Pragya Chalise and Douglas G. Pfeiffer, Dept. Entomology, Virginia Tech, Blacksburg, VA
While working on your grapevines or small fruit crops, you may come across small, white slow-moving insects. These are mealybugs, named for the white powdery secretions covering their bodies. They occur in perennial crops including grapevines and deciduous fruit crops. They use their piercing-sucking mouthparts to pierce through the tough tissues of the host to feed on the phloem sap. They can infest all plant parts including the roots. Although few can be seen on the underside of the leaves or on the new spurs, the number of insects might be higher especially on the cordon or trunk.
We have been surveying mealybugs in commercial vineyards in Virginia from 2019 through 2021. Grape mealybug, Pseudococcus maritimus (Ehrhorn), has been the predominant pest mealybug in the past. In an earlier survey of mealybugs in Virginia (part of a larger study on grapevine viruses by Taylor Jones and Mizuho Nita), grape mealybug, Gill’s mealybug, Ferrisia gilli (Gullan), and a low number of obscure mealybug, Pseudococcus viburni (Signoret) were recorded. Our survey indicates that the majority consist of grape and Gill’s mealybugs, with only 2-6 specimens of obscure mealybugs from some sites.
Gill’s mealybug is a newly described species of mealybug found in pistachio-growing regions of California and found infesting almonds, grapes, persimmons, and stone fruits as well as mulberry. Grape mealybug and obscure mealybugs are easily confused with each other, and both have been found in Virginia.
Identifying mealybugs in the field
In the field, species are usually identified based on the females which are more easily found. Females are elongated oval and lack wings. Females often have shorter filaments on the sides and longer filaments on the rear end. Males are small, gnat-like insects with a single pair of wings. The immature stage of mealybugs is called a nymph. The first nymphal stage, often called ‘crawlers’, are most active and quickly moves within the vine to find a suitable feeding spot. Grape mealybug and obscure mealybug look exactly like each other, except that when poked grape mealybug releases bright red/orange liquid towards the rear and top surface near the front of the body, while obscure mealybug releases a transparent liquid. Gill’s mealybug does not exude such liquid. Grape mealybug has four slender white tails, while Gill’s mealybug has two thick broad tails. Grape mealybug also has shorter filaments on the sides of the body, while Gill’s mealybug has none. It has also been observed that female grape mealybug lays eggs in egg sacs and immature disperse from their mother, while the immatures of gill’s mealybugs remain aggregated around the mother, especially on the underside of the body. The presence of wax-secreting pores gives Gill’s mealybug a distinct two-striped appearance on the back.
Mealybugs overwinter as crawlers in the cracks or crevices inside the bark. The overwintering crawlers migrate to the new buds during bud break and start feeding. Gill’s mealybugs were usually seen feeding on the base of new buds, while grape mealybugs were usually on the trunk. Although the population is not that high during this phase, one easy way to spot mealybugs on the vineyard is by looking at the base of the new buds.
Both sexes were seen both on sticky traps and the vines in mid-June. The number of mealybugs often increases rapidly in the second and the third generations. One of the most devastating effects of mealybugs is their appearance on the grape clusters before the harvest.
Ants and the mealybugs
The pavement ant, Tetramorium immigrans, and the smaller yellow ant, Lasius (Acanthomyops) claviger were found in close association with mealybugs in 2020, while pavement ant, the smaller yellow ant, odorous house ant, Tapinoma sessile, and cornfield ant, Lasius (Lasius) neoniger were recorded in 2019. While sampling mealybugs in the field, we observed ants picking up the nymphs when disturbed and transporting nymphs around. Some of the vines had ant nests within their crevices, and when disturbed, ants were observed to grab mealybug adults as well as every egg being laid into the inner portion of the nest hole. They were also observed carrying the insect, trying to find a suitable location (time range from 2 minutes to 10 minutes), when they dropped to the ground.
Effect on the vines
Mealybugs can often be found in aggregations when the population is relatively high.One of the main problems with this aggregation is the elevated level of honeydew on the trunk. Honeydew is a sugary liquid excreted by the insect after the removal of nutrients from its liquid diet. Honeydew attracts ants towards the vines and supports the growth of dark sooty mold fungus as well.
Mealybugs (especially longtailed, obscure, citrus and grape) have the potential to transmit the grape leafroll-associated virus (GLRaV-3 isolates). A study conducted in Virginia and surrounding states has reported the three most common grapevine viruses as grape leafroll-associated virus-3 strain, grapevine red blotch associated virus, and grapevine rupestris stem pitting associated virus. The widespread presence of virus-infected vines points out the importance of the control of the mealybug vectors.
Extra precaution must be taken in the sites with a history of mealybug infestations when planting new vines or replacing near next to an old vine. Mealybug monitoring can be a painstaking task, considering the fact that they have relatively clumped distribution. The location on the vines also varies throughout the season. A magnifying hand lens (10x-30x) can be useful in examining these insects carefully.
During budbreak, as mentioned earlier, the base of these buds can be checked with or without the hand lens. The insects can also be seen on the underside of the newly formed leaves. The population can be monitored by peeling a few pieces of bark off, especially if you see lots of ants moving up and down the trunk. Sometimes they may appear on the vines before harvest or in the case of small fruit crops, in the stem and stalk end of the fruit.
In cases of extreme infestations, when a single vine can have more than 20 egg-laying adults aggregated together in a single vine, the trunk is often darker in color with sooty mold growth due to large amounts of honey dew production. The younger nymphal stages in this case can be seen more on the leaves or the base of the new shoots/canes.
Sometimes vines with leaf roll-associated virus can appear stressed, although the mealybugs may or may not appear on the aerial plant parts. In this condition, a few vines can be dug out and checked for the presence of mealybugs on the roots. The movement to the roots is mostly assisted by the ants, and tendency to appear in the roots varies among species.
After harvest, overwintering populations can be assessed by checking the bark of the vines. Mealybugs often leave behind white cottony material in the bark. Live crawlers can often be seen around this cottony mass or in the trunk camouflaged with the color of the trunk. Egg masses can often be seen in bark crevices at this phase.
Sticky traps are often placed within the vine canopy to monitor the presence of mealybugs. Sex pheromone traps are often used to attract winged males and are species-specific. The trap may be used to signify the presence of mealybugs on the vines, especially when the population is low and building up. It does not necessarily indicate the level of infestation. As the adult males are feeble fliers, lack of males in the traps can be misleading. The number of males captured on the trap does not indicate the level of infestation on the vine or even their presence or absence. Males can also be seen on the trunk, even when they fail to be detected on the trap.
If summer pruning can be carried out, it can help in removal of any infested plant parts and provide better spray coverage.
Natural predators on the vines
From a variety of generalist predators found on the vines, we will mention a few that are interesting in the field. Two commercially available important predators are green lacewing and mealybug destroyer. The lacewing larva is a voracious predator, feeding on aphids and mealybugs. One interesting question asked in a grower meeting was, “is there a mealybug feeding on other mealybugs?” This could be the impression after viewing the larval stage of mealybug destroyer lady beetle, Cryptolaemus montrouzieri. The larval stage is covered with wax and resembles a mealybug, although they are much faster than mealybugs. The larvae feed voraciously on mealybugs and hence are important predators on your vines.
Another interesting predator in the field can be predatory midge larvae. These were one of the most numerous predators seen in our survey. Mealybugs can often be seen moving around trying to get rid of these larvae. As many as 4-5 were seen trying to feed on an adult female mealybug.
I also would like to mention another generalist predator – spiders! Leaves containing spider webs were often seen in the vineyard. Most of the spiderwebs were filled with mealybug crawlers, most likely used as food by these spiders.
Chemical control and future work
Our work on chemical control of mealybugs is ongoing; we well report on this later. The crawler stage is the most vulnerable stage when chemical control is being taken into consideration. More information on chemical control can be found in our ‘Virginia Cooperative Extension Pest Management Guide for Commercial Vineyards’ and pest management guides published by the Southern Region Small Fruit Consortium.