Luck be a Ladybug
Backyard gardeners know why the ladybird beetle brings good fortune: Its appetite for plant pests is extraordinary
Dave Stanley raises and sells beetles in western Massachusetts. His top product, which sells for as much as 40 cents apiece, is a small, darkish ladybug called Delphastus pusillus. Delphastus is an unrivaled devourer of whiteflies, which in turn are a scourge of cotton, broccoli, melons and assorted indoor plants.
To successfully breed such a pestivore, Stanley must also raise the pest-and at this endeavor he has, oddly enough, had some trouble. "I acquired a colony of whiteflies last year," he says, "but the population crashed because the beetles ate them all." To give his whiteflies some defensive help, Stanley replaced their smooth-leaved host, poinsettias, with eggplants. The latter's fuzzier leaves offer the whiteflies some slight refuge from the ladybugs' onslaughts, but the ravaging continues.
At first glance, ladybugs may not seem at all like deadly hunters, but pound for pound these insects are among the most ferocious predators ever to creep on Earth. More than 4,000 species of ladybugs-or ladybird beetles, as they are officially called-have been identified. The best known of these, the glossy red-backed, black-spotted convergent ladybird (Hippodamia convergens), can eat 100 aphids a day, day after day, month after month.
"They're marvels," says Richard Wetzler, an ecologist at Tufts University. "They're like little lawn mowers. It's a treat to watch them eat an aphid. Plus, they're about as benign a creature as you could ask for." (One exception is the Mexican bean beetle, the black sheep in the ladybug family, which feeds on bean foliage instead of insects.)
Though voracious, ladybugs are discriminating diners. Of the 475 or so species found in this country, the reddish ones prefer aphids, sometimes only certain types of aphids. Their darker cousins tend to stick to mealybugs, whiteflies or scale. In a pinch, a ladybug will change its diet-to pollen, for example-but its fertility suffers. The fast-crawling larvae, resembling miniature alligators, are even more gluttonous than their elders. After eating 40 aphids in a row in an hour or so, an adult will take a break; a larva can eat 40 aphids in an hour and will keep scuttling about searching for more.
Despite their predatory nature, ladybugs arouse affection in even confirmed insect-phobes. The familiar black-on-red pattern of the convergens' back-actually a pair of hinged wing covers - turns up on children's toys, tea sets, even toilet seats. Farmers, meanwhile, have long appreciated the beetle's pest-killing qualities; the name ladybird is a derivative of the reverent "Bird of Our Lady."
Ladybugs helped usher in the use of modern biological pest control in 1888. At the time, California's entire citrus crop was threatened with annihilation by the cushiony-cotton scale, an exotic insect that had probably reached America some years earlier by stowing away in a citrus shipment from Australia. A U.S. Department of Agriculture (USDA) entomologist, Arthur Koebele, was dispatched to Australia to find the natural enemy presumably keeping the scale under control in its native habitat. Among the candidates Koebele shipped home were 28 Vedalia ladybugs (Rhodolia cardinalis). This tiny band and its offspring, augmented by later shipments, went on a cushiony-cotton-scale rampage. Within 18 months, California's citrus plague was over.
Vedalia was especially suitable for pest-control work because it more or less stayed where it was released. Several other ladybugs, notably convergens, refuse to stay put. Come summer or early fall, convergent ladybird beetles fly straight up into the air, as high as a mile above their prime feeding grounds in California's central valleys. Being feeble fliers, they simply drift on the prevailing westerly winds into the foothills of the High Sierra, where, perhaps lured by pheromones, they aggregate in dense beds of forest litter for the winter. Warmed in the spring, the insects head aloft again, this time to drift on the prevailing northeasterlies back to California's crop- and aphid-filled valleys.
The conviviality of wintering convergens has made possible a brisk mail-order business in the creatures. A half pint runs about $8; a gallon $40, or one-twentieth of a cent per beetle, far cheaper than a cultivated bug-like Dave Stan-ley's hot-house Delphastus. The industry's backbone is made up of free-lance bug pickers who head into the High Sierra packing scoops, screens and 30-gallon garbage cans. A rich bed can be a solid mass of sluggish H. convergens several inches deep, enough to raise the ground temperature dramatically-and enough to reward the picker with several hundred dollars for a day's work.
As lucrative as ladybug gathering may be, biologists question the basic effectiveness of mail-order beetles. Ken Hagen, an entomologist at the University of California-Berkeley, probably knows more about these insects than anyone. He's even followed their migrations in a small plane equipped with sticky retractable pads that can catch samples on the wing. Most ladybird beetles are sold in the springtime, and once they warm up, notes Hagen, "they don't know they've been taken away from their aggregation site in the mountains. They just take offinstead of looking for food locally." In other words, the insects you let loose may not keep your garden free of pests, but they'll probably help someone else's.
The peregrinations of ladybugs can be unpredictable indeed. Between 1978 and 1981, USDA researchers in the Southeast released several batches of a ladybug imported from East Asia named Harmonia axyridis. The insect's mission was to combat the tree aphids plaguing the region's pecan orchards. "For 10 years, there was no sign of them," says Walker Louis Tedders of the USDA's research lab in Byron, Georgia. Then a few turned up in Louisiana in 1989. "Last year, they were showing up in astronomical numbers." Tedders surmises that only the passage of 10 years let natural selection produce a strain of Harmonia able to thrive in this country's unfamiliar habitat. And thrive they do, eating aphids from Texas to Virginia.
Especially Virginia. "I don't know if it was the weather or what, but suddenly on October 15 last year we were inundated with calls about ladybugs in people's houses," says Eric Day, manager of the Insect Identification Lab at Virginia Polytechnic Institute. "One lady claimed she had 50,000 to 100,000 ladybugs in her house. She said, 'I can't sleep at night because if I open my mouth, they crawl in.' "
Harmonia, like convergens, is an aggregating beetle. It apparently mistakes houses for its preferred winter gathering sites-granite outcroppings in East Asia. "The beetles were probably up in the trees feeding," says Day. When the trees suddenly shed their autumn foliage in mid-October, Harmonia may have figured it was time to bed down for the winter.
As home invasions go, Harmonia's have been largely benign. Some homeowners don't seem to mind at all. "Some people just said, 'I must be having good luck,' " recalls Day. "One person told me: Now if I could just get these ladybugs to stay in my garden this spring.' "
Massachusetts writer Doug Stewart answered 10 commonly asked questions about wildlife in the April-May issue.