By Jawwad Qureshi
A wide range of pests colonize citrus crops. Most of these species target tree canopies, and a few feed on the roots. Damage is either direct through feeding on the plant tissues or it can be twofold if the pest is also responsible for transmitting the pathogens of a disease or making conditions favorable for the disease to spread.
Common pests include Asian citrus psyllid (ACP), citrus leafminer (CLM), aphids, whiteflies, scales, mealybugs, thrips, mites and root weevils. ACP and CLM cause twofold damage and are major areawide threats. ACP vector pathogens that spread citrus greening (HLB) disease. The larval feeding of CLM exacerbate the spread of citrus canker disease. These pests also cause direct feeding damage.
Damage associated with other pests, such as scales, mealybugs, aphids, thrips, mites or root weevils, is also of major concern. This damage is generally localized and not as widespread as ACP and CLM. Managing these pests in groves and areawide is essential for suppressing their damage.
Natural enemies such as predators and entomopathogens are beneficial organisms that attack multiple pests. Natural enemies can also be pest specific, such as parasitoids. This method of control is called biological control. These beneficial organisms have been reducing pest populations across commercial and urban environments for decades. This is achieved through their introduction, conservation or augmentation. Their abundance and distribution over landscapes may vary with prey availability, habitat and human intervention of pest management.
Natural enemies suffered after the discovery of HLB in Florida in 2005. Chemical control against ACP was increased, harming natural enemy populations. This was a major intervention for the citrus agroecosystem that previously depended heavily on biological control, oils or selective insecticides. However, natural enemies continue to exist in citrus and other environments and help suppress ACP, CLM and other pests. They also keep several pests at less damaging levels, preventing their areawide spread.
PREDATORS OF MULTIPLE PESTS
Several naturally occurring predators are key to managing citrus pests in commercial groves and urban environments. Lady beetles are one of the largest groups, with at least 10 species that suppress citrus pests. They generally feed on multiple pests, which is beneficial for their survival and growth.
However, studies have also documented their impacts concerning specific citrus pests. Lady beetles such as southern twospotted lady beetle (Olla v-nigrum), metallic blue lady beetle (Curinus coeruleus), multicolored Asian lady beetle (Harmonia axyridis) and blood red lady beetle (Cycloneda sanguinea) were some of the first responders against the ACP invasion in Florida (Figure 1). They continue to reduce its populations. Southern twospotted lady beetle populations significantly increased in citrus groves following ACP infestations.
The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) conducted a study that spanned 17 generations of ACP after the discovery of HLB in Florida. The study found that lady beetles and other predators such as lacewings, spiders, syrphids, mirids and anthocorids reduced 80% to 100% of ACP immatures in developing colonies. These predators also prey on additional pests, including aphids, mites, scales, mealybugs, thrips and leafminers. They keep several of these pests at levels that are not economically damaging, at least on an areawide basis.
Green citrus aphid (spirea aphid) and brown citrus aphid (BrCA) were acceptable prey to seven species of lady beetles in citrus groves. These aphid species are known for their potential to transmit plant viruses, particularly BrCA, a highly efficient vector of citrus tristeza virus (CTV), that is a serious concern for Florida citrus. C. sanguinea and H. axyridis completed development and C. sanguinea laid eggs while feeding on an exclusive diet of BrCA, suggesting that besides preying, the aphids aid in the predators’ population growth. The presence of natural enemies such as lady beetles, lacewings and syrphids is one of the reasons that threats like BrCA are rare in Florida citrus.
CLM is another serious pest that is established in Florida citrus. Its larval feeding exacerbates the spread of citrus canker disease by providing opportunities for the casual pathogens of the disease to land and grow. CLM larvae are attacked by various predators, particularly ants, spiders and lacewings. Studies report the impacts of natural enemies on CLM populations in Florida. For example, natural mortality of its larvae averaged 85% to 89% in the southwestern region, of which about 60% was attributed to predation.
PARASITOIDS SPECIALIZING IN SPECIFIC PESTS
Parasitoids are tiny wasps that attack different life stages of a pest (egg, nymph or larvae) to disrupt their growth to the adult stage, which is responsible for producing the progeny and then spreading the disease. Most pests of citrus crops, such as ACP, CLM, scales, mealybugs, whiteflies and root weevils, are attacked by parasitoids, which are indigenous toFlorida or introduced from other regions of the world.
Two parasitoids of ACP, which are native to Asia, were introduced to Florida soon after the pest’s invasion. These parasitoids coevolved over time with ACP in tropical and subtropical Asia. Only the Tamarixia radiata parasitoid was established in Florida.Later efforts included additional importation of the same parasitoids from that region.
The mass-rearing facility established to raise this parasitoid by the Florida Department of Agriculture and Consumer Services Division of Plant Industry provided millions of adult T. radiata to commercial citrus producers and homeowners for release against ACP. A T. radiata female is capable of destroying 500 nymphs during her lifetime through a combination of host feeding and parasitization.
A survey of commercial groves shows that T. radiata’s contribution to natural mortality of ACP has increased threefold to fourfold during the last decade from less than 10% observed in 2006–07. T. radiata performs better in ACP management programs using organic or integrated approaches than in conventional programs.
Eight or more species of indigenous parasitoids of CLM have been reported in Florida. Most are generalist ectoparasitoids. Pnigalio minio dominates this native complex by 70% to 80%. Ageniaspis citricola, an egg-prepupal parasitoid from Thailand, was introduced from Australia; it was established in Florida soon after. Parasitism levels by A. citricola at monitored groves increased from 2% in May 1994 to 86% in October 1995, apparently unhindered by competition from native parasitoids. This parasitoid is well established in the state and still provides high levels of parasitism.
Other introduced parasitoids, Semielacher petiolatus and Cirrospilus ingenuus, also established, attack larval to pre-pupal stages of CLM and contribute to its natural mortality. Similarly, several other pests are attacked by indigenous or exotic parasitoids. For example, two introduced parasitoids of diaprepes root weevil were established in Florida, and parasitism rates ranging from 70% to 80% were recorded in South Florida.
ENTOMOPATHOGENIC FUNGI AND NEMATODES
A naturally occurring fungus, Hirsutella citriformis, that attacks ACP adults is observed in citrus groves. ACP adults had an average mortality of 23% and a maximum of 75% from H. citriformis. This fungus is unavailable commercially.
Other species of fungus such as Beauveria bassiana and Cordyceps fumosorosea were also identified as potential candidates and are under investigation. B. bassiana and C. fumosorosea are available commercially.
Research on controlling below-ground larvae of diaprepes root weevil using the commercially available nematode Steinernema riobrave demonstrated that its applications provided significant mortality of the root weevil larvae, averaging 80% to 95% compared to 15% to 40% in the untreated plots. See here for more.
Jawwad Qureshi (jawwadq@ufl.edu) is an associate professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.