Ferrisia gilli : A New Mealybug Pest of Pistachios and Other Deciduous Crops

Author(s): Haviland, David; Beede, Robert; Godfrey, Kris; Daane, Kent | Abstract: Ferrisia gill is a mealybug species in California that affects pistachio plantings in 11 counties. Find out how to monitor and control the pest's spread using biological and chemical means.


INTRODUCTION
Ferrisia gilli, Gill' s mealybug, is a newly described species of mealybug that is spreading throughout pistachio production regions in California (Gullan et al. 2003). In the late 1990s, Gill' s mealybug was first found infesting pistachios near the town of Tulare in the southern San Joaquin Valley of California. By 2006 it had spread to at least 3,000 acres (1,214 ha) of pistachios in 11 counties (Butte, Colusa, El Dorado, Fresno, Glenn, Madera, Tulare, Sacramento, Shasta, Stanislaus, and Tehama). Gill' s mealybug has also been found, and occasionally caused problems, on almonds, grapes, persimmons, and stone fruits as well as several deciduous ornamentals such as fruitless mulberry.

IDENTIFICATION
Adult female mealybugs are flat and oval (2 to 5 mm, or 0.08 to 0.20 inch, in length), and have a pink body covered with white wax. Wax patterns on the body surface give the mealybug a striped appearance. They may also be covered in crystalline filaments (5 to 10 mm, or 0.2 to 0.4 inch) that have the appearance of long, glassy rods ( fig. 1). Adult males, rarely noticed in the field, are smaller than females and have wings ( fig. 2). Immature mealybugs, called nymphs, pass through three developmental stages (instars) and look like smaller versions of the adult females.
Gill' s mealybug may be confused with the grape mealybug (Pseudococcus maritimus), a nondamaging mealybug that is sometimes found on pistachios. The grape mealybug can be distinguished from Gill' s mealybug by its longer tails, filaments around the edge of its body, and lack of long, glassy rods ( fig. 3).

DAMAGE
Mealybugs feed on carbohydrates by sucking plant juices through their strawlike mouthparts. During the late spring and summer they prefer to feed on the rachis and cluster, respectively, where they rob the tree of carbohydrates intended for fruit development ( fig. 4). Feeding damage to the pistachio hull also causes shell staining that can demote high-quality split-inshell nuts to less valuable shelling stock ( fig. 5). Severe feeding on the rachis or hull can also cause nuts to dry up and shrivel prior to harvest. Many of these nuts do not split and remain in the tree during shaking.

SEASONAL BIOLOGY
Gill' s mealybug has three generations per year in California. After harvest, adult female mealybugs migrate to the main tree scaffolds and trunk where they aggregate and give the wood a white, bearded appearance as if draped in cotton candy ( fig. 6). Then they produce crawlers that seek out protected places in cracks and crevices to overwinter. During budbreak, the overwintering nymphs migrate to the swelling buds and begin to feed (fig. 7). They continue feeding at the interface between the previous year' s wood and the current year' s growth until May, when the overwintering mealybugs reach maturity and move to the rachis ( fig. 8). Between late May and mid-June the adult females give live birth to crawlers of the first of two in-season generations that feed on the pistachio hull ( fig. 9). The first generation is present from early June through mid-July and the second from mid-July through Figure 3. The grape mealybug is sometimes found on pistachios but is not of economic importance. It can be distinguished from Gill's mealybug by its longer tails, lateral filaments, and lack of long, glassy rods.      harvest. Whereas the overwintering generation has low survival rates throughout the winter, the two in-season generations are noted for their exponential growth rates such that one mealybug per cluster in May can result in hundreds of mealybugs per cluster at harvest.

FINDING GILL'S MEALYBUG Late Summer through Harvest
Look for wet, shiny, or black areas in the tree canopy ( fig. 10). Wet, shiny spots are the result of large amounts of honeydew excreted by the mealybugs, and the black areas are where honeydew has acted as a substrate for the growth of sooty mold. When honeydew or sooty mold is anywhere in the canopy, look for mealybugs within the clusters.

Fall and Early Winter
Look for white, fuzzy aggregations of mealybugs on the undersides of the main scaffolds, in knotholes, or on the trunk ( fig. 11). Even after these adult females have produced offspring and died, their carcasses will remain on the trunk as evidence of an infestation for up to a year. If white aggregations of mealybugs are found, mark the location and return the following spring to determine the need for an in-season insecticide application.

BIOLOGICAL CONTROL
Several species of predators and parasitoids can suppress Gill' s mealybug densities. Predators include green lacewings ( fig. 12) and a small brown coccinellid (ladybird) beetle whose larva mimics the appearance of a mealybug (figs. 13 and 14). Parasitoids include wasps in the genera Pseudaphycus, Chrysoplatycerus, and Anagyrus ( fig. 15). These parasitoid species have been shown to effectively reduce Gill' s mealybug populations on almond, persimmon, and grape crop systems where pyrethrin-based insecticide use is very limited. However, they are rarely found in pistachios where these insecticides are often used for control of true (hemipteran) bugs.     Mealybugs also have very close associations with ants, which "tend" the mealybugs for their honeydew. One example is the native gray ant (Formica aerata), found in the San Joaquin Valley (fig. 16). It is not certain, however, whether this association with ants is beneficial or harmful to the mealybugs. Certain observations suggest that ants feed on mealybug honeydew and may protect the mealybugs from becoming parasitized; other observations suggest that ants may also feed directly on the mealybugs during certain times of the year.

CHEMICAL CONTROL
In cases where biological control is insufficient, foliar insecticides can be used to control Gill' s mealybug. Optimal application timing is when the majority of the mealybugs are in the crawler stage (first-instar nymph), which occurs from early through mid-June in the San Joaquin Valley. Treatments earlier or later in the season are not as effective, and postharvest treatments have not been evaluated. Ongoing research has documented that insecticides containing buprofezin or acetamiprid are highly effective against the mealybug, and that insecticides containing phosmet or carbaryl can provide partial suppression. A listing of current control options can be found in the UC IPM Pest Management Guidelines for Gill' s Mealybug in Pistachios (http://www.ipm.ucdavis.edu).

PREVENTING MEALYBUG SPREAD
You can help limit the spread of Gill' s mealybug by taking the following simple steps: • Become educated on the mealybug by attending meetings to learn more about its biology and how to control it. • For orchards not infested, initiate monitoring programs to ensure that pest control advisors, irrigators, pruning crews, and others working in the orchard are trained to recognize and report mealybug infestations. Figure 15. Mealybugs parasitized by one or more wasps will stop feeding, begin to bloat and turn yellow (arrows), and then turn an amber brown color. When parasitoids are fully developed, they chew a round exit hole in the side of the mealybug mummy. Figure 16. Ants, such as this native gray ant on an almond hull, collect honeydew from mealybugs and protect them from being parasitized. At certain times of the year they may also feed directly on the mealybugs.
• For infested orchards, ensure that equipment is washed down prior to leaving the site, especially during harvest. Hitchhiking on equipment is the primary means of mealybug spread. • Ensure that pruning and other field crews do not enter uninfested orchards after working in an area infested with mealybugs. • When grafting new pistachio orchards, use only budwood collected from locations that do not have mealybug infestations.

SUMMARY
• Gill' s mealybug, Ferrisia gilli, is a newly described species of mealybug that infests pistachios and other deciduous crops and woody plants. • It feeds on plant juices and can cause significant economic losses by lowering the quality of pistachio nuts. • Monitor for the mealybug by looking for honeydew and sooty mold during the summer and for white aggregations of mealybugs on the trunk and scaffolds in the fall and winter. To simplify information, trade names of products have been used. No endorsement of named or illustrated products is intended, nor is criticism implied of similar products that are not mentioned or illustrated.
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