There are different species of mealybugs. Some live in the roots of plants, others on the leaves and stems. Either way, the presence of mealybugs is a major concern in greenhouses.

The citrus mealybug, longtailed mealybug, obscure mealybug and root-feeding mealybugs are the primary types responsible for damage in greenhouses, conservatories and in house plants.

Mealybugs are “the number one pest of greenhouse production,” Dr. Raymond Cloyd, Kansas State University entomologist, said. They are a “sneaky insect pest” that can rapidly cause so much damage to a wide variety of plants and are so difficult to control that Cloyd advises growers that the threshold for mealybug tolerance is “very minimal.”

Even one or two mealybugs are cause for action. Scouting is an ongoing necessity, and treating incoming plant material before bringing it into the greenhouse is the best management strategy there is.

Mealybugs are able to hide quite well among the crevices and undersides of leaves and stems, making them difficult to scout – and difficult to treat with contact sprays. A waxy coating that develops during one of the instar (larval) stages makes it less susceptible to any contact sprays. Without a natural enemy that can keep pace with their exponential growth, Cloyd isn’t able to recommend any specific biocontrol agent.

Across the spectrum – from scouting to physical, cultural, biological and insecticidal controls – “options are substantially limited compared to other insects,” he said of mealybugs.

Lifecycle is Key

The citrus mealybug was the primary focus of Cloyd’s presentation for the American Floral Endowment’s Grow Pro series. Adults have soft, small white bodies with a gray stripe down the back. The full lifecycle from laid egg to adult is 25 to 60 days, depending on environmental conditions. A female citrus mealybug lays up to 400 eggs. When she dies, her body acts as a covering to protect the eggs.

Unlike the females, male citrus mealybugs have wings, but they are destined only to mate, and then to die, as they have no mouths for feeding. The females and the nymphs (or “crawlers”) feed on the plant phloem, causing visible damage with stilting, wilting or leaf distortion.

The larvae molt as they mature from one instar stage to the next, and during this process their waxy coat develops, protecting the older larval stages and the adults from insecticides.

There are several instar stages, all of which move around seeking food from the phloem. Like aphids and whiteflies, mealybugs damage plants’ vascular systems by withdrawing fluids which contain nutrients. The carbohydrates in this fluid causes the sticky honeydew residue as the mealybugs feed on the plants. The honeydew attracts ants and wasps and predisposes the plant to black sooty mold, which can block photosynthesis and is an indicator of a very high mealybug population.

The lifecycle of mealybugs is short, and not all the eggs will hatch at the same time, so there are multiple lifecycle stages present at once.

“I cannot stress the importance of understanding this lifecycle when it comes to managing mealybugs throughout the growing season,” Cloyd said.

Mealybugs. Photo courtesy of U.S. National Collection of Scale Insects Photographs, USDA-ARS, Bugwood.org

Control Efforts

Physical control includes washing crawlers off of plants using water sprays or removing them by hand. Doing this proactively, before any plant material enters the greenhouse, is the first step to stopping mealybug infestation.

Cultural controls include not applying excess fertilizer. Mealybug females will lay more eggs, producing more offspring, in high nutrient environments. Proper water management, using less susceptible cultivars if there are any and maintaining healthy plant growth are best practices.

Contact insecticides will only effect the nymph stages prior to waxy coat development and only if the spray saturates all parts of the plant. Timing, coverage and frequency of application are all important.

Rotating insecticides with differing modes of action is important and needs to be done every one or two weeks, depending on how quickly the mealybugs are maturing. TriStar® and Altus® insecticides, which have both contact and systemic activity, can have 70% to 80% efficacy if used properly, Cloyd said.

“Use the same mode of action within a generation. With the next generation, you want to use a different mode of action,” Cloyd said.

Systemic insecticides have not been shown to be effective against the citrus mealybug, even in trials where the product was applied up to eight times over label rates. Trials show less than 50% mortality, regardless if the systemic insecticides were applied prior to mealybug infestation or after mealybugs were already present.

“Systemic insecticides are not an option,” he said. “It’s basically spray applications, which mean that thorough coverage and spray frequency are going to be really, really critical.”

Horticultural oils can be effective in slowly dropping mealybug populations, if thorough coverage and frequent applications once or twice per week are utilized. The oils are only effective on the nymph stages prior to formation of the waxy coating, and the oils have a short residual period.

Biocontrol agents are lacking too. Green lacewings can’t consistently manage mealybugs. Mealybug destroyers (ladybird beetles) require high populations, and these have to be released before mealybugs are present in order to have any effective control. Some parasitoids which prey on citrus mealybugs are being studied for effectiveness in greenhouses.

Cloyd also cautioned growers not keep any plants that are more than 40% infested with mealybugs. If attempting to treat large plants with lesser infestations, prune out visibly affected parts and place them in a plastic bag for disposal outside of the greenhouse. “Pet” plants are often hosting mealybugs populations, which then spread to the rest of the plants being propagated, he cautioned. If reusing pots, bleach them to kill any mealybugs that may be present.

Even in cold temperatures, mealybugs will hide in plant crowns, so lowering the greenhouse temperature does not effectively kill mealybugs. Mealybugs can also survive in hot temperatures, with one grower mentioning that 120º F did not kill the citrus mealybug.

“Insects have numerous mechanisms to enhance their survival,” Cloyd concluded.