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.
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.
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.
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.
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.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Centaur WDG)||34.5 oz||12||60|
|MODE-OF-ACTION GROUP NUMBER1: 16|
|COMMENTS: Very effective when used while mealybugs are in the crawler stage of the first in-season generation of mealybugs (early to mid-June in the San Joaquin Valley). Other timings may be effective but have not been evaluated. Apply by ground only for a maximum of one application per season. Use allowed by FIFRA Section 2(ee) Recommendation, which must be in the possession of the user at the time of application.|
|(Movento)||6–9 fl oz||24||7|
|COMMENT: Apply at crawler emergence in early June.|
|(Assail 70 WP)||2.3–4.1 oz||12||14|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: If mealybugs are not treated in early June, which is optimal time, this material can be effective against second-generation crawlers in mid to late July.|
|(Admire Pro)||2.8 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Less effective than other treatment options. However, ease of application and low product cost make it a viable option when mealybug numbers are low.|
|**||Unless otherwise noted, apply with enough water to ensure adequate coverage.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals 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; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.|