Description of the Pest
Adult female Gill's mealybugs are roughly 1/12 to 1/5 inch (2–5 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 about 1/5 to 2/5 inch (5–10 mm) that protrude from the back of the insect.
Gill's mealybug has three generations per year. Third generation crawlers are born live in mid-September to November and then overwinter in small cracks and crevices as small nymphs. During budbreak, overwintering nymphs migrate to the newly forming buds and begin to feed. They mature in May and produce crawlers in early June. The crawlers reach maturity in about 6 weeks and produce the second in-season generation, occurring from mid-July through harvest (mid-September). From May through September mealybugs are found almost exclusively in the clusters. After harvest adult females migrate out of the clusters to the trunk and main scaffolds where they form aggregations that give the bark a white fuzzy appearance.
Pistachio growers should be cautious not to confuse Gill's mealybug with grape mealybug. Grape mealybug is sometimes found on pistachios, but does not cause economic damage and does not need to be treated. Grape mealybug has four slender white tails (two long ones and two short ones), whereas the female of F. gilli has two broad white tails. Grape mealybug also has short white lateral projections that extend from the sides of the body whereas F. gilli has none. Immature grape mealybugs hatch from egg sacs and usually crawl away from their mother after egg hatch whereas F. gilli have live birth with immature stages remaining aggregated around their mother. Lastly, when poked, adult females of grape mealybug extrude a bright red liquid through structures called ostioles towards both the rear and front of the top of the body. F. gilli does not extrude such a liquid.
Damage
Mealybug feeding results in the production of large amounts of honeydew that acts as a substrate for black sooty molds. Thick layers of sooty molds on leaf surfaces can reduce photosynthesis.
Gill's mealybug has 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. Marketability 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. Monitor for mealybugs the following spring. If adult females are found in clusters, 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 Acerophagus sp., Chrysoplatycerus 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 numbers 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, note the locations for further evaluation the following spring.
At budbreak, determine if the overwintering populations survived by searching for mealybugs at the bases of new buds on trees and mark infestations for further sampling in May.
In mid-May start monitoring weekly, checking for adult females on the rachis and for the presence of crawlers. Make treatment decisions by determining the number of adult female mealybugs per cluster. One 3-year study showed that a treatment in early June was economically justified if one mealybug was found per every 10 clusters in May.
The most effective timing for insecticides is when most mealybugs are in the crawler stage of the first generation. For the lower San Joaquin Valley, this typically occurs in late May to early June. Applications later in the season are more variable in effectiveness.
Do not apply postharvest treatments in the orchard because this is when predators are most active, no damage occurs to the crop in winter, and there is already very high winter mealybug mortality.
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(human) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
Assail 70WP | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4A |
2.3–4.1 oz | 12 | 14 | If mealybugs are not treated in early Ju... | moderate | — 13 | — 14 | moderate/high | moderate | very low | very low | low | very low | low | 10/2014 | ||
|
|
Centaur WDG | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
16 |
34.5 oz | 12 | 60 | Very effective when used while mealybugs... | narrow | low | high 15 | low | long | intermediate | low | high | low | intermediate | 10/2014 | ||
|
|
Admire Pro | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
4A |
2.8 fl oz | 12 | 7 | Less effective than other treatment opti... | narrow | moderate | moderate | high | short to moderate | — | — | — | — | — | 10/2014 | ||
|
|
Movento | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
23 |
6–9 fl oz | 24 | 7 | Apply at crawler emergence in early June... | narrow | low | low | low | short | low | very low | low | very low | low | 10/2014 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
- 13 ↩ May cause an increase in spider mite numbers.
- 14 ↩ Does not kill adults but sterilizes females.
- 15 ↩ Kills lady beetles.
UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
R.H. Beede (emeritus), UC Cooperative Extension Kings County
W.J. Bentley (emeritus), UC IPM and Kearney Agricultural Research and Extension Center, Parlier
K.M. Daane, Kearney Agricultural Research and Extension Center, Parlier
D.R. Haviland, UC IPM and UC Cooperative Extension Kern County
Acknowledgement for Contributions to Invertebrates
R.E. Rice, Kearney Agricultural Research and Extension Center, Parlier
