Plant Health: Hard to control

Mealybugs are major insect pests of greenhouses, interiorscapes, and conservatories because they feed on a diversity of plant types, and they are difficult to control or regulate with insecticides. The host plant range of mealybugs depends on the mealybug species but, in general, includes tropicals or foliage plants, herbaceous annuals and perennials, orchids, ferns, vegetables and herbs. Some of their favorite host plants include aglaonema, begonia, coleus, croton, false aralia, marigold and transvaal daisy. There are a number of mealybug species that may be encountered in greenhouses (depending on geographic location), but the predominant species are the citrus mealybug (Planococcus citri) and the longtailed mealybug (Pseudococcus longispinus). In addition to these aboveground feeders, greenhouse producers may experience problems with root mealybugs (Rhizoecus spp.), which are a concern because they are typically noticed too late, when plants are already exhibiting damage symptoms, and are extremely difficult to control with insecticides. Here are some questions to help you combat mealybugs.

1. Why are mealybugs so difficult to control with insecticides?
This is one of the most frequently asked questions. There are a number of factors that impact the control or regulation of mealybugs with insecticides. First, mealybugs have a cryptic behavior or clumped spatial distribution, and they tend to aggregate or establish in concealed or protected areas of plants. Second, mealybugs may have frequent overlapping generations with an age structure that consists of all the life stages present (eggs, crawlers and adults) simultaneously. Third, mealybugs in the later stages (instars) possess a hydrophobic waxy body covering that repels hydrophilic insecticides.

2. When is the appropriate time to apply insecticides?
This is a great question, and one of the reasons why control or regulation with insecticides fails. The susceptible life stage is the crawler stage, which does not possess a waxy covering, and as such may be effectively dealt with by applying insect growth regulators, insecticidal soaps, horticultural oils and other contact insecticides. Adults, however, are harder to deal with because they form a white, waxy protective covering that is nearly impervious to most contact insecticides. In addition, because most insecticides have minimal or no activity on the eggs, at least two to three weekly applications will be required to satisfactorily reduce established mealybug populations, especially when dealing with overlapping generations.

One of the main problems when dealing with mealybugs is their small size, especially the crawlers or early instars, which are less than 2.0 mm in length. What generally happens is that greenhouse producers will not notice mealybugs until they are adults, which then limits options when attempting to use insecticides or biological control. At this point, it is almost best to simply rogue-out and dispose of heavily infested plants.

3. Are systemic insecticides effective against mealybugs?
It may simply be that mealybugs are less susceptible to systemic insecticides than other phloem-feeding insects. However, it is important to understand that the feeding behavior of mealybugs is not necessarily the same as aphids and whiteflies. For example, there are differences in the number and length of time associated with intracellular punctures, intervals between phloem-ingestion, and stylet motion during the phloem- searching process. In addition, mealybugs tend to feed on plant stems, which may not contain lethal concentrations of the systemic insecticides. We have been evaluating a variety of systemic insecticides and have obtained mixed to minimal control against the citrus mealybug, which may be associated with feeding location or insecticide label rates (actual rates may be too low).
 

Mealybugs123: Citrus mealybugs feeding on a coleus plant.

4. What about using biological control against mealybugs?
Using biological control agents, or natural enemies such as parasitoids and predators, has shown to be successful in managing mealybugs under specific crop production systems and in conservatories and interiorscapes. The natural enemies commercially available are primarily used against the citrus mealybug. These include the predatory ladybird beetle, Cryptolaemus montrouzieri, which is commonly referred to as the “mealybug destroyer” and the parasitoid, Leptomastix dactylopii. The larval stages of the mealybug destroyer resemble adult mealybugs. The parasitoid females only attack the third instar and young adult female life stages of the citrus mealybug. Both natural enemies are effective in regulating populations of the citrus mealybug, and they can be used together under certain systems. Furthermore, it is important to manage ant populations as ants will protect mealybugs from any natural enemies. In the presence of ants, mealybugs tend to ingest larger quantities of plant sap, resulting in greater plant damage.

Raymond Cloyd is a professor and extension specialist in ornamental entomology/integrated pest management for the department of entomology at Kansas State University.

Have a Question? You can contact Raymond at rcloyd@ksu.edu.