Figure 1: Adult Asian citrus psyllid (ACP) and nymphs (Warnert, 2013) [above]
Contributors: Ing. Helen Theresa Choco, Manuel Garcia, Veronica Manzanero-Majil
The presence of the Asian Citrus Psyllid (ACP), a tiny insect about 4 mm in size (figure 1) was first detected in Belize in 2005. Later, in 2009, the presence of Huanglongbing (HLB) (formerly citrus greening) was confirmed in Belize. ACP is the most efficient vector responsible for the spread of HLB in the Americas. Considering the potential gravity of HLB based on experiences from other countries, the Citrus Research and Education Institute (CREI), the research arm of the Citrus Growers Association (CGA) in collaboration with the Belize Agricultural Health Authority (BAHA), the International, Regional Organization of Animal and Plant Health (OIRSA) and other interested parties adopted major parts of the three pronged approach for the management of HLB from China, Brazil and other countries battling the disease. This three-pronged strategy to manage HLB evolved into a four-pronged approach in 2013 with the interventions of a technical cooperation program funded by the Food and Agricultural Organization (FAO) of the United Nations. Through this assistance from FAO, the use of a biological control program to manage the ACP vector of HLB was integrated into the four-pronged approach to manage the disease.
The four components of the management strategy for HLB are to (1) remove infected trees in groves with low incidence of HLB, (2) suppress ACP population using chemical and biological control methods in citrus groves and biological control exclusively for backyard trees, (3) use certified citrus plants for starting new plants or for replanting, and (4) use a robust fertilizer management programme and improve control of other pests (especially Phytophthora, Leprosis, and greasy spot) in the groves and backyard citrus trees.
Now let’s take a look at what biological control really is! Biological control is the beneficial action of parasites, pathogens and predators in managing pests and their damage. This group is the primary group used in biological control of insects. Bio-control provided by these living organisms, collectively called “natural enemies”, is especially important for reducing the numbers of pest insects (Dreistadt, 2014). Most parasites, pathogens and many predators are highly specialized and attack a limited number of closely related pest species (Dreistadt, 2014). In the case of HLB management, a tiny wasp called Tamarixia radiata is the natural enemy of ACP. This wasp is present in Belize.
According to Dresitadt (2014) types of natural enemies include:
Parasites: an organism that lives and feeds or develops in or on the host’s body. Adult females such as wasps feed on and kill their hosts but often only the immature stage of the parasite feeds on the host. NOTE: Although the term “parasite” is used in bio-control, true parasites do not typically kill their hosts. Species useful in bio-control kill their hosts; they are more precisely called “parasitoids”.
Pathogens: microorganisms including certain bacteria, fungi, nematodes, protozoa and viruses that can affect and kill host.
Predators: insects that kill and feed on several/many individual prey during their lifetime. Predatory lacewings and wasps feed on various pest insects or mites.
At CREI, the biological control program aims at mass producing parasitoids and entomopathogens to combat the ACP vector of HLB, particularly in backyard citrus and unmanaged citrus groves. Currently, the two major components under the biological control program are (1) mass production of the parasitoid T. radiata and (2) the collection and culturing of entomopathogens such as Hirsutella citriformis and species of the Isaria and Bovaria genus.
Mass Production of the Parasitoid Tamarixia radiata
Figure 2: Adult Tamarixia radiata
female (above) and male (below) .P
n October 2013, the Belize CGA, in collaboration with the FAO, established facilities at the CGA’s compound to start the rearing of the parasitoid Tamarixia radiata (figure 2). The program entails the mass production of the Tamarixia wasp on its host, the ACP reared on Murraya paniculta (orange jasmine). After an adult female Tamarixia wasp mates with an adult male and becomes fertile, it deposits an egg in the lower abdomen of the fourth and fifth instars (developmental/juvenile stages) of the ACP nymph. The nymph is killed as the wasp develops inside and then exits by making a hole on the top area of its thorax. The adult Tamarixia wasp also feeds on the first to the third instar stages of the ACP. A single female Tamarixia wasp can kill up to 500 psyllids in its life span. Currently, CREI has a clean collection of orange jasmine plants and an HLB-free colony of psyllids. Work is now underway to scale up the production of Tamarixia wasp at CREI’s facilities. Figure 3 shows the rearing facilities for T. radiata at CREI donated by FAO.
Figure 3. Facilities donated by FAO for rearing T. radiata at CREI. (CREI/CGA 2014)
Entomopathogen Collection and Production
The collection and production of entomopathogens aims at determi ning the different pathogenic fungi that attack and kill ACP in various areas of Belize. The fungi that are identified to cause the highest mortality rate of ACP are selected and cultured in large scale in the laboratory for application by means of spraying in backyard citrus trees and unmanaged and abandoned citrus groves to control ACP populations.
Figure 4: Hirsutella citriformis on ACP cadaver
Psyllid cadavers (dead psyllids killed by pathogenic fungi) are collected from groves and brought to the laboratory for identification of the fungal pathogen (figure 4). Fungal spores from the psyllid cadavers are isolated with the use of a microscope and cultured in special media conducive to growth of fungal pathogens. Upon obtaining a clean fungal culture, scale-up culturing is done using a rice medium. When sufficient spores are obtained, they are collected and released onto backyard trees and groves by means of sprays. If mortality is observed within five to seven days after the application is made, the fungal pathogen is considered to be effective in controlling ACP. Currently, CREI is conducting laboratory trials with two fungal pathogens already identified in Belize and confirmed to kill ACP, the Hirsutella citriformis and the Isaria fumosarosea.
Biological control of ACP using Tamarixia radiata to suppress ACP in Belize is a complement to the already existing ACP control program using insecticides, a component of the four-pronged approach to HLB management. While the use of Tamarixia radiata is a more environmentally friendly approach to controlling the ACP, experiences from other countries that use biological control on its own in citrus groves have shown that this practice is not sufficient to fully suppress ACP population to a level required to successfully manage HLB. The Tamarixia wasp population will always lag behind the ACP population because in nature the wasp has to ensure its survival and thus will seldom eat out all its food. One exception to this example occurred in the Reunion Island as reported by Etienne & Aubert, 1980 whereby conditions on the island and in the grove were more controlled.
Etienne J., Aubert B., 1980. Biological control of psyllid vectors of greening disease on Reunion Island. Proceeding of 8th Conference IOCV, IOCV, Riverside 1980, 118-121.
Dresitadt, S.H. Biological Control of Natural Enemies. 2015. University of California Statewide IPM Program. Agriculture and Natural Resource. University of California. Publication 74140
Hoy,M. Nguyen,R. Jeyaprakash,A. 2006. Classical Biological Control of Asian Citrus Psyllid in Florida. Univeristy of Florida. IFAS Extension. Integrated Pest Management Florida. Gianesville, FL.
Warnert, J. E. 2013. Newly Found Asian Citrus Psyllids Prompt Quick Action in Tulare. ANR News Realeses. Published August 2, 2013. Division of Agriculture and Natural resources. University of California.