Photo above shows the adult parasitic Thripobius semiluteus, which attacks only cer- tain species of plant-feeding . Photo be- low shows a typical distorted parasitized thrips larva (right), a black parasitoid pupa (center), and a normal thrips pupa (left).

Immature stages of greenhouse thrips are pictured above. Upper left is small, first stage larva preferred by the parasite for egg deposition; at center is larger, second stage larva. Others are ei- ther prepupae, with antennae directed forward and short wing pads; or pupae, with antennae di- rected backwards and long wing pads.

Imported parasite of greenhouse thrips established on California avocado

James A. McMurtry o Horace G. Johnson o Sheldon J. Newberger

The parasitic wasp, Thripobius the coast, now are common in more inte- duced to California. Goetheana pawipennis semiluteus, introduced for biologi- rior districts. This causes scarring of (Gahan), introduced from Jamaica in 1962 fruit, especially on the Hass variety, reduc- and the Bahamas in 1983, failed to become cal control of greenhouse thrips, ing its market value. It also affects fmit established. However, Thripobius has been established and spread- quality if infestation occurs when the fruit semiluteus Boucek, introduced from Aus- ing for up to four growing seasons is small. The problem has brought about tralia in 1986 and Brazil in 1988, has be- at some sites in Southern Califor- an increased use of pesticides by Califor- come established in Southern California. nia avocado orchards. Studies nia avocado growers, who generally have The following is a progress report of our avoided spray applications and encour- research on this species. show that declines in thrips num- aged biological control of most insect and The Australian introduction of T. bers coincide with increasing para- mite pests by resident natural enemies. semiluteus was provided through the cow- sitization by this wasp, which Only two native natural enemies of tesy of Dr. Andrew Beattie, New South could become an important mortal- greenhouse thrips are relatively common Wales Department of Agriculture, ity factor of greenhouse thrips if its in California avocado orchards: a preda- Rydalmere, N. S. W. in August 1986. An tory thrips, Franklinothrips vespiformis exploration trip by the senior author in six widespread establishment is (Crawford),and a trichogrammatid egg southern states of Brazil in April-May of achieved. parasite, mymaripenne 1988 resulted in only one collection of Timberlake. Apparently, neither of these parasitized greenhouse thrips. Parasites natural enemies provides effective control. emerging from this material were hand- Therefore, the introduction of exotic para- carried to California and processed Greenhouse thrips, Heliothrips sites, especially those attacking the larval through quarantine at UC Riverside, at haemorrhoidalis (Bouchb), has increased in stage, was considered to be the best ap- which time they were found to be the importance as a pest of avocados in South- proach to biological control of greenhouse same species as the one from Australia. ern California during the past decade. thrips. The Brazil collection is the first New Damaging infestations, previously con- Two species of parasitic , both in World record for T. semiluteus, known pre- fined mainly to areas within a few miles of the family Eulophidae, have been intro- viously only from India, Australia and the

CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 1991 31 island of Sao Tome, off the coast of West Africa. Biology of the parasite Both the Australian and Brazilian strains of T. semiluteus are uniparental, which means all individuals are female. The adult female parasite inserts an egg inside the body cavity of a thrips larva, usually the first or early second stage (the stages of the thrips are egg, first- and sec- ond-stage larva, prepupa, pupa and adult). The parasitized thrips develops to a mature second-stage larva (apparently, it does not complete transformation to the within 2 years, following a release of ap- parasitization to about 60% (table 1).In prepupa) before it is killed by the develop- proximately 11,000 wasps in 3 release comparison, the check area, with no or ing parasite. At this time, the entire body sites. The rate of spread throughout an or- little parasite activity, had consistently cavity of the parasitized thrips is filled by chard probably is related to the density high populations of thrips that persisted the wasp larva. The parasite then trans- and extent of the distribution of the thrips even beyond the dates shown. forms to a black pupa. populations. A heavy, widespread infesta- Declines of thrips populations to low The life cycle of T. semiluteus averaged tion serves as a natural "insectary" provid- levels also have been noted in plots in sev- 23.6 (range 22-25) days at 73°F (23°C). The ing host material for rapid multiplication eral other orchards when the estimated pupal stage comprises about 50% of this of the parasite. percent parasitization increased to about developmental time. In comparison, the 60%.Results to date suggest that T. greenhouse thrips has a longer life cycle: Potential impact of T. semiluteus semiluteus can suppress thrips populations, 35.6 (range 34-39) days at 73°F. The late To acquire information on the potential although thrips numbers probably will not second-stage thrips apparently are less fa- effectiveness of T. semiluteus, populations/ always be reduced below the level at vorable for parasite development, as more of thrips and parasites were monitored at which some fruit scarring occurs. than 50% of the pupae developing from 2- to 3-week intervals in a 3-ac block of these thrips hosts failed to survive. No mixed experimental avocado cultivars at Conclusions and future plans parasites developed in the thrips prepupal South Coast Field Station. Samples of 20 These preliminary studies suggest that or pupal stages. leaves were taken from each of four thrips- Thripobius semiluteus could become an im- infested areas where parasites were estab- portant control agent of greenhouse thrips, Colonization program lished and one infested area where they if it becomes widely established.Wide- From late September 1986 to April had not become well established (check). spread colonization now is possible be- 1990, more than 500,000 T. semiluteus were Additional check areas were not available cause the parasite is commercially avail- liberated in a total of 50 different orchards because the parasite had spread through- able. in Orange, San Diego, San Luis Obispo, out most of the block. Leaves were trans- Future investigations are planned to Santa Barbara and Ventura counties. Peri- ported to the laboratory and examined un- determine: (1)the ability of T. semiluteus to odic surveys in release orchards have re- der a dissecting microscope, recording control thrips in terms of reduced fruit sulted in recoveries of T. semiluteus in greenhouse thrips larvae, prepupae plus scarring; (2) the potential rate of spread of many of these plots. Plots in which para- pupae and adults, as well as distorted, ob- the parasite within and between orchards; site recoveries have not been made are viously parasitized thrips larvae and para- and (3) its ability to carry over when thrips mainly those in which thrips subsequently site pupae. A rough estimate of percent are reduced to extremely low levels from declined to such low numbers that a suffi- parasitization was calculated by the for- fruit harvest, high parasitization, or severe cient number of infested fruit or leaves mula: weather conditions. Results from such could not be found for an adequate studies should suggest whether or not sample. Parasitization in some plots has parasite pupae + distorted thrips larvae augmentative or supplementary releases been observed for as many as four con- (parasitized) might be needed to reestablish the para- secutive seasons after the parasite releases. site, and, if such releases appear to be ben- Our surveys indicate that Thripobius is parasite pupae + distorted thrips larvae eficial, what release strategy (rates, timing established in Southern California, and + second-stage larvae + pupae and number of release sites per acre) that it has survived both cold and hot would be feasible. weather extremes of the region (several As parasites could not be detected in degrees below freezing and in excess of the hosts until a few days before pupation, 100°F). Extensive surveys in avocado this source of underestimation of parasit- J. A. McMurty is Professor, and H. G. John- blocks in the UC South Coast Field Station ized hosts is probably partially compen- son and s. J. Newberger are Staff Research As- at he,where approximately20,000 T. sated for by an overestimation due to sociates, Department of Entomology, UC Rip semiluteus were released between 1986 and longer duration of parasite pupae com- erside. 1988, showed that within three growing pared to the duration of the thrips pupal Funding for this research was provided in seasons, the parasite had spread through- stages. Some laboratory rearing of thrips part by the California Avocado Commission out two adjacent blocks (total 5.3 ac) and larvae in the samples for parasite emer- and the UC Statewide IPM Project. dispersed across an open field to become gence is in progress to help interpret these The authors thank John R. LaSalk for iden- established throughout another block of 2 data. tification of Brazilian parasites, as well as nu- ac. In a 6-ac orchard in San Diego County, A marked reduction in numbers of merous growers, pest control advisors and Co- widespread establishment and up to 60% greenhouse thrips on September 5,1989 operative Extension personnel, who obtained or parasitization by T. semiluteus occurred coincided with an increase in estimated provided field plots.

32 CALIFORNIA AGRICULTURE, VOLUME 45, NUMBER 6