Gill's Mealybug

Adult female Gill's mealybugs are flat and oval, approximately 0.08 to 0.2 inch (2–5 mm) in length with pinkish grey bodies covered by long white waxy filaments, up to twice the body in length. The waxy filaments are interrupted by two rows of sparse wax on the dorsal side, giving the appearance of stripes.

Nymphs and the adult female stage can be distinguished from other grape-infesting mealybugs by their abundance of the glasslike waxy filaments, lack of lateral filaments around the body, and two broad white “tails” (caudal filaments). Adult males are rarely seen; they are very small, less than 0.08 in (2 mm), are wax-less and have wings.

Gill’s mealybug’s biology and life cycle can also provide clues to distinguish it from other mealybug species. Female Gill’s mealybugs give birth to live crawlers (first instar nymphs) that remain under the female’s body for a short time before dispersing to leaves to feed. The tiny orange crawlers are indistinguishable from those of other mealybug species. Mealybug nymphs molt through three instar stages, with each successive instar resembling the previous, but slightly larger and with increasing amounts of the waxy filaments covering the body.

In the Sierra foothills, Gill’s mealybug completes two discrete generations and may begin a third generation in grape cultivars with later harvest dates. Gill’s mealybug overwinters as nymphs primarily under bark of the trunk including the trunk base and, to a lesser extent, under the cordon armpit. By May, late-instar nymphs and adult females are found under the bark of arms and spurs and, most conspicuously, at the base of new shoots. Mating occurs and adult females move back down the grapevine to shelter under the bark of spurs, cordons and upper trunks and in developing clusters. They spin a filamentous waxy sac and begin to produce crawlers in mid-June to early July (first generation) and from late August to September (second generation). Crawlers pass through their nymphal development, moving out to leaves to feed and then back down the grapevine, developing into adults during late July to early August (first generation) and during late September to early November  (second generation). Honeydew, produced by feeding, is most evident with increasing populations in the fall. Ants foraging the honeydew may be associated with infestations.

Gill’s mealybug, Ferrisia gilli, is a pest in wine grape vineyards in the Sierra foothills (El Dorado County) and the Northern Coast (Lake County). First reported on grape in 2003, it has also become a pest of pistachio in the San Joaquin Valley but has rarely been found infesting vineyards there or in other California wine grape regions.  Though uncommon, it has been recorded as a pest of almond and of ornamentals.

Gill’s mealybug, like other mealybug species affecting grape, causes damage in two ways: by contaminating the crop with its bodies and honeydew production, making the crop unmarketable, and by transmitting grapevine leafroll-associated virus (GLRaV). While Gill’s mealybug numbers may take time to rise to a noticeable level, if left untreated its presence can reduce marketable yield. Studies conducted in untreated plots within commercial vineyards where Gill’s mealybug damage was monitored over two harvests showed an increase in the percent of unmarketable fruit, ranging between 7% to 29% of sampled clusters rated “unmarketable” at harvest in the second year without treatment. Greenhouse studies have confirmed that Gill’s mealybug is a vector of GLRaV, which can delay fruit ripening, and reduce quality and yield.

Generalist predators including lady beetles (Coccinellidae), green lacewings (Chrysopidae), praying mantises (Mantidae), damsel bugs (Nabidae), snakeflies (Raphidiidae) and spiders will attack and feed on Gill’s mealybug. The parasitoid Acerophagus sp. has been identified from Gill’s mealybug mummies where it oviposits (lays its egg) inside the mealybug, rendering the mealybug immobile until eventually killed by the feeding Acerophagus larva. Mummies are typically found under the trunk or cordon bark or within clusters and can be examined for exit holes where fully mature parasitoids have emerged leaving evidence of active mealybug parasitism. It is likely that other parasitoids also attack Gill’s mealybug, but none are well studied. Generally, natural enemy populations are too low to provide economic control alone, but they can help reduce Gill’s mealybug populations over time and in combination with selective insecticides. When spraying to control Gill’s mealybug, choose the most effective insecticide with the least impact on biological control, and consider leaving a few rows untreated to serve as a refuge for natural enemies. As with other mealybug species, controlling ants, will help prevent the disruption of biological control.

Lightweight horticultural oils can be applied to smother Gill’s mealybug crawlers and young instars when applications are timed correctly, and mealybugs are exposed to direct contact by the spray. Oils are not effective against larger mealybug stages that are protected by a layer of wax. Good spray coverage is essential and multiple applications may be necessary. Some oil products include botanical oils (peppermint oil, rosemary oil and others) that may irritate the mealybugs and cause them to move, possibly increasing their contact with the oil.

Gill’s mealybugs are often limited in number, cryptic, and difficult to find. Traps do not exist and a sex pheromone has not been identified. Management is based on monitoring for nymphs and adults and making treatments, when needed, at timings that target one of the two seasonal discrete peaks of crawlers when they are exposed and feeding on leaves. There are no set treatment thresholds. Treatment decisions should consider the extent of infestation and level of threat from grapevine leafroll-associated virus.

Begin monitoring for Gill’s mealybug during canopy shoot thinning, which typically occurs in May to early June in Sierra foothill vineyards. At this time mealybugs are fairly large (3–4 mm as an adult) and visible at the base of growing shoots. If adults are found, flag the vines for subsequent monitoring to determine the emergence timing of crawlers on leaves in mid to late June. Apply pesticide at the initiation of crawler emergence. If the crawler emergence is drawn out over time, a second application may be needed two weeks after the first. If control is warranted and the first generation peak of crawlers was missed, monitor for the emergence of second generation crawlers on leaves in late August to early September. Pay attention to applicable pre-harvest intervals of insecticides.

Break open grape clusters and inspect for Gill’s mealybug during harvest when populations and honeydew production are most noticeable. Focus inspections on vines with the presence of ants as an indicator of mealybug infestations. Vines with Gill’s mealybug infestations found late in the season should be flagged for monitoring during shoot thinning the following season. Rarely are infestations so severe that they require a post-harvest treatment.