors suggested that these mites may eat fungi, nematodes, or even various life stages of the beetles.
Bark beetles are better known for carrying fungi from one host tree to another. These microorganisms ride in a special structure on the beetle’s thorax called the mycangia. After arriving at a new host tree, the bark beetles chew tunnels (called “galleries”) underneath the bark of these trees and the fungi are shed and develop in this new moist environment.
Over the years, researchers have discovered that when bark beetles carry certain fungi the new beetle colonies are more successful compared to non-transporting bark beetles. These fungi appear to alter host tissues to make nutrients more readily available to the developing insect life stages.
In the quest to learn more about insect phoresy, other examples surfaced:
• Scarab beetles leaving dung carry mites, nematodes and fungal spores.
• Burying beetles and carrion beetles, which help recycle animal carcasses, transport a number of mite species. These mites feed on fly eggs and maggots, thus decreasing the competition for the beetle offspring.
• Harvestmen (“Daddy-Long-Legs”) carry mites attached to their bodies.
• Bird lice hitch a ride on louse flies when their avian host dies.
• Certain aquatic species of midges undergo a life cycle involving phoresy of the larval stage on dragonflies, caddisflies, stoneflies, true bugs, mayflies and snails.
• Bat bugs cling to the fur of their host animal when it moves to a new roosting site.
• A unique example of familial phoresy occurs in a gall-forming scale insect. Newly hatched females are carried from the maternal gall on the abdomens of their adult brothers, which are in search of receptive adult female scales.
• A type of itch mite lives on pollen in sugarbush, Protea spp. The mites move from plant to plant on sugarbirds, sunbirds and green Protea beetles, which aid in plant pollination.
• A lady beetle feeding on scale insects often carries along a mite competitor.
• Scelionid female wasps cling to the leg of an adult host (insects or spiders) female, and then drop off once the host lays eggs. After the wasp lays her eggs in the host eggs, she hops back on the adult host as it goes to new egg-laying sites.
• Subterranean termites seem to have an unusual relationship with phoretic mites. These innocuous mites ride on their heads until the termite dies, when the mites ingest the carcass.
• Honeybees have phoretic mites that are parasitic on several life stages. Varroa mites and/or tracheal mites can weaken and destroy affected colonies of domestic and wild populations, thus impacting pollination and honey production.
And, finally, to prove that phoresy is not a new phenomenon, mites have been found on several insects preserved in amber, the fossilized remains of tree resin. The transporting insects included a type of fruit fly and a round fungus beetle.
Somehow, the song “Ticket to Ride” by the Beatles now takes on a double meaning.