Mealybugs

Pest Notes: Introduction

Mealybugs are soft, oval, wax-covered insects that feed on many plants in garden, landscape, and indoor settings. Usually found in colonies, they are piercing-sucking insects closely related to soft scales but lack the scale covers. Like soft scales, they can produce abundant honeydew and are often associated with black sooty mold. Mealybugs are favored by warm weather and thrive in areas without cold winters or on indoor plants.

Identification and Life Cycle

Mealybugs are in the insect family Pseudococcidae, part of the superfamily Coccoidea, which also includes armored scales, soft scales, and cottony cushion scale.

Mealybug bodies are distinctly segmented and usually covered with wax. Older individuals may have wax filaments around their body margins. In some species the filaments are longer in the rear and can be used to help distinguish between different species.

Mealybugs are usually found feeding in colonies in somewhat protected areas such as between two touching fruits, in the crown of a plant, in branch crotches, on stems near soil, or between the stem and touching leaves. A few mealybug species feed on roots.

While adult females are wingless and similar in shape to nymphs, adult male mealybugs, which are rarely seen, are tiny two-winged insects with two long tail filaments. Many mealybug species can reproduce asexually without mating.

Life cycles vary somewhat by species. Adult females of most mealybugs lay 100-200 or more eggs in cottony egg sacs over a 10- to 20-day period. Egg sacs may be attached to crowns, leaves, bark, fruit, or twigs. An exception is the longtailed mealybug, which produces eggs that remain within the female until they hatch.

Newly hatched mealybug nymphs (called crawlers) are yellow to orangish or pink, lack wax, and are quite mobile, but they begin to excrete a waxy covering soon after settling down to feed. Although older nymphs and adults have legs and can move, they don’t move very far or very rapidly. Nymphs molt through several instars before becoming adults.

Depending on species and environment, mealybugs may have two to six generations a year. Where climates are warm or plants are growing indoors, all stages may be present throughout the year. On deciduous plants such as grapevines, mealybugs may overwinter on or under bark as eggs (within egg sacs) or as first-stage nymphs.

Mealybugs are sometimes confused with other pests that produce waxy coatings, honeydew, and black sooty mold, including the cottony cushion scale, woolly aphids, and even some soft scales and whiteflies. Be sure to carefully examine the insect beneath the wax to identify it properly.

Over 170 species of mealybugs occur in California. Only a few have become major pests. Some of the most common problem species are pictured and described in Table 1.

Table 1. Common Pest Mealybugs in California.

Species	Description
Obscure mealybug Pseudococcus affinis	Distinct filaments around the body, covered with powdery wax. Hosts: Many hosts, primarily outdoor plants.
Longtailed mealybug Pseudococcus longispinus	Two tail filaments that are longer than its body. Gives birth to live nymphs and produces no egg masses. Hosts: Citrus, grapes, nursery stock, and indoor ornamentals.
Grape mealybug Pseudococcus maritimus	Almost identical in appearance to obscure mealybug. If poked (not punctured), it will release a reddish orange defensive secretion. Obscure mealybug secretion would be clear. Hosts: Grapes, pears, pomegranate, and other fruit trees.
Citrus mealybug Planococcus citri	Short, equal-length waxy filaments around the body. A dark stripe may be visible down its back. Hosts: Citrus, several landscape shrubs. Most common mealybug on indoor ornamentals.
Vine mealybug Planococcus ficus	Similar to citrus mealybug with shorter filaments than other mealybugs in grapes. Has a dark stripe on its back. May be found on roots as well as aboveground. Hosts: Mostly grapes in California, although its potential hosts include other fruits and ornamental trees.
Cypress bark mealybug Ehrhornia cupressi	Round, bright orange or red and surrounded with a ring of wax. Found beneath bark plates. Hosts: Monterey cypress, other cypress, cedar and juniper.
Ground mealybugs Rhizoecus spp.	No obvious filaments. Live in soil. Hosts: Many plants, but mostly damaging on potted plants such as African violets.

Damage

Mealybugs suck sap from plant phloem, reducing plant vigor, and they excrete sticky honeydew and wax, which reduces plant and fruit quality, especially when black sooty mold grows on the honeydew. Large accumulations of mealybugs, their egg sacs, and wax can be unattractive. High populations feeding on foliage or stems can slow plant growth and cause leaf drop; however, healthy plants can tolerate low populations without significant damage. Ground mealybugs, which are not very common in landscapes and gardens, feed on roots and can cause plant decline but are generally not seen until plants are dug up and roots are exposed.

Many types of perennial plants are affected by mealybugs. Among fruit trees, citrus has the most problems, but mealybugs may sometimes be found on stone fruits or pome fruits, although rarely at damaging levels. Mealybugs can build up in grapes, especially the vine mealybug, a new invader that attacks roots as well as aboveground parts, but the grape, obscure, and longtailed mealybugs also occur.

Many woody ornamental plants and some herbaceous perennials can be infested including cactus, coral bells (Heuchera), figs (Ficus), flax grasses (Phormium), fuchsia, gardenia, hibiscus, jasmine, mimosa, Miscanthus grasses, and oleander. The cypress bark mealybug can be a serious pest on Monterey cypress in urban areas and also attacks other species of cypress, cedar, and juniper.

Plants growing indoors or in greenhouses are especially vulnerable because year-round mild temperatures favor mealybug populations, and indoor plants are usually not exposed to the natural enemies that often keep mealybugs under control outdoors. Among houseplants, aglaonema, coleus, cactus, dracaena, ferns, ficus, hoya, jade, orchids, palms, philodendron, schefflera, poinsettia, and various herbs including rosemary and sage often have problems with aboveground mealybugs. Ground mealybug infestations are most often reported on African violet and gardenias.

Some mealybugs, such as those infesting grapevines, can transmit viruses, but these aren’t usually a major problem in gardens and landscapes. The pink hibiscus mealybug, Maconellicoccus hirsutus, which currently is only established in Imperial County in California, has saliva that is especially toxic to plants.

Management

Mealybugs are very difficult to manage with insecticides. Fortunately most species have natural enemies that keep their populations below damaging levels in outdoor systems such as landscapes and gardens. The best approach to managing mealybugs is to choose plants known to be less prone to problems, inspect plants for mealybugs before bringing them onto your property, and rely on biological control and cultural practices to keep mealybug numbers in check.

Cultural Practices

Mealybugs are often introduced into landscapes (and especially into indoor areas) on new plants or on tools or pots. Because adult females can’t fly and mealybugs can’t crawl very fast, they don’t rapidly disperse in the garden on their own. Inspect any new plants thoroughly for mealybugs before installing them. If you can’t remove all the mealybugs present, discard and destroy the plant or, if possible, take it back to the source.

Regularly inspect mealybug-prone plant species in your landscape or indoor plantings for mealybugs. If you find an infestation, physically remove the insects by handpicking or prune them out. Toss out older “grandmother” plants that may be a source of infestation for new plants. Check pots, stakes, and other materials for mealybugs and their egg sacs and dispose of any infested items.

If mealybugs are somewhat exposed, it may be possible to reduce populations on sturdy plants with a high-pressure or forcible spray of water. Repeat applications at several-day intervals may be necessary.

Avoid unnecessary applications of nitrogen fertilizer on plants with mealybugs. High rates of nitrogen coupled with regular irrigation may stimulate tender new plant growth as well as mealybug egg production.

If your landscape or interiorscape has a history of serious mealybug problems, consider using only plant species that are not prone to mealybugs for at least a year or two to reduce mealybug density and harborage potential.

Ground mealybugs are even more difficult to control than those that feed aboveground. Prevent introduction of ground mealybugs and quickly dispose of infested plants before the pests can move onto clean plants.

Biological Control

Many natural enemies feed on and kill mealybugs on fruit trees and woody ornamental plants in the landscape. These beneficial insects generally can be relied upon to keep numbers at tolerable levels. Natural enemies include a number of species of parasitic wasps that lay their eggs in or on developing mealybugs. Common parasites (or "parasitoids") include species in the genera Coccophagus, Leptomastix, Allotropa, Pseudaphycus, and Acerophagus. Look for parasite pupae within mealybug colonies, or emergence holes in mummified mealybugs. Leptomastix dactylopii is sold commercially for release in greenhouses, citrus groves, and interiorscapes, but it kills only the citrus mealybug.

Naturally occurring predators of mealybugs include lady beetles, green and brown lacewings, spiders, minute pirate bugs, and larvae of predaceous midges. The mealybug destroyer lady beetle, Cryptolaemus montrouzieri, is the most important of these predators in many areas. It does not tolerate cold winters, so it is more common in southern California and in coastal areas.

The mealybug destroyer can be purchased for augmentative release and is often released in greenhouses and interiorscapes or in citrus orchards after a cold winter has killed off native populations. Adult beetles are bicolored with reddish-brown heads and hind ends and black in the middle; older mealybug destroyer larvae are covered with white wax, which makes them look somewhat like large mealybugs. When releasing mealybug destroyers, focus on periods when there are many mealybug egg sacs, because the lady beetles require mealybug eggs as food to stimulate their own reproduction. There is little point in releasing them when mealybug numbers are low or when they are not reproducing.

Operators of greenhouses or interiorscapes with regular mealybug problems can establish their own mealybug destroyer colonies for self-release. The lady beetle can be reared in wide-mouth jars on mealybugs grown on sprouted potatoes or other hosts. A ring of petroleum or other sticky material smeared inside jars around the top will prevent the flightless mealybugs from crawling out but allows the lady beetles to fly out into the greenhouse.

Preserve naturally occurring biological control agents by avoiding use of broad-spectrum insecticides for any pests in the area. Also keep ants out of mealybug-infested areas and plants because ants protect mealybugs from their natural enemies.

Chemical Treatment

Nonchemical methods usually provide sufficient control for outdoor plantings in gardens and landscapes. Home and garden insecticides are not very effective for mealybugs, especially on larger plants. The mealybugs’ waxy coating repels most contact insecticides, and their habit of aggregating in hidden locations makes them hard to reach.

For houseplants, greenhouses, and interiorscapes where it is not physically possible to remove mealybugs and where biological control may not be feasible, spot treatment may be used to suppress populations of aboveground feeding mealybugs.

Spot Treatment with Isopropyl Alcohol

On small infestations on houseplants, a 70% or less solution of isopropyl (rubbing) alcohol in water may be dabbed directly on mealybugs with a cotton swab to kill them or remove them. Test the solution out on a small part of the plant 1 to 2 days beforehand to make sure it does not cause leaf burn (phytotoxicity). In some cases, a much more diluted solution may be advisable. Where infestations are extensive, a 10-25% solution of isopropyl alcohol can be applied with a spray bottle. You will need to repeat this procedure every week until the infestation is gone.

Insecticides

Insecticidal soaps, horticultural oil, or neem oil insecticides applied directly on mealybugs can provide some suppression, especially against younger nymphs that have less wax accumulation. Be sure to test for phytotoxicity of these materials prior to treatment as well.

Products containing the systemic insecticide dinotefuran may reduce mealybug numbers on some landscape plants, and plant spikes or granules containing the related insecticide imidacloprid may reduce mealybug crawler numbers on houseplants. These neonicotinoid products are less reliable against mealybugs than against other piercing-sucking insects in many situations. Their use should be avoided when possible, especially on flowering plants, because of potential negative impacts on natural enemies and pollinators.

Other insecticides, including pyrethroids, are also labeled for some situations but may not be much more effective than soaps and oils and can be devastating to natural enemies. Be aware that none of the available insecticides will likely provide complete control of all individuals, and that you will need to monitor and treat again as needed. When infestations become severe, consider discarding houseplants rather than repeatedly treating them with insecticides. On outdoor plants, cultural practices and biological control should be adequate for suppressing mealybugs in most situations.

References

Bettiga, L. J. (editor). 2013. Grape Pest Management. UCANR Publication 3343. Richmond, CA. (See chapters 37-42 for information on mealybugs on grapes).

Cloyd, R. A. 2011. Mealybug Management in Greenhouses and Interiorscapes (PDF). Kansas State University AES and CE Bulletin MF3001. 4pp.

Dreistadt, S. H. 2001. Integrated Pest Management for Floriculture and Nurseries. UC ANR Publication 3402, Oakland, CA.

Dreistadt, S. H. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 3rd edition. UC ANR Publication 3359, Oakland, CA.

Godfrey, K. E., K. M. Daane, W. J. Bentley, R. J. Gill, and R. Malakar-Kuenen. 2002. Mealybugs in California Vineyards. UCANR Publication 21612, Oakland, CA.

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