Classical Biological Control of Asian Citrus Psylla - Update on Tamarixia radiata releases and first releases of Diaphorencyrtus aligarhensis

By: Marjorie A. Hoy, Ru Nguyen, and A. Jeyaprakash

The Asian citrus psyllid, Diaphorina citri, was first discovered in two counties in south Florida in June 1998. It had spread to 12 counties by July 1999, and as of May 2000 can be found in 21 counties. This pest is now well established and is expected to colonize all of Florida's citrus. The psyllid also feeds on orange jasmine (Murraya paniculata), which is commonly planted as hedges in south Florida.

The Asian citrus psyllid can transmit the bacterium that causes greening disease, which is considered the most serious disease of citrus in Asia. Greening disease results in reduced fruit production and death of the trees, typically within 6-8 years after infection. If greening disease ever is found in Florida, this vector could rapidly disperse it throughout the state. Both adults and nymphs of the Asian citrus psyllid feed on young foliage. High populations of psyllids feeding on young flush can kill the growing tip while moderate populations can distort shoots and leaves. The psyllids also produce wax and honeydew, which allows the growth of sooty mold.

Classical biological control programs to suppress foreign insect populations take several years to complete because they involve identifying the pest, identifying potentially effective natural enemies in the country or region of the pest's origin, importing the natural enemies, evaluating them in quarantine, applying to regulatory agencies for permission to release the natural enemies after carrying out a risk analysis, mass rearing, release, monitoring for establishment, and monitoring to determine effectiveness of the released natural enemies.

Two parasitoids of this psyllid were imported into quarantine soon after the pest was first detected in Florida. The two host-specific parasitoids were imported from Taiwan and Vietnam in October 1998. The purpose of this paper is to provide an update on the project, which will take several years to complete.

Update on Status of Tamarixia radiata

A colony of Tamarixia radiata (Eulophidae) was obtained from both Taiwan and Viet Nam and each was reared separately in quarantine facilities in Gainesville, Florida. Permission to release the two T. radiata colonies was obtained on July 12, 1999, as reported in the September 1999 issue of Citrus Industry. Between July 15 and December 1999, approximately 12,000 T. radiata from both colonies were released in citrus and orange jasmine plantings where psyllid populations were present in Boynton Beach, Clewiston, Del Ray Beach, Ft. Pierce, Homestead, Indiantown, Miami, Moore Haven, and Port St. Lucie.

Females of T. radiata have a high reproductive rate and a female can deposit up to 300 eggs Single eggs are deposited on a psyllid nymph and the newly hatched parasitoid larva sucks fluid from the site where it attaches to the host, eventually killing it. By the time the parasitoid has matured, it has sucked out the contents of the psyllid and the psyllid has 'mummified', turning a characteristic dark brown color. The adult parasitoid emerges through a hole in the top of the thorax. Adult females of T. radiata also kill psyllids by host feeding; females insert their ovipositor into nymphs of the psyllid to make a hole, then suck up the liquid that oozes out. Host feeding provides the parasitoid female with protein so that she can deposit eggs. Thus, a single T. radiata female is able to kill up to 500 psyllids by a combination of host feeding and parasitism.

Tamarixia radiata is an effective parasitoid of the citrus psylla, Diaphorina citri Kuwayama, in Asia. These psyllid nymphs have been killed by the feeding of the immatures of this eulophid ectoparasitoid.

Monitoring of 1999 release sites at the end of the 1999 growing season indicated that T. radiata persisted and reproduced at several locations near Boynton Beach and Delray Beach during 1999. Additional releases of both colonies have been, and will be, made during the 2000 growing season. Evidence of persistence of T. radiata also was obtained in May 2000 in one site near Miami, but confirmation that this parasitoid has established has been difficult due to the drought during the spring of 2000, which has reduced flushing of both citrus and orange jasmine plants. Without young flush upon which the psyllid can oviposit, host populations (and thus potential parasitoid populations) have remained low. Confirmation of the establishment of T. radiata requires additional monitoring, but the persistence of T. radiata is a promising sign.

Quarantine Evaluation of Diaphorencyrtus aligarhensis

A colony of D. aligarhensis (Encyrtidae) was imported into quarantine from Taiwan in fall of 1998, with extra care taken to avoid the accidental importation of greening disease. For example, no plant material was imported into the quarantine and only adult parasitoids reared from psyllids reared on orange jasmine, which is considered an unsuitable host for greening disease, were imported. Once the D. aligarhensis colony was established in quarantine, multiple polymerase chain reaction (PCR) tests were conducted to determine if the greening disease agent was present. No parasitoids were ever found to be positive for the greening disease agent by these sensitive molecular tests.

Permission to release D. aligarhensis was obtained on March 10, 2000. Some D. aligarhensis were released during spring 2000 and additional D. aligarhensis will be released throughout the areas of Florida that have Asian citrus psylla. Some releases will be made in orange jasmine because it is often grown as hedges and pruned, making flushes (and thus host psyllids) regularly available to the parasitoids. Citrus groves will serve as release sites if the owner agrees to eliminate toxic pesticides for one year (oil and copper are permitted because they are relatively nontoxic), allow researchers back into the orchard to monitor establishment of the parasitoids, and provide approximately one to two acres for this purpose so that a buffer zone is present to reduce potential mortality by toxic pesticide spray drift in the adjacent citrus trees. The most favorable release sites in citrus groves may be where trees of mixed ages are present so that flushes can overlap, which helps to maintain host populations.

This adult female of Diaphorencyrtus aligarhensis inserts an egg into the nymphs of citrus psyllid, Diaphorina citri Kuwayama.

D. aligarhensis is being reared on Asian citrus psyllid, which is reared on orange jasmine because the psyllid's feeding is less detrimental to that plant than to citrus and we can maintain higher psyllid populations. Orange jasmine reportedly does not maintain greening disease, which provides an additional level of safety regarding accidental importation of greening with the parasitoids.

Biology of D. aligarhensis

D. aligarhensis is an endoparasitoid that kills psyllids, as does T. radiata, by two mechanisms: parasitism and host feeding. D. aligarhensis females prefer to oviposit within fourth instar psyllids, but females will host feed on the first through fourth instars. Each female, under laboratory conditions, can deposit up to 280 eggs. Adult D. aligarhensis females are relatively long lived, with some living as long as 100 days at 25ºC. One generation requires about 18 days. Once the parasitized psyllid becomes mummified, it turns a dark brown and adults emerge from the dorsal aspect of the abdomen. This will make it easy to determine in the field whether parasitism of the psyllid was by D. aligarhensis or T. radiata, which emerges from the dorsal aspect of the thorax.

The Taiwan colony of D. aligarhensis appears to have an unusual sex-determining system. The colony we imported from Taiwan has females only, but in Vietnam, this species has both males and females. The reason for the differences in sex determination in the different D. aligarhensis populations may be due to infection of the Taiwan population with a bacterium called Wolbachia. The use of all-female parasitoid strains with Wolbachia has been proposed as a method of improving biological control, because such populations will have higher population rates of increase and higher stinging rates, they are likely to be better colonizers and more easily established at low population densities because they don't need to find a mate they could be less costly to produce in mass-rearing programs because production is not 'wasted' on the males.

Information on the host range of D. aligarhensis is based on taxonomic records or field collections. D. aligarhensis has been observed parasitizing ONLY the Asian citrus psyllid, even when two other psyllid species were been found on the same citrus trees in Reunion Island. D. aligarhensis is not known to be a hyperparasitoid (a parasitoid of other parasitoids; hyperparasitoids are considered detrimental to effective biological control). There is no evidence from taxonomic records that D. aligarhensis attacks any psyllids other than Asian citrus psyllid, which reduces the likelihood that there would be any unexpected effects on nontarget species.

'No choice' tests were conducted with D. aligarhensis in quarantine to determine if this parasitoid would attack a psyllid species being evaluated as a potential natural enemy for biological control of the weed Melaleuca tree in Florida. Three replications, in which 10 D. aligarhensis were released into cylinders containing flushed Melaleuca trees containing all stages of the Melaleuca psyllid, yielded no progeny. Thus, this parasitoid is unlikely to attack this weed biological control agent should it ever be released in Florida as part of a classical biological control program against Melaleuca.

Based on our current knowledge of the biology of this population of D. aligarhensis, the expected geographic range of D. aligarhensis is where the Asian citrus psyllid becomes established. If the psyllid spreads to other citrus-growing regions in the USA, D. aligarhensis could move with the psyllid into Louisiana, Texas, Arizona, and California.

D. aligarhensis has a greater ability to endure long periods of host deprivation (20 to 60 days at 25ºC) than T. radiata. Females provided with honey can resorb their eggs and survive when held under laboratory conditions at 17ºC for up to 140 days and still produce progeny (L. Skelley and M. Hoy, unpublished).

Why Release Both Parasitoids?

Both T. radiata and D. aligarhensis have been released because we expect them to complement each other if both establish. They may perform differently in different climatic zones, which is the reason the two colonies of T. radiata from Taiwan and Viet Nam have been maintained separately. C. McFarland and M. A. Hoy (unpublished) found that the Taiwan population of T. radiata has a greater requirement for high relative humidity than the Vietnam population. The two species also may perform differently at different psyllid densities. In Taiwan, T. radiata had higher populations than D. aligarhensis during part of the year, but D. aligarhensis appears better adapted to low host densities there.

Both T. radiata and D. aligarhensis were imported for classical biological control programs on Reunion Island where they dramatically reduced the Asian psyllid populations so that the pest "...could only be detected in neglected groves or on occasional backyard trees". A review of this biological control program in Reunion was conducted after 25 years and was considered "highly successful". Success was attributed to growers having access to disease-free plants for planting and the biological control of the psyllid by T. radiata and D. aligarhensis. A similar classical biological control program was launched in 1991 on Mauritius island and also gave excellent results.

Beneficial effects of the establishment of D. aligarhensis in Florida could include: reduced populations of Asian citrus psylla in citrus groves and nursery trees, reduced frequency of transmission of greening disease should the disease ever be found in Florida, reduced applications of pesticides to control the Asian citrus psylla, reduced secondary pest outbreaks due to the negative effects of pesticides on other natural enemies of citrus pests. Most citrus pests in Florida are under substantial to complete biological control and applications of pesticides toxic to natural enemies could induce secondary pest outbreaks.

Help Wanted

We need help to find release sites for these parasitoids. Because this pest currently is most abundant in southern and central Florida and we are many hours from release sites, we would like to hear from people who know where abundant psyllids occur on citrus or citrus jasmine. If you are willing to release parasitoids of Asian citrus psyllid in citrus or orange jasmine, please contact us by telephone at (352- 392-1901, ext. 153 or 352-372-3505, ext. 430) or by email at: mahoy@ifas.ufl.edu or nguyenr@doacs.state.fl.us

Web sites with additional information about this pest, including photographs to help you identify it, and its natural enemies are at:
http://creatures.ifas.ufl.edu/CITRUS/acpsyllid.htm and
http://www.freshfromflorida.com/pi/pest-alerts/diaphorina-citri-pest-alert.html.

Hoy is in the University of Florida's Department of Entomology and Nematology at Gainesville; Nguyen is in the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (DPI) at Gainesville. We thank Dr. K. C. Lo, Taichung, Taiwan, for assistance in obtaining the colony of D. aligarhensis and T. radiata from Taiwan, Lucy Skelley, Larry Keal, Shane Hill, Jenny Marquez and Darlene George for assistance in rearing the host plants, psyllids and these natural enemies and A. Jeyaprakash for carrying out the PCR tests for greening. We are especially grateful to David Hall, Louis Lodyga, J. P. Michaud, Ellen Tannehill, Edwardo Varona, Gwen H. Myres and Edward T. Putland for releasing parasitoids into the field. This article originally appeared in Citrus Industry magazine, April 2000. Reproduced with permission of the publisher.

• Citrus