Menacing mealybugs

Mealybugs are easily recognized by their white, mealy-appearing waxy secretions covering their body. The citrus mealybug, (Planococcus citri) and longtailed mealybug (Pseudococcus longispinus) are two of the more common species. Less common and more difficult to control is the Madeira mealybug (Phenacoccus madeirensis). There are even mealybugs that feed upon plant roots.

As growers know, they are one of the most difficult greenhouse pests to control. Mealybugs’ wide host range, high reproductive potential, waxy coating, tendency to hide in protected locations, ease of spread and ability to survive on greenhouse benches, in cracks and crevices without live plant material for two weeks, all make them very challenging to eradicate.

In general, mealybugs feed upon tropical and foliage plants, spring annuals, orchids and herbaceous perennials. Some especially mealybug-prone plants include begonia, citrus, coleus, croton, dracaena, hoya, English ivy, ficus, fuchsia, stephanotis, schefflera, hibiscus, mandevilla, jade plants, palms, prayer plants and gardenia, as well as many other foliage plants. They tend to be a problem when plants are grown and maintained for long periods.

Scouting for mealybugs

As only the short-lived winged males fly, do not rely on yellow sticky cards to detect mealybugs. Look for white flecks or cottony residues along the leaf midribs, on leaf or stem axils, on stem tips, on the underside of leaves and near the base of plants. When scouting, visit infested areas last. Adult females may crawl off plants and be found in brick crevices and under benches where they lay eggs. Honeydew, sooty mold and the presence of ants may also be an indication of an infestation.

Citrus mealybugs (Planococcus citri) females are small, (less than 1/8 of an inch long), with a faint purplish stripe running down their back. They also have short, waxy filaments around the margin of their oval body, with a slightly longer pair of filaments at their rear. They produce cottony egg masses and females can lay up to 600 eggs. Longtailed mealybugs have distinctive long tails (about ¾ or more of their body length), hence their common name. Longtailed mealybugs give birth to live young. Madeira mealybugs have three rows of white waxy tufts down their back. Check out Lance Osborne’s University of Florida mealybug website for helpful photos and descriptions (mrec.ifas.ufl.edu/lso/mealybugs.htm). Send samples to an extension entomologist for identification of species to help give you more information to prepare your plan of attack.

Critical cultural controls

Always begin with cultural controls. Adult females can live from 10 to 19 days without a host plant and crawlers can continue to emerge for up to 45 days afterward, according to Casey Sclar (executive director of the American Public Gardens Association). Inspect incoming plants and avoid overfeeding plants, especially with high nitrogen fertilizers. Using a forceful jet of high-pressure water twice a week helps remove mealybugs from their hiding places, especially when you have small infestations. Females can lay their eggs under pots, on greenhouse benches and in plant debris. Power wash and sanitize your greenhouse benches between crops. If feasible, keep the greenhouse fallow or empty for two weeks. Do not re-use infested pots, for they can hide under the lip of pots or trays. Regularly remove plant debris, too. Of course, you do not have any alternative hosts (weeds or pet plants) in your greenhouse, do you? Keeping temperatures on the low side helps slow down their development. Throwing out heavily infested plants is very often the best option.

Biological control agents

Whether biological control agents can be used depends upon what mealybug species you have. Both adults and larvae of the mealybug destroyer (Cryptolaemus montrouzieri) feed upon mealybugs that lay eggs, so they are not effective against the longtailed mealybugs. “Crypts” like it warm, above 70°F, with 70-80% relative humidity where there are high mealybug densities. Their larvae feed on all mealybug life stages, but they do not move very far. It is helpful to release both larvae and adults, especially from April to October, according to Sarah Jandricic, greenhouse floriculture IPM specialist at the Ontario Ministry of Agriculture, Food and Rural Affairs. Use cardboard release boxes available from your biological control supplier. (See Floriculture IPM for Mealybug at bit.ly/mealybug-ipm.) Repeated releases of green lacewing (Chrysoperla sp.) eggs or larvae may also help suppress mealybugs. Control ants so they do not eat the lacewing eggs off the cards.

Insecticides for mealybugs

Chemical control is difficult because of the mealybug’s tendency to hide in protected locations and form dense colonies. Overlapping generations occur so that all life stages are present (eggs, crawlers and adults). Contact insecticides will kill young, immature crawlers (provided there is good coverage). However, as eggs hatch, repeated applications are needed. The waxy covering on the adult mealybug makes it very difficult for insecticides to be effective. However, the immature crawlers do not have this waxy covering, so consider the use of insect growth regulators such as azadirachtin (Azatin O etc.), buprofezin (Talus) (MOA 16) and kinoprene (Enstar) (MOA 7A). Horticultural oils (SuffOil- X, Ultra-Pure Oil) are standbys, provided you can obtain good coverage. Some growers report good results using pyrifluquinazon (Rycar) (MOA 9B) in rotation with buprofezin (Talus). Read labels carefully for plant safety precautions and follow all resistance management guidelines. Adding a spreader sticker such as Capsil may help improve needed coverage, if the label allows.

Systemic insecticides applied as a drench may not work as well as the same products applied as a spray. Researchers at Kansas State University (Raymond Cloyd, Nathan Herrick and Amy Raudenbush in 2018, bit.ly/systemic-insecticides) reported less than 50% mortality when using the systemic insecticides azadirachtin, dinotefuran, flonicamid, imidacloprid, spirotetramat and thiamethoxam against citrus mealybugs, whether products were applied preventively or curatively as growing medium drenches.

Systemic insecticides may also be applied as foliar sprays, including dinotefuran (Safari), thiamethoxam (Flagship), imidacloprid (Marathon) and acetamiprid (Tristar) (MOA 4A). Some neonic alternatives include flonicamid (Aria) (MOA 29) and flupyradifurone (Altus) (MOA 4D) as foliar sprays. Spirotetramat (Kontos) (MOA 23) has been reported to give excellent control of root mealybugs by foliar or drench applications. The recently introduced Pradia (cyclaniliprole and flonicamid) (MOA 28 & 29) has been reported to work well against longtailed and Madeira mealybugs. How well insecticides work depends upon the mealybug species you are dealing with. See Scale and Mealybug Efficacy - 2019 (bit.ly/scale-mealybug-efficacy) by the IR-4 Ornamental Horticulture project.

It always seems like managing mealybugs is a never-ending battle, for there is no silver bullet, but with a good plan of attack, and knowing what species you have, you can succeed.

This story originally appeared in the March issue of sister publication Greenhouse Management.

Consult and follow pesticide labels for registered uses: local restrictions may apply. No discrimination is intended for any products not listed. If any information is inconsistent with the label, then follow the label.

The author is an extension educator at UConn Extension specializing in Greenhouse IPM. She can be reached at leanne.pundt@uconn.edu