The natural way to control a pest species is through the use of a biological control (biocontrol) agent; one of the 3 Ps: predator, parasite or pathogen. Biocontrol is the normal ecological method nature uses for controlling the populations of native species. Within a local environment you find predator, prey, parasite and pathogen evolving alongside each other; their interactions keep each species’ population in balance.
Some native species may be annoying to us, but pest species are generally the alien invasive species, introduced either deliberately or accidentally to a geographical location they have never been before. Without the predators, parasites and pathogens of their native environment, their population can explode exponentially.
The Asian citrus psyllid (Diaphorina citri Kuwayama) is a herbivorous insect that drinks the phloem (sap) of several species of trees and shrubs in the taxonomic family Rutaceae, also known as the Citrus family. They particularly favor many of the Citrus species (spp.) and at least a couple of the Murraya spp., including the ornamental orange jasmine (Murraya paniculata).
Most trees can support a mild infestation of psyllids, although it will slow growth. However, the Asian citrus psyllid is a primary vector (carrier species) for three of the Liberibacter species of bacteria that cause “huanglongbing”, Chinese for yellow dragon disease, known more often as citrus greening disease or yellow shoot disease in the West. This can cause problems for citrus growers in its native China, elsewhere it can be a serious threat to the commercial viability of citrus orchards. They have been fighting them in India since the early 1970s, using a variety of insecticides and management practices, but recently die-back of citrus trees has been increasing, seriously threatening India’s citrus industry.
Asian citrus psyllids were detected in Florida in the USA on June 2, 1998. A survey at that time found them established in 12 counties of the southeastern citrus growing region. By 2001 it had spread to 28 counties in Florida and had been found in Texas, apparently hitchhiking a ride on a shipment of orange jasmine plants from Florida to Texas. By 2006 it was throughout the citrus growing region of Florida and found in 32 counties in Texas; in May of 2008 it was found in the Jefferson and Orleans parishes of Louisiana. It is also found throughout the islands of Guam, Hawaii and Puerto Rico. In 2005 it was confirmed that citrus greening disease was established in Florida.
Natural predators of Asian citrus psyllid include the larvae of some species of both lacewing and syrphus flies, at least a dozen species of coccinellid beetles, commonly called ladybirds, and several parasitic wasp species. Hirsutella citriformis Speare and Isaria fumosorosea Wize are two species of fungi that cause disease in both the instars (growth stages) and adults of Asian citrus psyllid.
The best form of biocontrol would be to encourage a native species to predate on the alien invader. Introducing another alien species to control the first needs to be carefully researched and considered to avoid possible problems. The main difficulty comes in finding one that will, at least predominantly, attack the pest species in the region where it has become a problem and won’t become a worse pest species itself. The cane toad (Bufo marinus) situation in the state of Queensland in Australia is one of the more notorious examples of failing, badly, to do that.
The introduction of cane toads to Queensland in 1935 was intended to be a biocontrol of French’s canegrub (Lepidiota frenchi
Blackburn) and the greyback cane beetle (Dermolepida albohirtum), which were having significant detrimental impacts on their commercial sugarcane production. The cane toads took to their new home well, and have steadily spread ever since. Unfortunately, the cane beetles are not a significant component of their diet, instead they are eating their way through endemic Australian species while poisoning others that might try to eat them, and have caused a number of species extinctions already. Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) is still trying to find some way to at least slow their steady progress across the nation, actually controlling their numbers would be a dream outcome; their eradication is a virtual impossibility. And the cane beetles are still a problem for farmers with sugarcane plantations.
So introducing a biocontrol to naturally manage the Asian citrus psyllid population is not a decision to make lightly or quickly. In Florida, the decision was made to investigate using two parasitic wasp species with the scientific names Tamarixia radiata Waterston and Diaphorencyrtus aligarhensis; breeding populations were obtained from Taiwan and Vietnam on October 21, 1998. T. radiata was chosen because they have been used previously on the islands of Reunion, Mauritius and Taiwan, where they are considered to do well as a biocontrol.
They were kept in high security quarantine while being cleared of carrying any of the Liberibacter bacteria species themselves. Permission for field release of T. radiata was received on July 12, 1999, and the first wasps were released on July 15, 1999. Permission for field release of D. aligarhensis was received on March 10, 2000. Further releases occur annually, in 2005 approximately 24,000 T. radiata and D. aligarhensis parasitic wasps were raised and released.
What, if any, research was done on the ecological impact of introducing these species into the US is not clear. T. radiata, at least, is also extant in Texas, although not deliberately, it hitched a ride in the same orange jasmine shipment in 2001 that snuck the Asian citrus psyllid in. There were just four months between the discovery of the psyllid and the importation of the parasitic wasps. It must be recognized that citrus greening disease is a significant threat to US citrus crops and obviously the relevant authorities wasted no time in getting to work on the problem. It might be hoped that some ecological impact studies were done in the eight months they were in quarantine before release. If so, links to the resultant reports on Florida’s Department of Agriculture and Consumer Services’ Division of Plant Industry’s website would be appropriate.
It would have been more prudent to consider species native to or already extant in Florida as possible biocontrol agents first, rather than going straight for another alien species. Hopefully it wasn’t just a panic reaction. It would certainly be worthwhile investigating the potential of native ladybirds for a start. Both of the fungi species, H. citriformis and I. fumosorosea, are native to Florida; it was research in the US from 2005 through to 2008 that found these fungi could cause disease in the psyllids resulting in a high mortality.
There is a history of human agencies jumping in with a biological control for dealing with a problem species, too quickly, with too little research and seemingly with too little thought. I just hope the introduction of these parasitic wasps doesn’t turn out the same. Considering the spread of Asian citrus psyllids, the responsible agencies do not appear to be managing the situation very successfully so far.
Sources:
Florida Department of Agriculture and Consumer Services Division of Plant Industry. (2007) Bureau of Methods Development & Biological Control.
http://www.doacs.state.fl.us/pi/methods/classicbio.html
Hoy, M., Nguyen, R. & Jeyaprakash, A. (2006) Classical Biological Control of Asian Citrus Psyllid in Florida.
http://ipm.ifas.ufl.edu/agriculture/citrus/psyllid.shtml
Mead, F. (2008) Featured creatures. Common name: Asian citrus psyllid.
http://creatures.ifas.ufl.edu/citrus/acpsyllid.htm
Meyer, J., Hoy, M. & Boucias, D. (2008) Isolation and characterization of an Isaria fumosorosea isolate infecting the Asian citrus psyllid in Florida. Journal of Invertebrate Pathology 99(1): p96-102.
Meyer, J., Hoy, M. & Boucias, D. (2007) Morphological and molecular characterization of a Hirsutella species infecting the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), in Florida. Journal of Invertebrate Pathology 95(2): p101-109.
Roberts, L. (2006) Unpublished Wild Animal Population Management lectures.
