San Jose scale sucks!
No, really! That’s how they feed. San Jose scales (Quadraspidiotus perniciosus) inject toxins into many of our favorite tree fruits in order to liquify our luscious fruits and suck them up. Twigs, leaves, and branches are not exempt.
While a single scale insect cannot cause much harm, a single female can produce thousands of offspring in a single season. Left to its own devices, a severe San Jose scale infestation can kill a mature tree in as little as two years.
San Jose scale symptoms
Scale insects often hide in the cracks and crevices of bark, making them difficult to see. Unfortunately, they are particularly fond of apples and pears, with plums, sweet cherries, apricots, peaches, nectarines, and almonds a close second. San Jose scale also attacks nearby ornamental trees and shrubs, where pests can overwinter in relative safety. Often, the first visible symptom of a San Jose scale infestation is purplish-red halos on fruit, leaves, and young bark. A single apple may have over a thousand scale insects on it. When cherry leaves are infested with San Jose scale, they tend to stay on the tree well into winter. Those leaves should be removed and destroyed.
San Jose scale lifecycle
Female San Jose scales cannot fly. In spring, winged males mate with the females. One month later, tiny yellow crawlers are born. Each crawler has six legs and a bristly beak that is three times the length of its oval body. Crawlers will move around for a few hours before selecting a permanent location. Sometimes they are moved to new plants by wind, birds, people, and garden tools. Once they move in, they build a white waxy house over themselves. This protective coating is about the size of a pinhead, with a nipple-like bulge in the middle. The shell hardens to black and then gray, keeping away predators and pesticides, as they go through several molts. Often, the scale color is hard to see because it is often covered with sooty fungus. Underneath this private bubble, immature males have eyes, but no legs or antenna, and immature females have no eyes, legs, or antenna. The females stay in this house, while the winged males, once they reach maturity, will take wing in search of females.
San Jose Scale control
So, how do we get rid of San Jose scale? First, you have to make sure you have a problem. Pheromone traps can be used February 25 - March 1 to monitor for the flying males. If they are present, spraying dormant oil can suffocate the crawlers and unprotected adults, as long as leaves are not present. If leaves and buds have already emerged, horticultural oil should be used. Larger, older trees are harder to treat because full coverage is more difficult. Sticky barriers can be used to reduce the migration of crawlers to other trees. Sticky barriers will also block the ants that may protect and farm scale insects. San Jose scale have many natural predators, but these beneficials are generally not able to provide enough protection. Compounding the problem, San Jose scale was the first U.S. insect to show resistance to a pesticide. According to UC Davis, San Jose scale rarely occurs in organic orchards. Trees should be checked twice a year, in spring and fall, for signs of infestation. One method for determining the best time to treat for San Jose scale is called the Degree Day model. You can look up your local degree days using the UC Davis CA weather data (assuming you live in California). We will learn more about Degree Days in a later post. Generally speaking, in the Bay Area, crawlers begin to emerge between mid-May and mid-June, depending on weather.
Contrary to its name, San Jose scale originated in China. It travelled to San Jose, CA on a flowering peach tree in the 1870s. Within 20 years, this pest was found nationwide. Ah, yet another example of why buying local is better...
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