Issue 8, June 11, 2010

Twicestabbed Lady Beetle

Cottony maple scale is a native insect, and it has a native predator. The adult twicestabbed lady beetle is one-eighth inch in diameter and black with two red spots on the back. It overwinters under loose bark as an adult beetle, emerging in the spring to feed on the overwintered cottony maple scales through the spring. Eggs are laid and larvae appear in late spring at about the same time as the cottony maple scale egg masses are being produced, at about this time of year. The larvae are covered with whitish, waxy filaments, appearing very similar to the cottony maple scale egg masses. Fully grown larvae are oval and about one-quarter inch long. They can be distinguished from the cottony maple scale egg masses because they move, albeit very slowly. Cottony maple scale egg masses have the old female scale appearing as a brown cap on one end, which twicestabbed lady beetle larvae do not have. The lady beetle larvae pupate in late summer and emerge as adults in the fall.

Twicestabbed lady beetles have a typical predator-prey relationship with cottony maple scale. That is, predator numbers increase in direct relationship to prey number increases. Twicestabbed lady beetles typically reduce cottony maple scale numbers to very low numbers. They normally take about three years to do so once cottony maple scales become numerous enough to be noticeable. In other words, we are probably in the first year, or perhaps the second, of having high numbers of cottony maple scale with associated silver maple canopy thinning and copious honeydew and sooty mold. If nothing is done, in the spring of 2013 there will likely be no cottony maple scale that can be found, and there will be large numbers of adult twicestabbed lady beetles crawling over trees, shrubs, and houses looking for something to eat before they die of starvation. About eight years later, cottony maple scale will be back as the cycle repeats itself.--Phil Nixon

Phil Nixon

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