Clusters of grayish-white, oval, soft-bodied insects on the underside of stems and leaves can mean a mealybug infestation.
You will probably never see a male mealybug. They are simply too small to notice. Orange or yellow egg masses are laid in cracks and crevices of bark and in the calyx (flower end) of fruit, and hatch in June in the Bay Area. Oblong crawlers, or nymphs, which emerge in spring, can be red, yellow or white, and they may or may not have the hairlike filaments seen on adults. Nymphs move from the bark to the base of new shoots or fruit clusters. As they mature, they turn purple and then gain their characteristic white powdery wax covering. They look like scale insects but without the armored covering. There are different mealybug species, including citrus, longtailed, obscure, golden, and pink, among others, along with the dreaded vine mealybug. Adult females are only 3/16 of an inch long, but they tend to congregate in large groups which makes them easier to spot (and more devastating to their host plants). Mealybugs are often mistaken for wooly aphids, and with good reason. Take a look at the images below to see why.
Cousin to aphids and whiteflies, mealybugs are sap eaters. They feed on new buds, shoots, and leaves, causing erratic or reduced budbreak, slowed growth, and twig dieback. Mealybugs are frequent pests to basil, grapes, stone pine, pomegranate, chamomile, apple, plum, pear, peach, ferns, orchids, and, well, quite honestly, pretty much everything growing inside or outside of your home. Mealybugs produce honeydew, which provides the perfect growth medium for sooty mold. They can also carry bean mosaic.
Prevention is, as usual, much easier than eradication. Placing new plants in quarantine areas can prevent the spread of mealybug infestations to the rest of your garden or landscape. Mild infestations can be tolerated by healthy plants, but water-stressed or otherwise unhealthy plants cannot. Mealybugs have many natural enemies, such as hoverflies, mealybug destroyers, pirate bugs, ladybugs, and several parasitic wasp species, so encouraging these beneficials can make your job easier. Globe allium is a good choice for attracting these natural helpers.
Heavy infestations can be treated with neem oil or insecticidal soaps, as they provide effective organic treatments against mealybugs. The trick is spotting these pests before they drain your plants of too many nutrients. Chlorosis and early leaf drop are common signs of mealybug infestation, but you will want to check underneath leaves regularly as a normal preventive maintenance.
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