PSI: The “Sneaky Insect Pest”

The basics

Mealybugs are sometimes considered the second most difficult insect pest to deal with, after only western flower thrips (Frankliniella occidentalis). Mealybugs are particularly resilient to insecticides. Several mealybug species may be encountered in greenhouses (depending on geographic location), but the primary species include the citrus mealybug (Planococcus citri) and the longtailed mealybug (Pseudococcus longispinus). In addition to these above ground mealybugs, greenhouse producers may experience problems with root mealybugs (Rhizoecus spp.), which can be a concern because they are typically noticed too late, after plants are already exhibiting damage symptoms. Furthermore, they are extremely difficult to suppress with insecticides.
 

What they damage

Mealybugs feed on many different types of greenhouse-grown horticultural crops with the host plant range dependent on the mealybug species. In general, mealybugs will feed on tropicals or foliage plants, herbaceous annuals and perennials, orchids, ferns, vegetables and herbs. Some preferred host plants include aglaonema, begonia, chrysanthemum, coleus, croton, false aralia, marigold, pothos, schefflera and transvaal daisy.
 

Stop and spot

One of the main problems associated with dealing with mealybugs is their small size (the nymphs or crawlers, or early instars are less than 2 millimeters in length), which makes it extremely difficult to scout or monitor, and thus detect early infestations. Subsequently, what generally happens is that greenhouse producers will not notice mealybugs until they are adults and heavy-infestations are present. However, this limits options, especially when using insecticides. At this point, it is best to simply and immediately dispose of heavily infested plants, as insecticides will not solve the problem. Furthermore, workers should wear disposable gloves when handling plants heavily infested with mealybugs to prevent the spread of mealybugs throughout the greenhouse.
 

Biological factors

1. Mealybugs have a cryptic behavior or clumped spatial distribution. Thus, they tend to aggregate or establish themselves in concealed or protected areas of plants. This may reduce insecticide effectiveness resulting in fewer mealybugs being killed.

2. Frequent overlapping generations with an age structure that consists of all life stages including eggs, nymphs (crawlers) and adults may be present simultaneously.

3. The hydrophobic (water-hating) waxy body covering of adults repels hydrophilic (water-loving) insecticides. This results in the insecticide never making contact with the insect body.
 

Coverage and recovery

Types of insecticide applications include foliar sprays and those directed at the growing medium as a drench or granule. Adult mealybugs are very difficult to kill because they form a white, waxy protective covering that is nearly impervious to most insecticides. Since many insecticides have minimal to no activity on eggs, with the possible exception of horticultural oils, at least two to three weekly foliar applications may be needed to achieve satisfactory suppression, especially when dealing with overlapping generations. The crawler stage does not possess a waxy covering so it is highly susceptible to most insecticides including insect growth regulators (Talus and Enstar), insecticidal soaps (M-Pede), mineral oils (Ultra-Pure Oil and SuffOil-X) and other contact insecticides. Although very few (if any) insecticides are able to penetrate the waxy covering of mealybugs, those containing ethyl alcohol (ethanol), such as certain oil-based insecticides, may penetrate through the waxy covering, killing mealybugs.

When applying high-volume sprays, thorough coverage of all plant parts is critical, particularly when using contact insecticides. Mealybugs are commonly located in areas of plants that are not easily accessible such as the base of leaf petioles, leaf sheaths and leaf undersides. The addition of a surfactant or spreader-sticker to the spray solution may help in improving coverage and penetration.

For highly susceptible plants, it may be prudent to routinely spray with either an insecticidal soap or horticultural oil to prevent mealybug populations from reaching outbreak proportions although this goes against the philosophy of integrated pest management (IPM). Moreover, it is critical to conduct multiple applications when crawlers are present because eggs will hatch (with the exception of the longtailed mealybug) over an extended time period. Insect growth regulators including those containing the active ingredients azadirachtin (Azatin, Ornazin, AzaGuard, and Molt-X), buprofezin (Talus) and kinoprene (Enstar) are only active on the crawler stage, so the application timing of these insecticides is important.

Raymond Cloyd (right) is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.