How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Adult female Gill's mealybugs are 2 to 5 mm in length and pinkish grey in color. They are often covered with white wax secreted from a pore, creating the appearance of 2 stripes (darker areas) on their backs. Larger nymphs and mature females produce a network of white filaments (5-10 mm) that protrude from the back of the insect.

After harvest adult female mealybugs migrate to the main scaffolds and trunk where they form large aggregations that give the bark a white fuzzy bearded appearance. These females produce crawlers that seek out and hide in cracks and crevices during winter. At bud break the overwintering nymphs migrate to the newly forming buds and begin to feed. The overwintering generation of mealybugs reaches maturity around the end of May and produces crawlers. This first generation develops in about 6 weeks (early June to mid July) and produces a second generation that develops between late July through harvest in mid-September. It is the females of this second generation that overwinter.

DAMAGE

Mealybug feeding results in the production of large amounts of honeydew that acts as a substrate for black sooty mold. Thick layers of sooty mold on leaf surfaces can reduce photosynthesis.

Mealybugs have a great affinity for feeding within the pistachio cluster. They use piercing-sucking mouthparts to suck out plant juices, extracting carbohydrates and other nutrients intended for nut development. This causes a decrease in nut quality because of increased shell staining and possibly smaller kernel size. Harvestability can also be affected when severe hull damage causes nuts to dry up and shrivel on the tree. These nuts may serve as overwintering sites for navel orangeworm.

Management

Look for mealybug infestations in fall after harvest, and mark areas in the orchard where they occur so their populations can be monitored the following spring. If adult females are found in clusters in May, a treatment aimed at crawler emergence may be warranted.

Biological Control
The most common predators of mealybugs in pistachios are brown lacewings and a lady beetle whose larva resembles a mealybug. There are also several parasites that attack Gill's mealybugs in California, such as Pseudaphycus sp, Chysoplatycerus sp. and Anagyrus pseudococci. However, these parasites have only been found in other crops such as almonds, grapes, and persimmons, and not in pistachios, likely because of broad-spectrum insecticide use for true bugs.

Cultural Control
There are no cultural controls known to affect the density of Gill's mealybug or the damage it causes to pistachios. However, cultural controls such as washing equipment (especially harvest equipment) when leaving infested orchards is essential for decreasing the rate of orchard-to-orchard spread of this new pest.

Monitoring and Treatment Decisions
The best time to find new mealybug infestations is the period from early fall through mid-winter when populations are at their highest. Before trees become dormant, look for sooty mold on leaves and for mealybugs within the clusters. Once the leaves have fallen, look for white aggregations of mealybugs on the trunks and undersides of main scaffolds. If mealybugs are found, mark and follow up on these locations the following spring.

At budbreak, monitor blocks known to be infested and again in mid-to late May. At bud break, search for mealybugs at the bases of new buds on trees known to be previously infested to determine overwintering survival. In mid-May return to these locations and begin monitoring weekly. Make treatment decisions by determining the number of adult female mealybugs per cluster in late May. Though no concrete treatment thresholds have been established, one research project showed that an average of 3 mealybugs per cluster in May is sufficient to cause a 15% reduction in the value of the crop at harvest.

The most effective timing for insecticides is when most mealybugs are in the crawler stage of the first generation, which for the lower San Joaquin Valley is around early to mid-June. Be sure to monitor clusters to determine crawler emergence. Applications later in the season are more variable in effectiveness. Postharvest treatments are not recommended because this is when biological control is most active, no damage occurs to the crop in winter, and there is already high winter mealybug mortality.

PUBLICATION

UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
Insects and Mites
W. J. Bentley, Kearney Agricultural Center, Parlier
R. H. Beede, UC Cooperative Extension, Kings County
K. M. Daane, Biological Control, UC Berkeley/Kearney Agricultural Center, Parlier
D. R. Haviland, UC Cooperative Extension, Kern County
Acknowledgment for contributions to the insect and mite section:
R. E. Rice, Kearney Agricultural Center, Parlier

Top of page