By Jawwad Qureshi
Managing the Asian citrus psyllid (ACP) is critical because of its role in spreading huanglongbing (HLB). Biological control is one method for ACP control. It is essential for developing integrated and sustainable pest management strategies. Biological control has always been a vital component of citrus pest management in Florida; understanding its contribution to the mortality of ACP is crucial.
Despite the scientifically proven impact that biological control has on reducing ACP populations, there has been no evidence to date that it is sufficient to manage HLB in Florida. However, it contributes significantly to ACP suppression in commercial groves and other habitats where chemical control is not appropriate, such as urban settings. Studies have shown significant contributions to the mortality of ACP and other citrus pests in Florida from several biological control agents, including predators, parasitoids and entomopathogens.
Several predators are known to attack ACP in Florida. These include multiple species of ladybeetles, lacewings, spiders, hoverflies and syrphid flies. Ladybeetles are one of the major groups that significantly manage ACP, with species including southern two-spotted ladybeetle, multicolored Asian ladybeetle, blood-red ladybeetle, little red ladybeetle and metallic blue ladybeetle. Laboratory studies confirmed that the larvae of all these species developed and the adults reproduced on a diet of ACP nymphs, except the blood-red ladybeetle, which developed but did not reproduce.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers conducted multiple experiments in the field to determine the natural mortality in the nymphal cohorts of ACP within two years of discovering HLB in Florida. Colonies of nymphs were left exposed to natural enemies or protected with sleeve cages. Predators such as ladybeetles, lacewings, syrphid flies and spiders were frequent visitors to the unprotected colonies and consumed 80 to 100 percent of the nymphs before they were able to turn into adults. Larvae and adult ladybeetles visited the colonies and fed on their prey voraciously.
Lacewings were the other significant predator of the nymphal colonies. There are two to three species of lacewings found in commercial groves; however, Ceraeochrysa cubana, a species of green lacewings, is abundant. Ladybeetles, lacewings and several other generalist predators also target citrus leafminer (CLM), scales, mealybugs, aphids and mites. They are useful in keeping several species of pests at lower levels that are not as damaging.
The adult southern two-spotted ladybeetle (Figure 1) feeds on ACP nymphs. Southern two-spotted ladybeetle larvae feed on ACP nymphs (Figure 2A) and CLM larvae (Figure 2B). Chemical or other pest control methods are not enough to eliminate the target and non-target pests for several reasons. For example, the pest may not be present when it is sprayed, or it may have developed resistance to some of the insecticides and could live through the treatments. Additionally, most pests have a high reproductive rate, and surviving females continue to reproduce and increase populations. Therefore, the consistent presence of beneficial organisms, such as predators, is warranted for the continued suppression of pests.
The tremendous increase in the use of insecticides to control ACP after the advent of HLB in Florida in 2005 negatively impacted the populations of several beneficial organisms. This includes predators such as ladybeetles, which were significant contributors to ACP mortality. Unfortunately, the species that inflict natural mortality on psyllid populations are not available commercially. Therefore, conserving these predators is essential.
UF/IFAS work with commercially available predators such as convergent ladybeetle, two-spotted ladybeetle and brown lacewings shows significant potential against ACP. A predatory mite, Amblyseius swirskii, also caused significant mortality to the eggs and first instar nymphs of ACP.
Tamarixia radiata is the primary species-specific parasitoid of psyllid nymphs. Females feed and lay eggs on the body of the psyllid nymph. The larvae consume the nymphs’ body contents and eventually kill them before they spread the disease as adults. Through combined feeding and parasitization behavior, a single female destroys several nymphs.
The limited releases made after the discovery of ACP in Florida in 1998 led to the establishment of T. radiata and parasitism rates averaging below 20 percent. Collaborations between the University of Florida, U.S. Department of Agriculture and the Florida Department of Agriculture and Consumer Services Division of Plant Industry resulted in the importation of these parasitoids from Taiwan, Vietnam, China and Pakistan. A mass-rearing facility that now provides stakeholders with these parasitoids for release in the environment was established.
The parasitism rates increased with repeated releases across multiple locations. A parasitism rate approaching 60 to 80 percent was observed in the urban and commercial plantings. Rates like this are not typical because of increased use of insecticides in commercial plantings and variations in the availability of nymphs. Repeated releases of T. radiata in the organic and conventional management programs for ACP showed its likely that parasitoids have established in the programs using oil, soaps and organic insecticides. The parasitism rates range between 20 and 40 percent in these programs.
Entomopathogenic fungi are another vital source of biotic mortality to psyllid populations. An entomopathogenic fungus, Hirsutella near citriformis, has reportedly attacked ACP in the field, averaging 23 to 75 percent adult mortality in Florida. Cordyceps fumosorosea also infected ACP at low levels.
Entomopathogenic fungi perform well at higher relative humidity. For instance, C. fumosorosea caused more than 95 percent mortality in ACP adults at 80 to 100 percent relative humidity under laboratory conditions.
CONSERVATION OF NATURAL ENEMIES
Predators, parasitoids and pathogens are all essential for reducing ACP populations in multiple environments. The intensive use of broad-spectrum insecticides negatively impacts parasitoids and other natural enemies. Therefore, conservation of biotic mortality is warranted through using selective insecticides, application methods and timing of insecticide application.
Selective insecticide approaches include 1) application of soil-applied systemic insecticides that avoid direct contact with natural enemies and 2) foliar sprays of insecticides directed mainly at adult psyllids, which are more vulnerable during winter when trees are producing little or no new growth which limits ACP reproduction. Predators are largely absent from the groves in winter due to the shortage of prey. Foliar sprays of broad-spectrum insecticides tested in the winter before spring growth suppressed ACP into the growing season and had no detectible impact on key natural enemies. An article (“Dormant sprays for Asian citrus psyllid management”) discussing details of this tactic was published in the January 2021 issue of Citrus Industry.
Potential options and supplements to conventional insecticides can be used to control psyllids during the growing season and maintain citrus production, even under conditions of high HLB incidence. For example, a program of organic insecticides sequentially mixed and alternated with horticultural mineral oil to control psyllids produced similar or better yields than a conventional program in a mature block of HLB-positive Valencia oranges.
Jawwad Qureshi is an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.