Are ladybugs worth buying?

If you've been an organic gardener for very long, you have probably heard that, (1), lady beetles are good to have in your garden because they eat aphids; and (2), it does little good to buy them because when released in your garden they'll just fly away.

There is little argument with the first statement, and not much agreement on the second. But according to a University of California study, maybe it doesn't matter. Although most beetles left the plants they were released into within one or two days, they "significantly reduced populations of melon aphids on potted chrysanthemums and of rose aphids on potted roses."

However, the background of the controversy (and the lady beetles' life cycle) is perhaps more interesting than their effect on aphids.

According to an article in California Agriculture, March-April 1995, the convergent lady beetle, Hippodamia convergens (Coleoptera: Coccinellidae) is the least expensive and most widely released aphid predator in the United States. This lady beetle is unusual because a large portion of its California population migrates to the mountain ranges when aphid populations in the Central Valley drop in summer. They spend the winter in massive mountain "aggregations," taking in water but consuming no aphids. They fly back down to the valley about March, when temperatures reach 65[degrees].

The habit of aggregating, or gathering together in huge swarms, allows collectors to harvest and package large numbers of the beetles for sale throughout the country.

"Despite widespread use, the University of California has historically recommended against releasing aggregation-collected H. convergens because they were believed to require a substantial flight (normally acquired when they return to the valley in the spring) before they will consume significant numbers of aphids." In other words, it was thought that if you released these collected beetles into your garden, they would just fly away.

(Although this information is still being handed out by some extension agents, it was "based mostly on field observations made in the 1910s and 1920s rather than on replicated field trials," according to the authors.)

Studies involved lady beetles removed from cold storage (40[degrees]-50[degrees]) such as you might purchase by mail; beetles that had been "pre-flown" in 10 x 10 x 8-ft. screened cages; and beetles that had been reared in laboratories.

There was no significant difference.

There's a big difference in cost, however. A gallon of lady beetles, or approximately 80,000 beetles, costs about $40, according to the researchers. Laboratory-reared mealy-bug destroyer lady beetles, Cryptolaemus montrouzieri, cost approximately 10[cts] each, or 200 times as much.

Do ladybugs, regardless of their cost or how long they hang around, reduce aphid populations? The researchers said, "Three days after releasing lady beetles, aphids were significantly less abundant (P< 0.001, t-test) on release plants in comparison with controls." Which to us means, heavens,yes.

In other research, effects of insecticides on lady beetles were evaluated. It was found that residues of malathion and carbaryl can kill newly exposed lady beetles for up to two weeks after application. Pyrenone residues had little or no effect after one day. "In contrast, dried residues of insecticidal soap or oil had no effect one hour after treatment."