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Biological Control of Glassy-Winged Sharpshooter in California

Prepared by the Applied Biological Control Research Laboratory,

Department of Entomology, University of California, Riverside, CA 92521 USA


Introduction
California Establishment
GWSSThe glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae) (Figure 1), is native to the southeastern states of the USA.  As an exotic invader, GWSS has become extremely pestiferous in southern California where it is thought to have established around 1990 (Sorensen and Gill 1996).  GWSS has now widely established in several counties of southern California [Figure 2 (CDFA 2003)].  GWSS has also successfully invaded French Polynesia (the Society Islands, Marquesas and Austral Island groups), [established 1999 (Cheou 2002)], Hawaii [established 2004 (Hoover 2004)], Easter Island [established 2005 (Sandra Ide pers. comm.)], and the Cook Islands (established 2007 [Disna Gunawardana pers. comm.]).  

An extraordinary population growth of the insect has occurred following its successful establishment in California.  This has been facilitated, in part, by a lack of co-evolved natural enemies coupled with irrigation of agricultural and urban areas in desert habitats normally too dry to support GWSS populations (Hoddle 2004a).

The GWSS feeds exclusively on xylem fluids, adults consume 1.8 - 2.9ml of fluid per day on cowpea and 0.2ml - 4.5ml when feeding on citrus, and its ability to spread the xylem-dwelling plant pathogenic bacterium Xylella fastidiosa is at the core of the insect’s classification as a pest in California.  In California, X. fastidiosa combination causes Pierce’s disease of grapes (Figure 3), almond leaf scorch (Figure 4), alfalfa dwarf, and oleander leaf scorch (Figure 5).  The number and type of plant maladies caused by X. fastidiosa vectored by GWSS is likely to increase as new hosts expressing X. fastidiosa induced diseases are identified.  The bacterium has already been found to be the causative agent of two previously unrecognized diseases in olive trees and liquidambar.  The GWSS-Xylella combination has devastated or has the potential to devastate many agricultural crops, urban ornamental and landscape plants, and native vegetation.  The potential effect of GWSS vectoring X. fastidiosa into native vegetation in California that previously has had no prior association with the bacterium is particularly worrisome as it may lead to new disease epidemics not previously seen.  Consequently, the establishment of GWSS in California has irrevocably changed the ecology of X. fastidiosa in California’s wilderness, agricultural, and urban landscapes.

PD GrapesAlmond Leaf Scorcholeander leaf scorch

The presence of Xylella in French Polynesia, Hawaii, Easter Island and the Cook Islands is currently unknown.  It is possible the bacterium has been introduced to these South Pacific Islands by means of the importation of ornamental plants from areas in the Americas where Xylella is native.  These potential silent Xylella reservoirs may harbor bacteria without expressing disease symptoms and, with the arrival of a vector such as GWSS, pathogen transmission to susceptible host plants could conceivably occur.
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Economic Impact

The economic cost to California caused by GWSS-Xylella is immense.  Oleander leaf scorch has been estimated to cause damages in excess of $52 million on 2,000 miles of freeway median plantings (Costa et al. 2000).  The year 2000 saw $6.9 million made available to supply pesticides to projects focused on area-wide spraying of GWSS habitats in an effort to manage populations migrating into vineyards in Temecula and Bakersfield.  Grape growers in Riverside and San Diego counties in 1998 and 1999 accrued estimated losses of $37.9 million because of GWSS-Xylella related diseases (Siebert 2001).  

In 2002 primary producers incurred additional economic costs resulting from containment activities such as inspections of export nursery stock, and shipments of bulk grapes and citrus from H. vitripennis infested counties (CDFA 2003).  There are currently in excess of 70 research programs on H. vitripennis or X. fastidiosa.  Because of the serious nature of this problem and the vast sums of money at stake, the National Academy of Sciences (NAS), a United States society of distinguished scholars engaged in scientific and engineering research with a mandate requiring it to advise the Federal Government on scientific and technical matters, has subjected these research programs to evaluation and assessment (CDFA 2003).

GWSS-Xylella has the potential to alter California’s economic trajectory akin to the impact cottony cushion scale had on California’s incipient citrus industry in the late 1880’s before natural enemies successfully brought the infestation under control.
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Biological Control of GWSS

Researchers at the University of California at Riverside (UCR), United States Department of Agriculture (USDA-ARS) and Californian Department of Food and Agriculture (CDFA) are pursuing classical biological control strategies to reduce populations of GWSS.  Natural enemies deemed safe and cleared from secure quarantine facilities are released into the environment and utilize the pest as food, thereby regulating its population growth and subsequent abundance. 
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Foreign Exploration Efforts for GWSS Natural Enemies

G. ashmeadiG. triggutatusIn the southeastern USA and northeastern Mexico, GWSS eggs are parasitized by several species of mymarid and trichogrammatid parasitoids.  Gonatocerus ashmeadi Girault (Figure 6), G. triguttatus Girault (Figure 7), G. morrilli Howard (Figure 8), and G. fasciatus Girault (Figure 9), all Mymaridae, are the most common natural enemies associated with H. vitripennis eggs in it’s native distribution (Triapitsyn and Phillips 2000) of southeastern USA.  In an effort to use natural enemies to control GWSS populations in southern California these parasitoids have been imported from the southeastern states, cleared through quarantine, and introduced into urban and agricultural areas.  These four parasitoids join populations of G. ashmeadi and G. morrilli in California that are established in the state.  While G. morrilli is native to California, G. ashmeadi is self-introduced and most likely established on incipient GWSS populations or, more likely, on the native Homalodisca liturata Ball, the smoke-tree sharpshooter (Vickerman et al. 2004).  Parasitoids from the GWSS distribution in the southeastern states have been introduced to bolster populations of the resident species because of inherent differences in elements of the parasitoid’s biology that may assist their establishment or efficacy as control agents.  It is possible that populations, while the same species, may exhibit a greater ability to G. morriliG. fasciatustolerate high temperatures, dry conditions or be generally more robust overall.  It is anticipated that genetic variability of native G. ashmeadi and G. morrilli populations will be increased through the release of new stock and this, in turn, may lead to improved biological control.  Over 90 separate recoveries of egg masses parasitized by G. triguttatus or G. fasciatus have been made in 23 sites over seven counties suggesting these control agents are establishing.  Biological control of GWSS is seen as a long-term control strategy for suppression of GWSS populations in areas where GWSS has already become established.  To this end, the CDFA has established two facilities based in Riverside and Kern counties to mass produce, release, and monitor introduced biological control agents.  The USDA-ARS with the CDFA has been evaluating the safety of mymarid parasitoids imported from Argentina for possible use against GWSS in California.  Quarantine work indicates Argentinean parasitoids will readily attack GWSS even though these parasitoids have not evolved in the home range of GWSS.  Evaluations are ongoing and no releases have been made.
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Determining the Safety of GWSS Parasitoids: Non-Target Impact Studies
 
Biological control has come under increased scrutiny because there is some evidence that under certain circumstances natural enemies released for the control of a pest species may attack non-target species and adversely affect the populations of these organisms (Hoddle 2004b).  To assist in the prediction and minimization of unwanted environmental effects that may be associated with exotic natural enemies released for the control of GWSS, native California sharpshooters have been studied to see if they are vulnerable to attack by parasitoids native to the southeastern USA, northeastern Mexico and Argentina.

All species of GWSS biological control agents have been screened for their ability to parasitize closely related non-target species of Homoptera.  These include the southeastern species H. insolita (Walker) and the southwestern species H. liturata (both proconiine sharpshooters), three sharpshooters of the cicadellini tribe, Colladonus montanus (Van Duzee) (Cherry mountain leafhopper), Graphocephala atropunctata (Signoret) (blue-green sharpshooter) and Xyphon fulgida (Nottingham) (red-headed sharpshooter), and other species of leafhoppers from a different subfamily Euscelidius variegatus (Kirschbaum), and Macrosteles fascifrons (Stål) (Aster leafhopper).  To date, the only non-target species susceptible to the agents introduced is H. liturata, a species that is implicated in X. fastidiosa transmission in agriculture, but exists primarily in desert habitats where mymarid parasitism is often low.

ST and GWSSST and GWPreliminary observations with native Californian sharpshooters (Cicadellinae) have revealed the GWSS parasitoids may impact their populations.  Homalodisca liturata, a native sharpshooter from the same tribe and genus and most similar to the GWSS (Figures 10 and 11) in its egg laying and generalist plant feeding habits, is expected to be utilized by introduced Gonatocerus species.  The habitats occupied by three other predominant native sharpshooters, however, have less overlap with the GWSS in addition to being from a different tribe.  The habitats of the cicadelline sharpshooters X. fulgida, and the green sharpshooter, Draeculocephala minerva Ball, both consist of grasses such as Bermuda and Johnson grass, Cynodon dactylon (L.) and Sorghum halapense (L.) respectively.  The blue–green BGSS Plantsharpshooter (Figures 12 and 13), G. atropunctata, also of the cicadellini tribe, prefers humid, partially shaded and densely vegetated habitats.  This sharpshooter is often found in coastal or riparian habitats consisting of trees, vines and succulent shrubs.  These unlikely foraging areas, combined with the differing tribal origins of the sharpshooter and the absence of any records indicating Gonatocerus emergence from any egg masses BGSSmay make this sharpshooter an improbable alternate host for the three Gonatocerus parasitoids.  Additionally, sticky card traps in the southern Californian habitat regions occupied by G. atropunctata have yielded no capture of the widespread and established parasitoid G. ashmeadi (Boyd, unpublished data).  Native sharpshooter eggs are approximately one-half the size of GWSS eggs, are laid singly, and embedded into the stem material rather than in groups just below the epidermal layer as is characteristic of GWSS egg masses (Boyd, unpublished data).  These characteristics, coupled with the different taxonomic placement, make the blue-green sharpshooter an unlikely host for any of the GWSS parasitoids. 
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GWSS and Natural Enemy Phenology in Southern California and Texas

demographyPhenological data on GWSS and G. ashmeadi populations have been collected for two full years in southern California and eastern Texas, part of the home range of GWSS.  In southern California, GWSS exhibits two distinct population peaks (Figure 14), the first occurring in spring during which an average of 12% of eggs were parasitized and the second in summer, where an average of 19% of eggs successfully parasitized.  This summer figure contrasts with reported parasitism rates of up to 100% parasitism in some regions (Triapitsyn and Phillips 2000).  A possible explanation of this discrepancy in the parasitism rates is this kind of data being collected in “snapshots” from any given season.  The data collected from citrus grown at Agricultural Operations at the University of California, Riverside often reflects near to 100% parasitism rates in individual sampling events but this level of oviposition is generally associated with low GWSS egg numbers and not reflected in the overall mean for any one season.  Of the egg masses surveyed, 17% had at least one egg parasitized by Gonatocerus spp. in spring, compared to 30% of egg masses utilized at some level in summer.  In Weslaco, Texas, summer parasitism rates have been observed to range from 38 to 100 percent of discovered GWSS egg masses having at least one egg parasitized, and G. triguttatus is the key natural enemy in this area.  These data from Texas demonstrate that GWSS populations might be successfully suppressed if efficacious egg parasitoids are successfully established in California.  Given that GWSS is such an effective vector of the X. fastidiosa pathogen, even highly successful suppression of GWSS may lead to population sizes that remain above acceptable levels for vineyard managers.  Regional lowering of GWSS populations will, however, assist greatly in all control efforts and management programs in many aspects of agriculture and the urban environment.
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Parasitoid Biology
 
Egg massMymarid parasitoids attacking GWSS eggs are small, approximately 0.5 – 1.5 mm (0.02 – 0.06 inches).  Parasitoid larvae pupate within GWSS eggs and then chew circular holes through which they emerge in search of mates and new host eggs to attack (Figure 15).  Gonatocerus ashmeadi, G. morrilli and G. triguttatus are solitary endoparasitoids that lay one egg into individual GWSS eggs within an egg mass.  Gonatocerus fasciatus is gregarious, and females deposit more than one egg per GWSS egg yielding multiple parasitoids per host egg (Triapitsyn et al. 2003).

The density of searching female parasitoids has a significant effect on the sex ratio of progeny produced.  When female Gonatocerus parasitoids fail to encounter other ovipositing females on a GWSS egg mass, progeny output is strongly female biased.  Laboratory experiments indicate that approximately 1 male : 8 females, 1 : 14 and 1 : 9 are produced for G. ashmeadi, G. triguttatus and G. fasciatus, respectively.  Increasing the number of conspecific ovipositing females from one to two per egg mass significantly reduces percentage of female offspring by up to 15.0% for all three Gonatocerus species.  These results suggest that local mate competition affects progeny production and more males are produced when females encounter conspecifics producing daughters with whom their sons may mate.

No-choice laboratory studies showed progeny production for G. ashmeadi, G. triguttatus, and G. fasciatus was greatest from GWSS eggs 3, 4 and 2 days of age, respectively.  Furthermore, each parasitoid species was able to utilize a range of egg ages around their most preferred age, these being eggs 1 - 4, 3 - 6, and 1 - 3 days of age for G. ashmeadi, G. triguttatus and G. fasciatus, respectively.  Parasitized GWSS eggs 8 to 10 days of age produced few parasitoid progeny.  This most likely occurred because of the advanced stage of development of the GWSS embryo and parasitoids that did emerge had been oviposited into sterile or dead host eggs lacking a GWSS embryo (Irvin and Hoddle 2004).  However, when given a choice between GWSS eggs 1, 3, and 5 days of age presented simultaneously to G. ashmeadi and G. triguttatus, no oviposition preferences were observed.  This suggests these two parasitoids will attack host eggs without preference as long as eggs are of a suitable age for oviposition.  The choice studies indicated G.  fasciatus had preference for GWSS eggs one and three days of age while eggs five days of age were not utilized.  The small size of G. fasciatus in comparison to G. ashmeadi and G. fasciatus possibly limits the range of GWSS egg ages available for parasitism.  This may occur because the smaller ovipositor of G. fasciatus may be unable to pierce the chorion of older eggs as they harden during maturation.
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Competitive ability and biological potential of Gonatocerus parasitoids

Interspecific competition between G. ashmeadi, G. triguttatus and G. fasciatus for GWSS egg masses was investigated in the laboratory by Irvin & Hoddle (in press) using three different experimental designs. Results showed that overall parasitism by G. ashmeadi was consistently higher (up to 76.0%) compared with G. triguttatus and G. fasciatus for all three studies. This suggests that female G. ashmeadi may show greater potential as a biological control agent of GWSS and could out compete G. triguttatus and G. fasciatus in the field and hinder their successful establishment and impact in California.

When one female of each species was simultaneously exposed to individual egg ages one, three and five days of age, G. ashmeadi parasitized a significantly higher (48.1%) proportion of GWSS eggs compared with G. triguttatus. In contrast, when females were exposed to all egg ages simultaneously, parasitism by G. ashmeadi and G. triguttatus was equivalent. This may be due to G. ashmeadi out competing G. triguttatus when females were only provided eggs one or three days of age, since these ages are more favorable for G. ashmeadi development, and G. triguttatus prefers older hosts. We speculate that when G. ashmeadi and G. triguttatus were exposed simultaneously to eggs one, three and five days of age, females parasitized egg ages most preferred by each species, thereby decreasing interspecific competition and resulting in equivalent parasitism rates between species. These results may have important implications for the field environment, where a range of host ages are present at one time, and may suggest that G. ashmeadi and G. triguttatus can coexist in California without significant interference competition.

Parasitism by G. fasciatus was consistently significantly lower (17.4-76.0% lower) than both G. ashmeadi and G. triguttatus for all three experimental studies. Gonatocerus fasciatus may have performed poorly because
    1/ This species is gregarious in nature and smaller in size compared with G. ashmeadi and G. triguttatus, therefore female G. fasciatus may require a longer period of time for each individual host handling;
    2/ Smaller eggs oviposited by female G. fasciatus conceivably give rise to smaller larvae which may be less competitive in multiparasitism;
   3/ Gonatocerus fasciatus larvae probably do not participate in larval combat, since gregarious larvae often frequently contact one another under normal development (Salt, 1961), therefore they may be at a competitive disadvantage in multiparasitism.

Alternatively, the competitive inferiority of G. fasciatus may simply be due to subordinate aggressiveness when competing for egg masses with congenerics. Results from a study which recorded visual observations on the behavior of one female G. ashmeadi, G. triguttatus and G. fasciatus simultaneously foraging on one GWSS egg mass showed that G. fasciatus allocated a significantly higher proportion (31.5%) of time to resting/grooming, compared with oviposition, whereas female G. ashmeadi allocated equal time to both activities. This may suggest that female G. fasciatus are less aggressive than G. ashmeadi and G. triguttatus. Furthermore, 39.6% of time allocated by female G. fasciatus was spent off leaves with GWSS egg masses, and it was observed that G. ashmeadi and G. triguttatus often aggressively protected the GWSS egg mass, sometimes excluding access by G. fasciatus.

Zwolfer (1971) proposed that due to life history trade-offs, parasitoid species that are intrinsically inferior should win in extrinsic competition, and vice versa. Therefore it is possible, that while G. fasciatus performed poorly in these competition studies, females may be extrinsically superior in the field due to potentially higher reproduction rates, oviposition preference for young GWSS egg masses, and greater host finding efficiency at low host densities. However, successful widespread establishment and impact by G. fasciatus on GWSS in California is unlikely unless this species can efficiently exploit low density populations of H. vitripennis in early spring when congeneric competition for egg masses is low.
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Enhancing Parasitoid Survival and Parasitism Rates in the Field

Most agricultural environments are unfavorable habitats for natural enemies because herbicides remove potential shelter, floral resources, and pesticide residues kill biological control agents (Gurr et al. 2003). In the laboratory it has been demonstrated that honey-water and buckwheat (Fagopyrum esculentum Moench) flowers significantly increased longevity of male and female G. ashmeadi, G. triguttatus, and G. fasciatus up to 94.6%, 92.4% and 93.1%, respectively, compared with water.  These results indicate resource procurement may be extremely important for enhancing parasitoid survival in agroecosystems, though until extensive field experimentation currently underway is completed this assertion is speculative.  Increased longevity of female parasitoids resulting from resource procurement may enhance biological control of H. vitripennis because increased female longevity may increase encounter rates with GWSS egg masses and subsequently increase parasitism.

Longevity of G. ashmeadi, G. triguttatus, and G. fasciatus on citrus flowers and H. vitripennis excrement was equivalent to that on water.  This indicates these field resources may not supply parasitoids with adequate nutrition to maximize survival. Understory management (i.e., the deliberate management of flowering plants beneath orchards and vineyards) is potentially one way to enhance parasitoid populations in agricultural systems thereby leading to improved pest control by natural enemies (Landis et al. 2000). Sowing flowering plants [e.g., buckwheat, dill (Anethum graveolens L.), or alyssum (Lobularia maritima L.)] as an understory in citrus orchards harboring H. vitripennis could potentially provide a food source to Gonatocerus species.  Additionally, softscale (Coccus hesperidum L.) excrement increased survival times by up to 85.2% for both sexes of G. ashmeadi, G. triguttatus, and G. fasciatus compared with citrus foliage alone, suggesting that in citrus orchards, low non-damaging C. hesperidum populations may also be beneficial for enhancing parasitoid survival and could enhance biological control of H. vitripennis. 
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The Invasive Potential of GWSS
 
GWSS has shown strong invasive potential having established outside of its home range in California, French Polynesia, Hawaii, Easter Island, and the Cook Islands.  Modeling that combines regional climate data and relevant biological information has indicated California’s premier wine growing areas of Napa, Sonoma, and Mendocino counties are vulnerable to invasion by GWSS while states north of California, also with substantial grape industries, may be too cold to harbor permanent populations of GWSS (Hoddle 2004a).  The major wine-growing regions of New Zealand, Australia, the Bordeaux region of France, most areas of Spain, as well as central and southern Italy have climates conducive to GWSS establishment and proliferation should it be accidentally introduced (Hoddle 2004a).

Data on GWSS in Tahiti is sobering where this pest has exhibited exponential population growth due to an abundance of suitable native and exotic host plants, mild climate permits year round breeding (in contrast to California where there are just two generations [spring and summer] each year), natural enemies are lacking, and no obvious competitors exist in urban or natural settings.  Naturally occurring parasitism of GWSS eggs is very low on the island of Mo’orea.  Surveys indicated that less than 2% of individual eggs were attacked in just 4% of egg masses collected.  Of those egg masses attacked, 44% of the eggs in those masses were parasitized (Table 1).  The wasp responsible for attacking GWSS egg masses is a platygasterid, a family that does not specialize on sharpshooters, but will parasitize various species of leafhoppers.  These data on parasitism in Tahiti indicate there are no specialized parasitoids attacking GWSS.  Only a few eggs in an egg mass are attacked, indicating inefficient and opportunistic exploitation, and only males were reared from GWSS eggs which suggest poor host quality as females did not oviposit fertilized female eggs.  These data clearly indicate GWSS populations in French Polynesia are free of the pressures associated with natural enemies.  A classical biological control initiative against GWSS has been launched and is a cooperative enterprise between the University of California at Riverside and Berkeley, and the French Polynesian Government.

No. of GWSS Eggs Examined
 No. GWSS Eggs Parasitized
 No. GWSS Eggs Eaten
 No. GWSS that Died of “Natural Causes” (bacterial,  fungal infection)
 No. GWSS Eggs that Nymphs Emerged From
2586 (246 egg masses studied)
 32
 444
 50
 2060
% of eggs
 1.24% (4% of collected egg masses were attacked)
 17.17%
 1.93%
 80%

Table 1.  Data on GWSS egg mass survivorship collected over the period September 3-9 2003 on the Island of Mo’orea in French Polynesia.

Classical Biological Control of GWSS in French Polynesia with Gonatocerus ashmeadi

A brochure on the GWSS biological control project in French Polynesia summarizing this highly successful classical biological control program with Gonatocerus ashmeadi is available as a downloadable PDF file. A summary of the biological control program in French Polynesia is also available in French.


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Figures 1 and 3 – 9 copyright UC Regents and Jack Kelly Clarke.  Figure 11 copyright Leigh J Pilkington.  Figures 10 and 11 copyright Michael Lewis.  Figures 12 and 13 copyright Elizabeth Boyd.

References

CDFA. 2003. Pierce's disease control program - report to the legislature, May 2003 [Online]. Available at: http://www.cdfa.ca.gov/phpps/pdcp/docs/2002LegReport.pdf. (verified 8 July 2004)

Cheou D. 2002. Incursion of glassy winged sharpshooter Homalodisca coagulata in French Polynesia. Plant Protection Service Pest Alert:1.

Costa H.S., Blua, M.J., Bethke, J.A., Redak, R.A. 2000. Transmission of Xylella fasitidiosa to oleander by the glassywinged sharpshooter, Homalodisca coagulata. HortScience 35(7):1265-7.

Gurr G.M., Wratten, S.D., Luna, J.M. 2003. Multi-function agricultural biodiversity: Pest management and other benefits. Basic & Applied Ecology 4(2):107-16.

Hoddle M.S. 2004a. The potential adventive geographic range of glassy-winged sharpshooter, Homalodisca coagulata and the grape pathogen Xylella fastidiosa: Implications for California and other grape growing regions of the world. Crop Prot., In Press

Hoddle M.S. 2004b. Restoring balance: Using exotic natural enemies to control invasive exotic species. Cons. Biol., In Press

Hoover W. 2004. New invader may threaten crops. Published in: The Honolulu Advertiser, May 14, 2004, Honolulu

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Irvin, N. A., Hoddle, M. S. (in press). The competitive ability of three mymarid egg parasitoids (Gonatocerus spp.) for glassy-winged sharptshooter (Homalodisica coagulata) eggs. Biological Control.

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Sorensen J.T., Gill, R.J. 1996. A range extension of Homalodisca coagulata (Say) (Hemiptera: Clypeorrhyncha: Cicadellidae) to southern California. Pan-Pacific Entomol 72(3):160-1.

Triapitsyn S.V., Morgan, D.J.W., Hoddle, M.S., Berezovskiy, V.V. 2003. Observations on the biology of Gonatocerus fasciatus Girault (Hymenoptera : Mymaridae), egg parasitoid of Homalodisca coagulata (Say) and Oncometopia orbona (Fabricius) (Hemiptera : Clypeorrhyncha : Cicadellidae). Pan-Pacific Entomol 79(1):75-6.

Triapitsyn S.V., Phillips, P.A. 2000. First record of Gonatocerus triguttatus (Hymenoptera : Mymaridae) from eggs of Homalodisca coagulata (Homoptera : Cicadellidae) with notes on the distribution of the host. Florida Entomol 83(2):200-3.

Vickerman D.B., Hoddle, M.S., Triapitysn, S.V., Stouthamer, R. 2004. Species identity of geographically distinct populations of the glassy-winged sharpshooter parasitoid Gonatocerus ashmeadi:  Morphology, DNA sequences and reproductive compatibility. Biol. Control In Press

Zwolfer, H., 1971. The structure and effect of parasite complexes attacking phytophagous host insects. In: denBoer, P. J., Gradwell, G. R. (Eds.), Dynamics of Populations: Proceedings of the Advanced Study Institute on ‘Dynamics and Numbers in Populations’ Oosterbeck 1970.  Centre for Agricultural Publicating and Documentation, Wageningen, The Netherlands, pp. 405-418.



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