In July 2021, Sandipa Gautam had just started as the area citrus IPM advisor in the San Joaquin Valley when the emails and phone calls started coming in. Pest control advisers were seeing citrus mealybug problems, which was unusual, and calling the Lindcove Research and Extension Center in Exeter, for advice. “They’re pretty bad infestations,” pest control advisers were telling her. “We don’t know what to do about it, and sprays aren’t giving it good control.”
Once established, citrus mealybug is hard to control. It is cryptic and hides in the navel, under the calyx, on the stem, and between fruits in clusters. Its overlapping generations pose a challenge to pesticide application timing. And the pest can shrug off insecticides because its eggs are protected within an egg sac while water-based insecticides roll off the signature waxy coating of the adults. It affects every kind of citrus—oranges, lemons, grapefruit, mandarins and more—and eats almost every part of the plant.
Citrus mealybug is not a new pest, having first been reported in California in 1900, and has historically been controlled by natural predators. So why is it now an emerging problem?
“We don’t have a clear answer,” says Gautam, but points to some possible factors. In December 2020, the Department of Pesticide Regulation (DPR) deregistered the organophosphate chlorpyrifos, which had been used to treat California red scale and may have kept mealybug numbers down. It could be the reduced use of systemic pesticides such as imidacloprid. Third, when citrus thrips became hard to control in 2021, the broad-spectrum pesticides used to treat them may have suppressed the natural enemies of mealybugs. Combine these factors with the warmer winters that reduced winter mortality, and Gautam was hearing increased reports about infestations and economic damage such as reduced tree vigor, fruit drop, abnormal fruit shape, honeydew and sooty mold contamination, and more.
To address the problem, Gautam looked at pest biology, scouting and monitoring practices, seasonal phenology, and chemical control options.
Scouting and monitoring are especially important because mealybug often escapes early detection efforts. For example, when she visited a grove with a known infestation, Gautam looked for an hour for mealybugs and did not find a single one. The pest control adviser who’d called in the problem said, go into the canopy and look up. “And there they were on the navels of fruit, on the calyx, and between fruits in clusters. Their tendency to hide makes monitoring difficult,” says Gautam. “There is not one single, simple, visual method. And it’s hard to achieve accuracy because they tend toward a clumped distribution.” She emphasizes to actively scout for it and to look inside the canopy, looking for signs of infestation such as sooty mold or mealybug wax on leaves, twigs, and trunks. “Once it’s in the canopy, it’s everywhere.”
Scouting and monitoring are especially important because mealybug often escapes early detection efforts.
Gautam’s research in 2022 showed that mealybugs move to different parts of the tree during the season. Early in the season, they are found on fruit (if present) and under the thick canopy leaves. In May and June, they move to the inner branches and trunk as well as new flush, especially in the tree interior. From July through November or December, they are present on fruit, calyx and navel ends, and between the fruits in clusters.
The seasonal phenology of citrus mealybug in California has not been well understood. To start developing IPM solutions, Gautam needed to know their movement patterns as well as how many generations there are per season, especially in the Central Valley. According to Gautam, “Like any scale insect, citrus mealybug is easier to treat when it is moving, or when it is young, without its protective waxy coating.”
Gautam and a research team from California State University, Fresno, are conducting studies, supported in part by the Citrus Research Board. To determine mealybug movement within the tree canopy, they applied sticky tape to the trunks and inner branches, changing the tape weekly and counting crawlers and adults stuck on the tape to determine the seasonal population changes over a 12-month period. Separately, they studied male flights using trap cards and pheromone lures.
Existing literature points to two to three generations per year, but Gautam, using male trap data, identified six distinct male flight peaks suggesting six generations. The first male flight occurred in the first week of April; crawlers were trapped approximately two weeks later.
Overlapping generations make the pest harder to control. Gautam’s initial research suggests that the best time to spray would be the last week of April (targeting the first generation) or the end of June or first week of July (targeting the second generation)—that’s when the crawlers and first instars (development stages) are out and moving to the fruit. Also, knowing when egg sacs may be present helps time the release of the mealybug destroyer predator that feeds on them.
Overlapping generations make the pest harder to control.
Regarding biological control, more study needs to be done. “We don’t really know what natural enemies exist for citrus mealybug in the Central Valley,” says Gautam. Although releases of mealybug destroyer can be effective, the insect does not tolerate the increasingly hot and dry weather found in the San Joaquin Valley.
For chemical control, Gautam conducted two field trials in 2022. The first trial targeted the first generation; the second trial targeted the second generation. “All treatments had [a] significant effect on mealybug populations compared to [the untreated control], especially on the first week,” reports Gautam.
Now citrus growers in the San Joaquin Valley have updated guidance for this emerging pest problem. Increased awareness of the pest could lead to more vigilant monitoring so that the younger, mobile stage of the pest is targeted for control. Preventing high pest numbers may reduce pesticide use; pesticides are often used when pest numbers need to be reduced quickly. The phenology of the pest is important for knowing when management practices will be most successful and lays the groundwork for a degree-day model for formal management timing. Knowing more about the biology and phenology of citrus mealybug is the first step in developing cultural control practices and finding natural enemies to help lower pest numbers, and thus reduce the economic harm of citrus mealybug to citrus growers in California.
View Gautam’s UC Ag Expert Talk webinar.
