Thursday, November 1, 2007

Greenhouse - Mealybugs and Their Control

Longtailed mealybugs have seventeen pairs of white waxy filaments around the periphery of the body (if you care to count them). The mature females have two tail-like projections projecting from their rear that are longer than the length of the body.

Citrus mealybug is recognized by a thin purple stripe that runs down the middle of the back. These mealybugs are densely covered with white wax and have very short tails.

Mealybugs can be a problem pest in greenhouses, expecially on longer term greenhouse crops. Once established they are very hard to control. The following is an article on mealybugs.

When you find mealybugs in a greenhouse be decisive and take quick action. Nip the problem in the bud. As soon as mealybugs are detected, throw away the infested plants. The old saying “Let them have an inch and they’ll take a yard” rings true with this pest. Once mealybugs become established, expect to be spraying for a quite a long while. Mealybugs cost growers and retailers millions of dollars each year in control costs and crop damage and loss.

Mealybugs feed on plant juices and inject toxins that cause stunting and distortion of new growth. Infested plants usually have premature leaf drop and dieback in heavy infestations. In addition, this scale insect secretes a waste product, called honeydew. They do not digest the sugars they extract and excrete this as a sticky material. The syrupy liquid coats the leaves making them shiny and sticky. Often sooty mold grows on this honeydew making the foliage appear dirty and interfering with photosynthesis.

Many growers run to the savior of insecticides, imidacloprid, when they get a tough bug. You are going to have to use a variety of control methods and probably use insecticide rotation to clean up a mealybug population. Trying to control this pest quickly and relying just on chemical control is like running with sharp scissors – someone is going to get hurt. Cultural control of mealybugs is the best defense. This means closely inspecting incoming stock plants (like you have the time). But, if you don’t make the time, mealybugs may come back to haunt you. Rejecting infested plants now prevents a big disaster from occurring in your greenhouse. Certain plants are more prone to infestation such as rosemary, coleus, sage Swedish ivy, artemesia, and gardenia. The most susceptible plants should be monitored the closest.

Taking cuttings from infested plants is the kiss of death. Young, immature mealybugs prefer to move to tip growth to feed – exactly the part of the plant from which you take cuttings. Exposing cuttings to low temperatures (33 –35 °F for 24 –36 hours before sticking) will often help reduce mealybug survival. This is based on work by Casey Sclar at Longwood Gardens, Kennett Square, PA. Check to make sure that your plant material can withstand this lower temperature without suffering irreparable damage.

Mealybugs, are one of the most common scale insects attacking ornamental plants. Yes, mealybugs are members of the scale family Pseudococcidae. Mealybugs are different from most scales in that they are mobile in all of their life stages. They actually move when it suits them. The young are the most restless, wandering about the plant looking for good feeding sites. If they find a suitable feeding site they usually settle down, insert their mouthparts, and don’t budge until the feeding site taps out or it’s time to mate.

There are 275 species of mealybugs in the continental United States, and I believe 272 of these species hang out mainly in Florida and southern California. In most greenhouses and nurseries in the United States you will encounter mainly citrus mealybug (Planococcus citri), longtailed mealybug (Pseudococcus longispinus), Madeira mealybug (also unofficially known as false Mexican mealybug) (Phenococcus madeirensis), and root mealybug (Rhizoecus falcifer).

Most mealybugs have an oval body shape and some sort of white waxy coating on the body or extending from the edge or end of the insect. Each mealybug species is slightly different, but basically the female goes through 4 developmental stages (instars). An adult female will lay 500 – 600 eggs, usually in a cottony-like ovisac beneath her body. The eggs hatch in 7 –14 days and the 1st instar nymphs disperse- usually on wind currents in the greenhouse, attached to workers clothing, or by natural bridges created by closely spaced plant material. Some mealybugs are slightly different, like the longtailed mealybug, which does not lay eggs but rather gives live birth to nymphs. Male mealybugs go through 5 instars and only feed in the first two. Males pupate on leaves, pots, or benches and emerge as winged adults. They then have only 1 – 2 days to make a date and mate before they die.

Madeira mealybug looks a lot like citrus mealybug. They are generally a dull gray color under the white wax and lack the single purple stripe on their back that citrus mealybugs posses. Their egg sacs are longer and denser than citrus mealybug and male pupal cases may be found in equal numbers to the females. This is one nasty, hard to control mealybug. There will be more on this pest later.

Root mealybug is found only on the roots and not on the plant parts above ground. You will have to flip the plant over and pull the pot off. They commonly hang out at the edge of the root ball. This mealybug has a layer of dusty to granular white wax covering its body.

Toughest of the Tough Mealybugs

There is always an Olympic marathon among the pest population to grab your attention. When we have fairly good control methods available and one pest falls by the wayside, others quickly rise up to take their place. Such is the case with the Madeira mealybug. This pest has cropped up in greenhouses throughout the United States on a number of pot-grown plants and is rapidly moving to the head of the pack of greenhouse pests. If you thought citrus mealybug and longtailed mealybug were nasty, you haven't seen anything yet until you've experienced the new kid on the block, Phenacoccus madeirensis. There is no accepted common name for this pest, but some entomologists are calling it the Madeira or false Mexican mealybug. This mealybug will kill a mandevilla in a matter of 2 -3 weeks if left unchecked. It looks very much like a citrus mealybug, but boy is it different. To tell it apartshort of sending a sample to a specialist in coccidiology (a scale expert) - look for large number of pupal cases among the white wax.

With citrus mealybug, generally there are a lot more females present than males and males are the only ones that pupate. With Phenacoccus madeirensis, there is about an equal number of males and females, hence the larger number of pupal cases being found on the plant in the white wax. The other external macro characteristics are the egg sacs, which are long. One final feature is that citrus mealybug has a dark line running down the middle of its back, whereas Phenacoccus madeirensis does not. Nothing appears to really control a population once it’s ‘ripping’ on a plant. Ron Oetting at Georgia University had good (but not outstanding) results with Distance IGR (Pyriproxyfen), Dursguard (Chlorpyrifos) and Talstar (Bifenthrin). However, they found poor control with Marathon (Imidacloprid) and Flagship (Thiamethoxam – not EPA labeled yet).

Like you really needed another mealybug

Florida was invaded by the pink hibiscus mealybug from the Carribbean. In June of 2002 the pink hibisicus mealybug was detected in Broward and Miami-Dade counties. You can view this pest and obtain more information at (a long URL, but worth the effort). Whenever Florida (the foliage plant capitol and plug producer of the world) gets a new pest, we all hold our breath hoping that they won’t spread it to the rest of us. In the fall of 2002 two parasitic insects, Anagyrus kamali and Gyranusoidea indica, were released in a 22 square mile area. These parasites were used in the Caribbean and in California during the 1990s with good results. The Florida Department of Agriculture is expecting a 95% or greater reduction in pink hibiscus mealybug with these parasite releases. The Department of Agriculture is trying hard to prevent this pest from spreading.

Biological Control of Mealybug

Mealybug destroyer (Cryptolaemus montrouzieri) can be an effective tool in controlling citrus and long-tailed mealybugs. Both the adults and the larvae of the mealybug destroyer feed on mealybugs. C. montrouzieri is a small, dark brown beetle with a tan head. Their wax-covered larvae resemble mealybugs, except they are twice as large as their prey. C. montrouzieri larvae feed on mealybug eggs, crawlers, and honeydew. Adults and young larvae prefer to feed on mealybug eggs, however older larvae will attack any stage.

Article reprinted from the Greenhouse TPM/IPM Weekly Report, November 15, 2006 issue, University of Maryland Cooperative Extension

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