Hanks, Lawrence M. : L.M. Hanks is Assistant Research Entomologist, Department of Entomology, UC Riverside

Millar, Jocelyn G.

Paine, Timothy D.

Tiny wasp helps protect eucalypts from eucalyptus longhorned borer Lawrence M . Hanks 0 Timothy D . Paine CI Jocelyn G . Millar Theeucalyptus longhorned borer ( ELB ) was first identified in Cali - fornia in 1984and has since killed thousands of eucalyptus trees . Now firmly established through - out Southern California , the borer continues to spread northward.A search for its natural enemies in Australia yielded a tiny wasp , Avetianellalongoi , that parasitizes ELB eggs . The wasp was im - ported , released and is now estab - lished in San Diego and Riverside counties . Results of releases in other areas are pending . A . longoi propagates quickly , disperses rapidly across wideareas , and is highly efficient at finding and parasitizing borer eggs . In 1984 , a eucalyptus tree in Orange County was discovered to be infested with the larvae of an insect species new to this continent , the eucalyptus longhorned borer ( ELB ) ( California Ag - riculture July - August 1986 ) . The ap - pearance of this large beetle in Califor - nia was alarming because of its reputation as a killer of eucalypts in most countries where these trees grow . ELB lay their eggs under the loose bark of the trunk and branches , and the larvae penetrate the bark to feed in the cambium . By consuming these critical tissues , which conduct water and nutrients , the larvae can de - stroy a tree in just a few weeks . ELB is an economically significant pest , af - fecting nearly every region in the world where eucalypts have been planted including Portugal , Spain , Italy , Israel , Egypt , Tunisia and South Africa . Damagedeucalyptus tree on suburban street . Without a natural ELB threatens the numerous , im - enemy to keep it in check , the eucalyptus longhornedborer has of eucalyptsthroughout the state . killed thousands portant roles of eucalypts in the Cali - CALIFORNIA AGRICULTURE , VOLUME 50 , NUMBER 3 14 A Avetianella longoifemales insert their eggs into eggs of the borer . Larvae of A . longoidevelop within the b A Avetianella longoiare attracted to egg eggs of the eucalyptus longhornedborer . of the eucalyptus longhorned batches Dark eggs contain adult parasites almost borer . ready to emerge . fornia landscape . First brought to the appear to be more resistant to beetle host - specific , we imported it into Cali - state in 1856 , eucalypts rapidly gained attack , and the beetle itself falls prey to fornia as a biological control agent of popularity as shelter , shade and orna - many natural enemies . No such eco - the borer . mental trees because they thrived logical checks and balances exist in Adult A . longoi have a flattened without care even on marginal sites . In California , where eucalypts experience body form that allows them to squeeze Southern California , eucalypts are subtle stresses , particularly water defi - under loose bark to reach ELB eggs . abundant in places originally treeless , cit , that may compromise their resis - Female wasps insert their eggs into the and , in some cases , comprise almost tance ; consequently , ELB populations borer eggs , and the developing wasp the entire urban forest - from inten - have exploded in the absence of natu - larvae consume the contents of the sively managed parks and golf courses ral enemies . borer egg . Each larva completes devel - to entirely unmanaged stands on opment within a single ELB egg in ELB parasitefound rangeland and undeveloped land . about 16 days , then the adult wasp Eucalypts are locally abundant in the Losses to the beetle can be miti - emerges by chewing a hole in the egg - central and northern parts of the state gated by bolstering tree resistance shell . As many as five wasps complete as well , where they serve as roadside through appropriate pruning and irri - development in a single borer egg , and and windbreak trees . They are also gation practices , and by planting euca - female A . longoi can parasitize about grown for use as biomass , pulpwood lyptus species that have higher innate 200 eggs during their life span , which and fuelwood . resistance ( see California Agriculture averages 26 days . This high reproduc - The eucalyptus pest has lived up to January - February 1995 ) . To reduce fur - tive capacity and short generation its bad reputation in the decade since ther losses of eucalypts , we have im - time suggest that A . longoi can quickly its colonization of California . On the ported a natural enemy of the borer build its population in response to an UC San Diego campus and in the com - from Australia for release in Califor - abundance of ELB eggs . munity of Rancho Santa Fe ( San Diego nia . This stingless wasp is a highly Wasp releases County ) alone , thousands of eucalypts specialized parasite of eucalyptus bor - have succumbed to attack . The beetle ers that poses no threat to native insect During summer 1993 , we released continues to spread northward , caus - species , nor can it sting people . more than 53,000 A . longoi in San Di - ing equally severe tree kills in Santa In 1992 , Dr . Qiao Wang of La Trobe ego and Rancho Santa Fe ( San Diego Barbara and the San Francisco Bay University in Melbourne , Australia , County ) , the city of Riverside ( River - Area , with new infestations reported working in collaboration with UC Riv - side County ) , Fontana ( San Bernardino in Fresno , Sacramento and Glenn erside entomologists , discovered ELB County ) , Will Rogers State Park ( Los counties . eggs that had been parasitized . From Angeles County ) , and on the Stanford Although ELB is effectively de - these parasitized eggs emerged tiny University campus ( Santa Clara stroying eucalypts in California , it wasps , no larger than grains of black County ) . By fall , ELB eggs parasitized rarely achieves pest status in its home - pepper , that were subsequently identi - by the wasp were found on eucalyptus land , Australia . There , eucalypts fied as Avetianella longoi Siscaro . After trees and fallen branches at sites dis - growing in their natural environments determining that the wasp was highly tant from all the release points . For ex - CALIFORNIA AGRICULTURE , MAY - JUNE 1996 15 ample , we released wasps on With fewer borer larvae at - the UC San Diego campus tempting to feed on the cam - and later found two egg bium , the trees will have a batches ( 44eggs total ) that greater chance of resisting and were completely parasitized surviving attack . about 9.3 miles ( 15km ) away The use of A . longoi as a bio - at the San Diego Zoo . In the logical control agent of ELB is field , we observed as many as also being explored in other re - four adult A . longoi on a gions of the world . This wasp single batch of borer eggs . was apparently accidentally in - These wasps did not appear Fig . 1 . Frequencydistribution of the per - troduced into the Mediterra - centage of eucalyptus longhornedborer to fly readily , but took short , hopping nean area , where it now parasitizes eggs within a batch that were parasitized flights . Nevertheless , the wasps spread high percentages of ELB eggs . We by Avetianella longoi . Egg batches were at a remarkable rate , especially given have also supplied A . longoi to re - collected in San Diego and Riverside counties in summer and fall of 1993 ( N = the seemingly inhospitable and mostly searchers for rearing and release in the 40 batches ) , 1994 ( N = 97 batches ) , and urbanized terrain they had to traverse . Republic of South Africa , where the 1995 ( N = 63 ) . Strong wasp dispersal ability would wasp has already spread more than be necessary for control of ELB be - 80 miles ( 130km ) in only 2 years . 4 . longoi is efficient parasite cause the beetles can fly long distances There the parasite also crossed 8.7miles in search of their eucalyptus host trees . A . longoi showed promise as a bio - ( 14 km ) of open sea , reaching an island Although populations of A . loiigoi logical control agent by surviving the where eucalypts were being attacked were spreading from all release sites , winter at most release sites , and by ef - by ELB . they still had to survive the winter . Be - fectively locating ELB eggs in varied Other natural enemies cause ELB are not active during win - habitats and on numerous eucalyptus ter , fresh host eggs are not available to species . Additionally , it proved a wor - We will continue to release A . longoi the wasps for several months . A . longoi thy adversary of ELB because of the in California in areas where it has not is known to pass the winter as pupae high proportion of eggs it killed . yet become established . In addition , inside the shells of the borer eggs . If We collected 40 ELB egg batches at we are also releasing three highly spe - the adult wasps were to become active the San Diego , Rancho Santa Fe and cialized braconid wasps that parasitize at the wrong time , however , they Riverside sites in summer and fall of ELB larvae feeding under the bark of would not find host eggs , and wasp 1993,97 batches in 1994 , and 63 eucalyptus . One species , Syngaster populations would go extinct . In the batches in 1995 . Most of the egg Brullk , already shows evidence lepidus spring of 1994 , adult A . longoi reap - batches in 1993were more than 75 % of reproducing in the field ; we have peared on eucalyptus logs at the Riv - parasitized ( fig . l ) , and 71 % of all eggs found hundreds of their emergence erside site , giving the first confirma - collected were parasitized . Even fewer holes . By establishing permanent tion that they had survived the winter egg batches escaped parasitism in 1994 populations of these natural enemies and were permanently established . We and 1995 , even though we had not re - in California , we hope to tip the eco - also found parasitized eggs in San Di - leased parasites at these sites since the logical balance in favor of the ego and Rancho Santa Fe , but not at previous year ; 81 % and 84 % of batches eucalypts , which have become such an Will Rogers State Park or Stanford collected in those years were more important feature of the California University . No ELB eggs could be than 75 % parasitized ( fig . l ) , with 91 % landscape . found at the Fontana site to confirm and 88 % of all eggs parasitized . The that the wasp was present , and trap decline from 1993to 1995in the per - logs put out to encourage egg laying centage of egg batches that were less L.M . Hanks is Assistant Research Ento - by the beetles had been pilfered . than 25 % parasitized ( fig . l ) , and the mologist , T.D . Paine and J.G.Millar are We continued to release A . longoi corresponding increase in the number Associate Professors , Department of Ento - through 1994and 1995at the Stanford of highly parasitized batches , are mology , UC Riverside . site where the wasp had failed to es - statistically significant ( XI = 17.3 , The authors wish to thank the Rancho df = 6 , P < . 01 ) . tablish , and at new sites at Chollas Santa Fe Association , Elvenia Slosson Lake ( San Diego County ) , in Irvine A . longoi The high rates at which Fund for Ornamental Horticulture , Cali - ( Orange County ) , Santa Barbara ( Santa parasitized ELB eggs , coupled with its fornia Department of Forestry and Fire Barbara County ) , Long Beach and strong powers of dispersal and effi - Protection , California Department of Santa Clarita ( Los Angeles County ) cient location of host eggs , suggest Transportation , UC Statewide Integrated and Fresno ( Fresno County ) . More that this wasp is likely to have an im - Pest Management Program and the Cali - than 121,000A . longoi were released at portant impact on the ELB problem in fornia Association of Nurserymenfor their these seven sites during 1994and 1995 , California . The wasp sharply curtails 1 . Gould , F . Ameeri , support . We also thank and we hope to confirm establishment ELB larvae that survive the number of A . Rust and S . Gilbertfor their contribu - in 1996 . to penetrate the bark of eucalypts . tions and assistance . 16 CALIFORNIA AGRICULTURE , VOLUME 50 , NUMBER 3