Opinion: how parasitoids can fight HLB-spreading insects
By University of California, Riverside, Centre for Invasive Species Research specialist and principal investigator Dr Mark Hoddle
Asian citrus psyllid (ACP), is a small four millimeter long juice-sucking insect native to central and southeast Asia. ACP will feed and breed on most types of citrus such as lemons, limes, Navels, Valencias, grapefruit, and citrus relatives like sweet orange jasmine or mock orange (Murraya paniculata) and Indian curry (Bergera koenigii).
ACP has gained notoriety because of its ability to acquire and spread a non-culturable bacterium, Candidatus Liberibacter asiaticus, that is lethal to citrus; some infected varieties of citrus can die in about five to eight years after infection. The disease caused by this bacterium is known as Huanglongbing (HLB) which is Chinese for yellow dragon disease and refers to symptoms of irregular yellow mottling of leaves of infested tree or citrus greening because fruit don’t ripen properly.
ACP and HLB, or a similar disease caused by a related bacterium, are now global pests of citrus with infestations occurring throughout Asia, parts of the Middle East, for example the Arabian Peninsula, Africa, the Caribbean, Central and South America, Mexico, the southeast and southwestern U.S., Australia, and the smaller New Zealand citrus market, appears to be one of the last citrus growing regions to be free of this vector-disease combination.
ACP and HLB are widespread in Mexico with infestations reported from around 17 of Mexico’s 23 citrus producing states (74%), and the disease is widespread in the state of Veracruz, the largest citrus producing state with 450,00 acres of citrus (about 48% of Mexican production). In the state of Colima, some estimates suggest that 40% of orchards are infected and growers are being advised to transition to alternative crops. This problem is only expected to worsen as producer, federal, and state government resources are limited in Mexico to combat ACP and HLB.
Even large growers who can afford pesticides and apply them regularly face an uphill battle with ACP dispersing from urban areas, small untreated groves, or abandoned properties into commercial areas. Research from Florida has demonstrated that abandoned citrus groves are significant sources of ACP and HLB that threaten surrounding commercial production areas and well coordinated area-wide management programs are needed if ACP and HLB are to be effectively managed.
ACP has been present in California since 2008. There are two infestation zones, a small contained one near the California-Mexico border, for example, parts of San Diego and Imperial Counties, and a much larger and uncontrollable urban infestation whose epicenter is Los Angeles County from which ACP has spread into urban citrus in the neighboring San Bernardino and Riverside Counties. The leading edges of this large urban invasion in Los Angeles are being aggressively treated with pesticides by the California Department of Food and Agriculture in an attempt to stop ACP spreading into the highly important commercial production areas of the Ventura County, and the Central and Coachella Valleys.
This ACP control program has taken on an even greater sense of urgency following the first discovery of HLB in Los Angeles County in April 2012. One infected tree was found, and it has been uprooted and destroyed. All citrus within an 800 meter radius have been treated with pesticides (1,442 properties were surveyed for citrus and 878 were treated with pesticides, a 99.9% treatment success), and additional ACP and plant material has been collected and analyzed for HLB. So far no additional HLB infestations have been detected as a result of these surveys. However, it is expected that another HLB detection will occur and it is only a matter of time now before this happens in California.
An alternative approach to controlling ACP in urban areas of Los Angeles that are no longer under the spray program has been to employ natural enemies of ACP, in particular, host-specific parasitoids from the Punjab of Pakistan that attack the nymphs of ACP. Parasitoids attacking ACP have been collected from this region on five different occasions by the author and tested for safety in quarantine at the University of California, Riverside. Releases of more than 6,000 parasitoids have recently occurred at almost 40 different release sites in Los Angeles and it appears that from these initial efforts the parasitoid Tamarixia radiata may have established in about 20% of the release sites and in one instance, it may have already move about 100 meters on its own to nearby ACP infestations. While not a silver bullet solution, biocontrol may be the best tool available to California to reduce ACP populations in heavily infested urban areas which in turn has the potential to reduce migration pressure towards commercial production areas.
California is not the only region pursuing biocontrol of ACP with parasitoids that attack the nymphs. Some of the first biocontrol programs against ACP using T. radiata were started in Reunion Island, Taiwan and China. Similarly, programs have been running in Florida since the 1990s. Interest in parasitoids, especially T. radiata, has risen in Mexico which, in the absence of large-scale pesticide application programs, has invested in mass production and releases of natural enemies to help small farmers combat this pest. Proactive biocontrol efforts are also underway in parts of Central America such as Costa Rica and South America, for example Brazil.
Future biosecurity risks
It is unlikely that commercial citrus will go extinct because of ACP and HLB - many countries with endemic populations of these pests still have commercial citrus industries for example, Pakistan, India, and China. As the ACP-HLB problem continues to spread citrus industries around the world will adapt with the development of new management programs, which will use novel combinations of natural enemies, pesticides, nutritional programs, and new citrus varieties that are either tolerant or resistant to ACP and/or HLB. One thing to keep in mind is that invasive species are an ever present menace.
There are many pests and diseases of citrus that we know about, but there are also the unknowns, the wildcards, organisms that we don’t know much about until they invade and cause problems. Invasive species issues are increasing at unprecedented rates and greater preparedness and proactive work, that meet identifiable problems head on before they arrive should be encouraged.