Gills Mealybug

Phoebe Gordon, UCCE Madera and Merced Counties

Gill’s Mealybug (Ferrisia gilli) ((Figure 1), first identified on pistachios in 1997, has become a major pest of this crop.  It’s most noticeable around harvest, however the effective treatment times are much earlier in the season so close monitoring of populations is essential to ensure its treated at the right time.  While Gills Mealybug has been documented attacking almonds, beneficial insects seem to keep this pest in check, most likely because early season pyrethroid use in almonds is becoming less common, preserving insects that can attack mealybugs.

Gills Mealybug Impacts

Gills mealybugs directly impact the pistachio crop by reducing yields and split percentage and increasing shell staining. They may cause increased sticktights and adhering hulls, though this has not been conclusively shown in research studies.  Mealybugs cause this damage by feeding on the phloem, consuming carbohydrates that the pistachio would otherwise use to fill the nuts or develop new growth.  They also produce honeydew that can harbor growth for sooty mold.  This is not considered detrimental to pistachios, though it does look dramatic at harvest time. 

Figure 1: Gills mealybug adults feeding on a pistachio rachis. Photographer: David Haviland.

Gills Mealybug Lifecycle

Gills mealybug has three generations in a year.  It overwinters as a nymph, and early season populations are low due to extremely high winter mortality.  Close to bud break (mid March to early April), the nymphs leave their hiding places and move to pistachio buds to feed.  It is thought that this may be triggered by the remobilization of carbohydrates from permanent woody structures during dormancy to expanding buds in the spring.  The nymphs complete development by late May/early June and produce the first generation of crawlers (the name given to the first of the three nymph stages of mealybugs).  These crawlers move to the developing nuts and rachis to feed, passing through the other nymph stages, and becoming adults in late July.  These adults produce the second generation of crawlers, which also feeds on the pistachio fruit.  As pistachio nuts finish developing in September, the mealybugs leave the fruit and move to branches, where they subsequently produce the third generation of crawlers.  It is this generation that overwinters in cracks and crevices.  By this time the mealybug population is at its highest, increasing exponentially with each generation.

Gills Mealybug vs. Grape Mealybug

Gill’s mealybug can be confused with another mealybug, Grape Mealybug, that can occasionally be found in pistachio orchards.  It is critical to distinguish between the two, as the grape mealybug does not cause economic damage in pistachios and should not be treated.  Luckily, there are a few key indicators that will help distinguish the two.  The three most obvious traits are seen in the adults:

·       Gills mealybug produces glassy rods (Figure 1, top), whereas grape mealybug does not (Figure 1, bottom). 

·       Gills mealybugs have two tails coming from the end of their bodies, the grape mealybug has four. 

·       When poked, the grape mealybug will produce two dots of red fluid near their tails, the Gills will not.

if you monitor around their reproductive times (late May/early June, late July, October), you will see that Gill’s mealybug produces live young (Figure 3), whereas the grape mealybug produces eggs, from which the crawlers hatch. 

Top: Gill’s mealybug. Bottom, Grape mealybug. Photographer: David Haviland.

Figure 3: Gill’s mealybug adults with first instar nymphs (crawlers). Photographer: David Haviland.

Treatment Options

Gill’s mealybug is best treated at the crawler stage; the waxy covering they create will protect older nymphs and adults from insecticides. Traditionally, Gill’s mealybug was controlled with a single insecticide application during the emergence of the first generation of crawlers in late May/early June.  UC guidelines suggest that you treat when you find 1 mealybug per 10 rachises at this time, however the economic threshold may change depending on the projected price of the crop and the projected yield.  Treating when there are fewer than 1 in 10 rachises with mealybugs may be warranted if the price of pistachios is high and the cost of an application is low.  However, if you are using an expensive product and nut prices are low, you may decide you can tolerate a higher mealybug infestation.  Be sure to check with your processor to see if there are any potential MRL issues with the product you plan on using, as some products have been causing export issues. You can find suggested products through the UC IPM link at the bottom of this article.

Recent reports from 2019 and 2020 have indicated that a single treatment in late May or early June may no longer give adequate control.  The reasons why have not yet been determined.  It is possible that the mealybug may be developing resistance to insecticides, however the reduced efficacy has been observed across several classes insecticides, making this explanation unlikely but still possible.  In order to improve insecticide control, it is suggested you:

·       Monitor populations closely to ensure sprays are timed to the crawler stage

·       Follow the application best practices

o   Drive slowly to ensure good coverage

o   Use enough gallons of mixed solution to ensure the spray gets to the top of the canopy and penetrates the canopy

o   Consider spraying when ambient humidity is higher to ensure droplets do not evaporate before they reach their target

A spray during the emergence of the second generation of crawlers (late July) may be needed.  While there is a third generation of crawlers late in the season, it is currently unknown if this is an effective spray time. 

Female insects cannot fly, so the way this pest is spread is by moving contaminated plant material or machinery.  Harvest activities can move Gill’s mealybug within an orchard, and equipment carrying infested plant material can move this pest between orchards.  Ensure that you clean your own equipment before moving between orchards and if possible, harvest infested orchards last.  Ensure that custom harvesters have cleaned their equipment before entering your orchard. 

For more information on Gill’s mealybug, you can visit:

https://www2.ipm.ucanr.edu/agriculture/pistachio/Mealybugs/

https://www.sjvtandv.com/blog/pistachio-short-course-videos (Listen to David Haviland talk about scale pests and mealybugs on Day 4.  His talk begins approximately 40 minutes in)