Has the Asian Citrus Psyllid Parasitoid, Tamarixia radiata, Established in California?

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Has the Asian Citrus Psyllid Parasitoid, Tamarixia radiata, Established in California?

By Mark Hoddle | July 19, 2012

Tamarixia radiata (female)

The Problem: Tamarixia radiata, a tiny parasitic wasp has been imported into California from the Punjab of Pakistan to attack nymphs of Asian citrus psyllid (ACP), a serious citrus pest that has established wide spread populations in the counties of Los Angeles, San Bernardino, and Riverside (significantly smaller populations are known in Imperial and San Diego Counties too). Releases of Tamarixia commenced in December 2011 after USDA-APHIS cleared this natural enemy for release from the Quarantine Facility at the University of California Riverside. Since these initial releases in December, approximately 8,500 parasitoids have been released at about 50 different sites in Southern California by July 2012.

The Biocontrol Release Program: Parasitoid releases have been made in the cities of Azusa, Bell Gardens, Chino, Duarte, Fontana, Los Angeles, Mira Loma, Montclair, Ontario, Pico Rivera, Pomona, Rialto, Riverside, San Bernardino and Whittier. In these areas, citrus in residential gardens with ACP infestations were selected for parasitoid releases. Ideally release sites had lemons or limes, and other types of citrus too (e.g., oranges or grapefruit.) Lemons and limes are good hosts for ACP because they tend to produce a lot of flush growth that is favored by ACP females for egg laying, and lemons in particular, tend to produce flush growth more frequently when compared to other types of citrus (e.g., oranges.)

A mixture of different citrus types in gardens is desirable for a release site because it increases the possibility that there will be flush present on different citrus types at different times of the year that will be available for ACP can infest. We’ve also noticed that semi-regular pruning of citrus, good fertilization, and decent watering schedules can also help garden citrus produce strong flush.

Tamarixia Recoveries: Parasitoids have been recovered at about 4-6 release sites in Azusa and Bell Gardens, and some of these sites have not received parasitoid releases for 2-3 months suggesting that Tamarixia has likely established and is breeding on its own (the life cycle of the parasitoid is about 12-14 days depending on the temperature). DNA analyses suggest that the parasitoids that have been recovered from release sites have a unique genetic signature that is very similar to the parasitoids imported from Pakistan for the biological control of ACP in southern California. This result tentatively suggests that the parasitoids recovered from release sites are most likely those that were mass reared and released by the

Tamarixia radiata parasitizing an Asian citrus psyllid nymph in Bell Gardens Los Angeles County

University of California Riverside. Further, at one site in Azusa, it appears that Tamarixia has self-dispersed about 65 m from where it was released on citrus and it established new populations on ACP infested curry plants (curry plants are really good hosts for ACP too.) Additionally, the genetic signal from captured parasitoids is reasonably diverse which suggests that the foreign exploration, rearing, and release program at UCR has preserved a lot of genetic variation which could be very important for local adaptation by parasitoids to citrus growing areas from the coast to the inland valleys.

How Does Tamarixia Kill ACP? Tamarixia can kill ACP nymphs in two different ways. First is parasitism, and in this instance a female parasitoid lays an egg underneath a fourth or fifth instar (instar refers to the developmental stage of the nymph, so a fifth instar is the fifth nymphal stage before the nymph becomes an adult) nymph. These larger nymphs are most preferred by Tamarixia for parasitism. When the egg hatches, the parasitoid larva begins to feed on the under surface of the ACP nymph. Eventually the Tamarixia larva will completely excavate the body cavity of the ACP nymph, and it will pupate inside the empty shell of its host. Often you will see beige colored silk strands radiating out from the edge of a mummified ACP nymph. The parasitoid larva spins this silk to hold the ACP shell onto to the twig that the nymph was feeding on. This ensures that the husk of the host won’t fall off the twig prematurely exposing the parasitoid pupa to predators or inclement climatic conditions. Once the parasitoid has finished pupating, the adult wasp chews a perfectly circular hole near the head of the mummified ACP husk and the parasitoid emerges. After emergence it will mate and if it is a female, it will hunt for more ACP nymphs to attack. The presence of these emergence holes near the head of dried up ACP nymphs is very strong evidence that Tamarixia emerged from that host.

The second way Tamarixia can kill ACP nymphs is by host feeding. When Tamarixia host feeds, the female uses her ovipositor or egg laying tube at the posterior end of her abdomen to stab and mutilate the ACP nymph. This physical injury causes hemolymph (the equivalent of insect blood) to leak from the body and the parasitoid feeds on this fluid. Hemolymph is an important source of protein for female parasitoids, and the trauma of being stabbed then feed upon is sufficient to kill ACP nymphs. Only females can attack ACP nymphs in this manner because males lack an ovipositor because they don’t lay eggs.
Laboratory studies on the biology of Tamarixia suggest that through the combined actions of parasitism and host feeding, individual female parasitoids have the capacity to kill several hundred ACP nymphs during their life time.

Argentine ants tending an infestation of Asian citrus psyllid nymphs. Ants may hamper biological control of ACP by Tamarixia

Ants and ACP Biocontrol: At some release sites, ants, in particular the invasive Argentine ant, may have the potential to interfere with the biological control of ACP. Field observations strongly suggest that ants tend ACP nymphs and as a reward for guarding them, the ACP provide the ants honeydew, a sweet waste product that they excrete. We’ve also seen ants capture and eat Tamarixia parasitoids foraging in clumps or patches of ACP nymphs, and in some instances the ants have chased Tamarixia off the patch if they could not catch it. It is possible that when ACP infestations are heavily tended by ants, some sort of ant control may be needed if the natural enemies are to attack the pests. This problem is not unique to ACP, honeydew producing scales and mealybugs, for example, are also tended by ants, which in turn hampers effective biological control of these pests too.

Future Plans: Monitoring and release programs are ongoing, and UC Riverside is now ramping up the mass production of Tamarixia for expanded releases throughout ACP infested zones. It is hoped that as more Pakistani Tamarixia are released in southern California greater establishment rates will occur and natural spread will begin to fill in areas between release sites.

Topics: Asian Citrus Psyllid, Mark Hoddle, Tamarixia radiata | 4 Comments »

4 Responses to “Has the Asian Citrus Psyllid Parasitoid, Tamarixia radiata, Established in California?”

  1. Residential & nursery Asian citrus psyllid blog: Biological control of Asian citrus psyllid Says:
    July 23rd, 2013 at 3:52 pm

    It is important to make sure natural enemies attack primarily (ideally only) ACP. One of the most promising of these wasps is Tamarixia radiata . This tiny insect, which poses no threat to people, has a strong preference for ACP. This is really useful information on the ACP-Tamarixia program in Southern California.

  2. Randy Black Says:
    April 29th, 2014 at 9:33 am

    Beautifully written article. Wishing you the best in this important effort against ACP-HLB to protect California’s citrus industry from the fate of Florida’s.

  3. Al Says:
    November 1st, 2014 at 6:45 pm

    My comment is great work! I am pleased to know about this so I can protect my plants from getting the disease. They should provide the Tamarixia for household growers to kill ACP to avoid the disease. Prevention is easier than eradication of the plant once it is diseased as this will cost thousands of dollars for home owners as well as millions for citrus farmers.
    Where can I get these beneficial insects? It is easier to implement control now versus planning a program later that may hit pest targets and kill beneficials. I would be happy to pay for Tamarixia for release in our area. Any help we can get is great as I love citrus fruits and if we do not make large efforts to eradicate this disease it means high danger and threatens the availability of an important food stuff for us human beings. We must act like it is Ebola with focus strong control efforts and continued research of this disease associated with this bug that spreads it.

  4. Mark Hoddle Says:
    November 2nd, 2014 at 7:56 pm

    Thank you for your post – I am glad that you found this information useful. Tamarixia is being mass reared by the California Department of Food and Agriculture and more than 750,000 of these ACP parasitoids have been released in southern California for the biocontrol of ACP. There are efforts underway to commercialize this natural enemy, but these companies are making slow progress and it is uncertain as to when Tamarixia will be commercially available. At this time CDFA and UC Riverside can not sell Tamarixia. Instead, we are releasing this natural enemy in urban areas and organic citrus orchards where ACP is present and computer models suggest that there is a high risk of HLB being present in that area. I agree with you that we must keep up a sustained effort in our fight against ACP and HLB. Biocontrol is just one part of new management plans that are being developed to save California citrus from this very serious threat. Scientists at UC Riverside and in Florida are working on developing better traps to monitor ACP, we have screened another natural enemy collected in Pakistan for safety, and it is likely that this will be released in southern California in November 2014, plant breeders are hoping to develop new citrus varieties that are resistant or tolerant to ACP and/or HLB. These are all medium to long-term projects. I am confident that California will develop new management plants for ACP-HLB and our iconic citrus industry will survive this threat and improve because of it.


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