Control of two important fruit tree pests, European red mite and San Jose scale, begins early in the growing season at bud swell with an application of superior oil. In very early springs, such as that of 2012, this spray was easily missed. As in most Missouri springs, fluctuating late winter temperatures increase the need for daily monitoring fruit tree bud stages. Apricot, cherry, peach, and plum are generally the first fruit tree buds to begin growth, followed by apple and pear. Dormant oil is applied when the temperature is above 40°F. Its primary purpose is to smother overwintering mite eggs and young scale crawlers.
For red mites, the number of overwintering eggs is important as there are multiple generations during the growing season. In favorable climates where mites have about eight generations per year, a single pair of mites could potentially produce more than 227 million offspring by the end of the season. Fortunately, low humidity, low winter temperatures, natural enemies, and miticides prevent mites from attaining their reproductive potential.
Because of their fecundity, control of the initial population of mites in overwintering eggs is important. Thorough coverage of the tree with oil is important as mite eggs overwinter on the underside of small branches, twigs, and fruiting spurs. Although eggs are red, they are hard to find with the naked eye as they are only about 0.15 mm in diameter. Usually, winter eggs hatch very quickly, within a period of 7 to 10 days. They begin to hatch at the tight cluster stage of floral development and about half of them hatch by the pink stage. By late petal fall, females have oviposited their summer eggs.
Mite feeding damage is generally the most severe on apple, pear, and plum trees. As mites feed, they withdraw cellular contents from leaves, including chlorophyll. Damaged leaves appear bronze in color and generally persist on the tree. However, partial leaf drop of injured leaves can occur during periods of water stress. Heavy infestations (more than 30 mites per leaf) in early summer adversely affect flower bud initiation and subsequent fruiting in the next growing season.
In contrast to mites, scales do not overwinter as eggs as females produce six-legged offspring called crawlers. Most overwintering scales are first nymphal instars that cling to tree bark. In early spring, young crawlers travel about the tree for a few hours and then begin sucking on plant tissue. After about three weeks, they molt and begin to secrete a waxy covering. Females never leave their scale, but males emerge from their covering as winged adults by May and subsequently mate. Females generally produce about nine to ten living crawlers per day for about six weeks. Females may have up to generations per year and a single pair of scale could hypothetically produce 300 million crawlers per year.
San Jose scale attack bark, leaves, and fruit. Scale control is essential as it can kill young trees in two to three years. Older trees may take longer to die but fruit is severely blemished. Scale damage on fruit is easily detected by the red spots encircling a white center. It also attacks bark and leaves on trees. Crawlers are spread by wind and on orchard equipment, workers’ clothing, and birds’ feet. Although parasitic wasps and lady-bird beetles are natural enemies of scale, additional control is needed to prevent severe infestations. While additional pesticides can be used later in the growing season to limit San Jose scale and European red mite eggs, an application of dormant oil at bud swell is an effective method for controlling the initial population of these pests.
REVISED: January 3, 2013