Article written by Mark Hoddle.
Revised August 31, 2018.
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What is biological control?
Biological control is the intentional use by humans of natural enemies, predators, parasitoids, and pathogens to reduce pest populations to less damaging levels. Classical biological control is the importation and release of a natural enemy species into an area where it is lacking. An example of classical biological control is the importation, release, and establishment in California of Tamarixia radiata, a parasitoid that attacks Asian citrus psyllid (ACP) nymphs, an invasive pest infesting citrus. The parasitoid was collected from Punjab Pakistan, which is part of the native range of ACP.
Punjab Pakistan was selected for foreign exploration because it has a very good (about 70%) climate match with major citrus production areas in California. Following release from quarantine in December 2011, Tamarixia established readily in California’s urban areas, it spread naturally into ACP-infested areas up to eight miles from the nearest release sites, and it has provided significant control of ACP.
Studies across numerous sites over several years indicate that ACP densities have declined in urban citrus by at least 70% and Tamarixia is one of the dominant natural enemies contributing to this mortality. Predatory hover fly larvae are important native natural enemies of immature ACP.
How does Tamarixia kill ACP?
(Fig. 1) Female Tamarixia lay eggs underneath the ACP nymph (A & B), the egg hatches, and the parasitoid larva consumes the inside of its host (C). The parasitoid larva pupates inside the shell of the ACP nymph (D & E), and after pupation is complete, the adult parasitoid chews a circular exit hole in the head region of the nymph and emerges (F). Tamarixia can also kill ACP nymphs by eating them, this is referred to as host feeding. To host feed, a female parasitoid uses her ovipositor to mutilate the nymph. This causes insect “blood” to leak from the host. The female parasitoid feeds on this liquid which provides protein for egg maturation. The trauma of being stabbed and fed on is sufficient to kill the ACP nymph.
Who is working on the Project?
The Joint Agency Biocontrol Taskforce for ACP biocontrol in California is comprised of team members representing UC Riverside (Mark Hoddle, Beth Grafton-Cardwell, Matt Daugherty, and Richard Stouthamer), CDFA (David Morgan, Victoria Hornbarker, and Mike Pitcairn), Citrus Research Board Scientists (Ruth Henderson, Raju Pandey, and Rick Dunn), Cal Poly Pomona (Anna Soper and Valerie Mellano), citrus growers (Jim Gorden and John Gless), pest control advisors (Joe Barcinas, Jim Davis, and Brett Chandler), and USDA-CPHST (Greg Simmons).
What are the challenges and opportunities?
An additional challenge is that ACP natural enemies often lack food in citrus orchards, especially nectar and pollen, and the absence of these resources reduces their life span and how many pests they can kill. Flowering plants, like alyssum (Lobularia maritima) and buckwheat (Fagopyrum esculentum), attract adult hover flies. Experiments have demonstrated that the presence of flowering plants significantly increases mortality of ACP nymphs by hover fly larvae (Fig. 3). More work is needed to determine the feasibility of growing flowering plants to attract hover flies in citrus orchards.
Funding source: ACP biocontrol has been supported by funds from the Citrus Research Board, USDA-Multi Agency Coordination, USDA-Citrus Health Response Program, and UCR’s Office of Research and Economic Development.