An Interview with Mark Hoddle
What is citrus greening, and why are people worried about it?
Huanglongbing (HLB), more commonly known as citrus greening, is a bacterial disease that kills citrus. HLB is spread by a sucking insect, the Asian citrus psyllid (ACP). ACP and HLB are thought to be native to the Indian subcontinent, and humans have inadvertently spread them to nearly all major citrus-producing regions. In Florida, for example, ACP was discovered in 1998, HLB was found in 2005, and more than 60,000 acres of citrus have been lost to this disease. It has been estimated that 6,600 jobs have been lost, and the impact on the Florida citrus industry has been valued at $1.3 billion. ACP was found in California in 2008, and HLB was detected in 2012. California citrus growers are very worried that the situation in Florida will repeat itself there. There is no cure for HLB.
What is one method that could be used in organic systems to control citrus greening without the use of pesticides?
One method to reduce densities of ACP that spread the bacterium causing citrus greening is biological control, which is the use of natural enemies, like parasites, predators, and pathogens, to suppress vector population growth. This is not a silver bullet solution to the ACP-HLB problem as natural enemies will not kill 100 percent of the psyllids and it is likely that some of the survivors could be infected with HLB-causing bacteria. However, by reducing pest population densities, other control practices (such as the use of organic materials) may be more efficacious.
In 2010, you went to Pakistan to look for natural enemies of the citrus psyllid. Why did you choose Pakistan?
Surveys for natural enemies in California, especially parasitoids that attack ACP nymphs, indicated that these life stages were not being parasitized, and this lack of attack may have been one reason why ACP populations were so high in southern California. Finding ACP-specific parasitoids and introducing them in California may provide control that is currently absent.
Punjab Pakistan is thought to be part of the home range of ACP and HLB. Biological control theory states that natural enemy diversity should be highest in the center of origin for a pest that is the target of a biological control program, as pest and its natural enemies have had the longest time to co-evolve together. The second reason Punjab Pakistan was selected was due to this region having a 70 percent climate match with the major citrus production regions of California. When natural enemies are imported from other parts of the world for use in a biological control program, they ideally should be pre-adapted to the prevailing climate of the area into which they are to be introduced.
Did you find any natural enemies in Pakistan that could be useful in the United States?
Two species of parasitoid were found attacking ACP in Pakistan: Tamarixia radiata and Diaphorencyrtus aligarhensis. Safety tests conducted in quarantine over an 18-month period indicated that T. radiata provided little undue risk to the environment. An Environmental Assessment Report was prepared for USDA-APHIS to review. This concluded that T. radiata should be released in California for the biological control of ACP. Similar studies have been completed for D. aligarhensis, and USDA-APHIS is currently reviewing the Environmental Assessment Report for this parasitoid.
What hurdles do you think the wasps you collected might face when released into the wild, and what steps did you take to avoid them?
Preserving genetic diversity of natural enemies is a big challenge in a biological control program. This is because biological control agents may pass through genetic bottle necks when relatively few individuals are collected, and then these small populations are maintained in quarantine for long periods. A reduction in genetic diversity may result in natural enemies that are not very effective. To avoid this potential issue, we made six collecting trips to Pakistan over two and a half years. In quarantine, natural enemy populations were kept separate from each other and were not allowed to interbreed. We established 17 lines of parasitoids that represented unique locality and collection times. When releases into the field are made, individuals from these lines are removed from the respective cages and put into a large mating cage where random mating occurs. This helps to reconstitute the genetic variation captured from different places and times in Pakistan. These hybrids are then released into the field. It is assumed that natural selection will select for genetic strains of ACP parasitoids best adapted to different citrus-growing areas in California (e.g., hot arid desert interiors to cooler, more humid coastal zones).
Do you have any information you can share with us about how the released parasitic wasps are doing?
So far, more than 300,000 t. Radiata have been released in California at more than 400 different release sites. Parasitoids have been recovered at some release sites. DNA testing has confirmed the Pakistani origin of the natural enemies, and tests indicate that there is quite a bit of genetic diversity in recovered parasitoids. Parasitism rates have been as high as 80 percent in some areas, but tend to be averaging about 25 percent across all sites. Additionally, some parasitoids have been found attacking psyllids up to eight miles from the nearest release site, which suggests that they can spread pretty well on their own.