In last week’s episode, we met contented flocks of aphids (LINK TO JUNE 1, 2009) sipping sap while their bodyguards, ferocious ants, protected them from would-be predators. Did you ever wonder what might happen to aphids when their bodyguards were not around? On a lush crabapple tree, I discovered thousands of aphids sucking away on buds and leaves. As ladies imbibed, they excreted copious amounts of sweet sticky liquid called honeydew and leaves glistened with the shiny brew. The odors of honeydew and smells emanating from the aphid-ridden plant provide airborne signals to many kinds of aphid hunters including flower flies – a.k.a. hover flies or syrphid flies. These curious flies are well known to most gardeners. Often brightly colored and sometimes hairy, many hover flies resemble bees or hornets. This mimicry affords protection from birds and nosey humans.
Adult flower flies are important native pollinators of many kinds of flowering trees, shrubs, and herbaceous plants. Flower flies have the unique ability to hover like a helicopter and fly forward and back as they search plants for aphids. Honeydew creates an aroma that acts like a dinner bell ringing “come and get it” to flower flies. The more aphids and honeydew on a plant, the more likely it will be discovered by flower flies. Once the infestation is detected, the female fly lays a small white egg near the colony of aphids.
The egg hatches into a gelatinous, wriggling, maggot whose sole purpose is to hunt and eat soft-bodied prey. With no true eyes, this predator discovers victims by searching to and fro with sensory structures on the front end of its fleshy head. When it bumps into an aphid, it quickly snares its victim and sucks the fluids from its body. Flower fly maggots have prodigious appetites. In the laboratory I have watched these predators consume more than 25 aphids each day. Reports of aphid carnage in the literature put the casualty figures in excess of 200 aphids during the course of development for each maggot.
Aphid peril does not end with flower flies. Several species of lady beetles were attracted to the aphid explosion on the crab apple. Alligator-like larvae roamed quickly over the leaves and stems searching for tasty aphids. Without much stealth or finesse, larvae captured aphids in their jaws and proceeded to munch their hapless prey. Small aphids disappeared in just a minute or two, but large, plump aphids required several minutes to eat. A period of rest often followed before the hunt for aphids resumed. A single larva of the multicolored Asian lady beetle may devour 1,200 aphids during the course of development. Adult beetles are also aphid-eating machines and may consume more than 250 aphids daily. Each female beetle may live more than one year and produce more than 700 eggs in a season. This ability to produce so many young with the potential for eating so many aphids makes the multicolored Asian lady beetle one of the most effective biological control agents in our gardens. Reports abound of people being “bitten” by these lady beetles and I confess that I have gotten a small nip every now and then. The “bite” was something less than that of a Doberman and more than a tickle. No break in the skin is “no foul” in my book. Handling lady beetles can result in the release of a smelly, bitter, secretion that may leave a faint yellow stain on your skin, wall, or curtain. This neat trick is reflex bleeding and it is their way of delivering a nasty surprise to a would-be predator. All is not sweetness and light with these exotic aphid eaters. Many folks are concerned that this very successful exotic lady beetle may be displacing some species of our native lady beetles. In addition, there is a somewhat disconcerting behavior of entering homes with the approach of winter (see Ladybug, ladybug, fly away home, LINK TO November 5, 2006). Overall, these ladies provide great service in my garden and I welcome them each spring. So, when aphids appear on trees, shrubs, or herbaceous plants, before you reach for the aphid spray, carefully look to see if the maggot brigade or lady beetle battalion is at work. It is wickedly entertaining to watch grotesque larvae hunt and capture aphids and to observe aerial acrobatics of adult hover flies and curious ambles of lady beetles as they search for food and mates.
The interesting articles, “Oviposition site selection of Episyrphus balteatus” by D. Scholz and H.-M. Poehling, “Managing the dispersal of ladybird beetles (Col.: Coccinellidae ): Use of artificial honeydew to manipulate spatial distributions” by E. W. Evans and D. R. Richards, and “Overwintering, phenology and fecundity of Harmonia axyridis in comparison with native Coccinellidae species in Italy” by Bazzocchi, Alberto Lanzoni, Gianumberto Accinelli and Giovanni Burgio served as a references for this Bug of the Week. For more information on hover flies and multicolored Asian lady beetles, please visit the following web sites.