Mealybugs are destructive and economically important insect pests of many greenhouse-grown horticultural crops, including ornamentals and vegetables. They feed within the vascular system (phloem sieve tubes or food-conducting tissues), removing plant fluids. Consequently, the damage associated with mealybug feeding includes leaf distortion, plant stunting, and plant wilting. Mealybugs tend to feed in cryptic (concealed) areas on plants, such as leaf undersides, plant stems, beneath leaf sheaths, and the juncture where the petiole meets the main stem, which makes control difficult with spray applications of insecticides.
Insecticide resistance is the genetic ability of some individuals in an insect pest population to survive an application or applications of insecticides. In other words, the insecticide(s) no longer effectively kills a substantial number of individuals in the insect pest population. The amount of “selection pressure” or frequency in which insecticides are applied is the main factor that influences the ability of an insect pest population to develop resistance to insecticides. Insects can develop resistance by means of specific mechanisms that are associated with certain insect pests. These mechanisms include metabolic resistance, physiological resistance, physical resistance, behavioral resistance and natural resistance.
The primary mealybug species that feeds on greenhouse-grown horticultural crops is the citrus mealybug, Planococcus citri. The citrus mealybug exhibits what is known as “behavioral” or “natural” resistance based on their ability to avoid contact with insecticides due to the waxy covering on the body of later life stages, which prevents insecticides from contacting the cuticle (skin). In addition, the waxy covering prohibits or repels water-based insecticides from penetrating and entering the internal tissues. The crawler stage of citrus mealybugs is the most susceptible life stage because the crawlers do not have a waxy covering. However, the crawler stage is only active for a few days.
Poor control (based on low mortality) and multiple generations per cropping cycle may exacerbate the rate by which citrus mealybugs develop resistance to insecticides. However, just because an insecticide that used to provide sufficient suppression of citrus mealybug populations is no longer effective does not mean that the citrus mealybug population has developed resistance to that insecticide. The development of insecticide resistance may be associated with inappropriate insecticide use (e.g. poor coverage, wrong rate used, and improper timing of applications). Furthermore, using higher label rates and applying insecticides in shorter time intervals between applications, resulting in more frequent applications and thus higher “selection pressure,” enhances the speed of insecticide resistance.
Mealybugs, including the citrus mealybug, are capable of developing resistance to insecticides although there is minimal information associated with insecticide resistance in citrus mealybug populations. Regardless, studies have shown that citrus mealybug populations have developed resistance due to extensive applications of certain insecticides, including: diazinon (formerly sold as Knox Out), malathion, chlorpyrifos (DuraGuard), and kinoprene (Enstar AQ).
Furthermore, another factor to consider is that the feeding behavior of mealybugs may be affiliated with resistance. In our research program at Kansas State University, we found that systemic insecticides applied to the growing medium as a drench are not effective in suppressing citrus mealybug populations on coleus plants even at four times and eight times the label rates. The potential reasons for the lack of sufficient efficacy (>80 percent) may be associated with the following: 1) although mealybugs feed in the phloem sieve tubes (food-conducting tissues), similar to aphids and whiteflies, they feed differently; and 2) feeding involves variations in the number and length of time affiliated with intercellular punctures, intervals between the first phloem-ingesting periods, and stylet motility or movement during the phloem searching process. Therefore, feeding behavior could impact the ability of systemic insecticides to suppress mealybug populations.
Another possibility is that citrus mealybugs can detoxify the active ingredient of systemic insecticides. Studies associated with citrus mealybugs in regard to metabolic detoxification of insecticides are limited compared to other insect pests such as aphids, whiteflies, and thrips. Future studies are needed to determine if the failure of systemic insecticides to suppress citrus mealybug populations is related to resistance based on the presence of detoxification enzymes. Therefore, to avoid the potential for resistance, always rotate insecticides with different modes of action within a generation of citrus mealybugs to minimize the prospects of resistance developing in a citrus mealybug population.