The role of natural enemy guilds in Aphis glycines suppression

Generalist natural enemy guilds are increasingly recognized as important sources of mortality for invasive agricultural pests. However, the net contribution of different species to pest suppression is conditioned by their biology and interspecific interactions. The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is widely attacked by generalist predators, but the relative impacts of different natural enemy guilds remains poorly understood. Moreover, low levels of A. glycines parasitism suggest that resident parasitoids may be limited through intraguild predation. During 2004 and 2005, we conducted field experiments to test the impact of different guilds of natural enemies on A. glycines. We contrasted aphid abundance on field cages with ambient levels of small predators (primarily Orius insidiosus) and parasitoids (primarily Braconidae), sham cages and open controls exposed to large predators (primarily coccinellids), and cages excluding all natural enemies. We observed strong aphid suppression (86- to 36-fold reduction) in treatments exposed to coccinellids, but only minor reduction due to small predators and parasitoids, with aphids reaching rapidly economic injury levels when coccinellids were excluded. Three species of resident parasitoids were found attacking A. glycines at very low levels (<1% parasitism), with no evidence that intraguild predation by coccinellids attenuated parasitoid impacts. At the plant level, coccinellid impacts resulted in a trophic cascade that restored soybean biomass and yield, whereas small natural enemies provided only minor protection against yield loss. Our results indicate that within the assemblage of A. glycines natural enemies in Michigan, coccinellids are critical to maintain aphids below economic injury levels.     

The biology of two Eretmocerus spp. (Haldeman) and three Encarsia spp. Forster and their potential as biological control agents of Bemisia tabaci biotype B in Australia

The performance, as measured by daily rate of parasitism and total parasitism, of five aphelinid species found in Australia parasitising Bemisia tabaci were compared on cotton, hibiscus, rockmelon, soybean and tomato. Two Eretmocerusspp., both indigenous to Australia, gave the highest levels of parasitism on each of the plant host species tested. The tritrophic interactions between B. tabaci, host plant species and Eret. mundus(Australian parthenogenetic form) (APF) were also examined. In general, more whiteflies were parasitised when cotton was the source host or rockmelon the test host. Parasitism was always low when tomato was either the source or test host. When parasitoids were transferred from rockmelon to cotton, parasitism declined. In contrast, parasitism increased when parasitoids were transferred from cotton to rockmelon. Parasitism also increased when parasitoids were transferred from soybean to rockmelon, yet failed to do so when shifted from soybean to cotton despite cotton normally being a better host. However, when parasitoids were transferred from cotton to soybean there was a marked increase in parasitism. Possible causes are discussed. The field cage trial demonstrated that parasitism by both Eretmocerus spp. increased with increasing whitefly density. Further, the increase in parasitism was not due to the presence of more parasitoids as neither the parasitoid-whitefly ratio nor the total number of parasitoids present had a significant effect on parasitism. The combination of the two species, gave similar levels of parasitism to that achieved by Eret. mundus(APF) alone. Subsequent identification of the emerged individuals indicated that over 50% of the parasitism was due to this species suggesting that it out-competed Eret. queenslandensis. Despite this competition, there was no evidence that overall control was compromised.     

Effects of fine-grain habitat complexity on egg parasitism by three species of Trichogramma

Effects of microhabitat complexity on host foraging by three species of Trichogramma (T. deion, T. ostriniae, and T. pretiosum) were evaluated under simulated stored product conditions. All three species have been considered as potential biological control agents for the Indianmeal moth, Plodia interpunctella, in retail stores and warehouses. Trials were conducted with single naïve female parasitoids in 10-cm Petri dishes that were either empty, contained flour, or contained millet. Empty 15-cm Petri dishes, which served as a surface area control, comprised a fourth treatment. Females were allowed to forage for sentinel egg disks for 2 h, after which percentages of parasitism and egg mortality were computed. In addition, behavioral observations were made on a subset of the trials. Trichogramma deion parasitized more eggs than the other two species in the empty dishes and in the dish containing flour. For T. deion, rates of parasitism and egg mortality were significantly greater in both the small and large empty dishes than in the small dishes containing flour or millet. Parasitism was consistently low for all three species in the grain-filled dishes. Among species, T. ostriniae spent the most time walking, while T. pretiosum was the most sedentary. In addition, T. pretiosum spent significantly more time on the first egg visited compared with subsequent eggs. T. deion may be the best-suited for use as a biological control agent for P. interpunctella. However, the potential negative effects of fine-grain habitat complexity should be taken into account when developing a release protocol for Trichogramma spp.     

Occurrence and activity of <Emphasis Type="Italic">Bemisia tabaci</Emphasis> parasitoids on cassava in different agro-ecologies in Uganda

Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD), which in turn causes devastating yield losses. Surveys were conducted from October 2000 to November 2001 in four agro-ecologies in Uganda to enhance the understanding of parasitoid fauna and parasitism of B. tabaci in cassava fields. Such an understanding is an essential prerequisite for the development of biological control methods of B. tabaci to complement current CMD control practices. Parasitoid abundance and parasitism efficiency varied between locations and sampling dates within the locations; highest parasitoid densities were observed at Namulonge in the Lake Victoria crescent while the lowest was at Kalangala. In all locations, parasitism was mainly due to Encarsia sophia Dodd and Girault and Eretmocerus mundus Mercet (all Hymenoptera: Aphelinidae). Two occasionally observed species included Encarsia mineoi Viggiani (Hymenoptera: Aphelinidae), only observed at Namulonge, and blackhead Encarsia (Hymenoptera: Aphelinidae) observed at Bulisa, Namulonge and Lyantonde. Parasitism efficiency was highest at Bulisa (57.9%), but ranged from 40.2 to 46.9% at the other three sites. This paper discusses the possible causes of variations in parasitoid abundance and parasitism efficiency, and proposes further studies that might be carried out to assess the potential for augmentation of parasitoids to control B. tabacipopulations and CMD.     

Effect of a turnip rape <Emphasis Type="Italic">(Brassica rapa)</Emphasis> trap crop on stem-mining pests and their parasitoids in winter oilseed rape <Emphasis Type="Italic">(Brassica napus)</Emphasis>

Concerns about the negative effects of chemical control of oilseed rape (Brassica napus L.) pests on non-target species, human safety, and development of insecticide resistance, require alternative control strategies such as the use of trap crops and biocontrol to be developed. Psylliodes chrysocephala(L.) (Coleoptera: Chrysomelidae) (cabbage stem flea beetle) and Ceutorhynchus pallidactylus (Marsh.) (Coleoptera: Curculionidae) (cabbage stem weevil) are two major stem-mining pests of oilseed rape. This study investigated the phenology of these pests and their main parasitoids in the UK, the potential use of turnip rape (Brassica rapa L.) as a trap crop to reduce oilseed rape infestation, and the effects of insecticide treatment on pest incidence and larval parasitism. Water trap samples, plant dissections and pest larval dissections were done to determine: the incidence of adult pests and their parasitoids, the level of plant infestation by the pests and percentage larval parasitism, respectively. The turnip rape trap crop borders reduced P. chrysocephalabut not C. pallidactylus infestation of oilseed rape plots. Treatment of the trap crop with insecticide had little effect on either pest or parasitoid incidence in the oilseed rape. TersilochusmicrogasterSzép. andT. obscurator Aub. (Hymenoptera: Ichneumonidae) were the main larval parasitoids of P. chrysocephalaand C. pallidactylus, respectively. Tersilochus microgasteris reported for the first time in the UK. The implications for integrated pest management are discussed.     

Effect of parasitism on flight behavior of the soybean aphid, Aphis glycines

Many aphid species possess wingless (apterous) and winged (alate) stages, both of which can harbor parasitoids at various developmental stages. Alates can either be parasitized directly or can bear parasitoids eggs or larvae resulting from prior parasitism of alatoid nymphs. Winged aphids bearing parasitoid eggs or young larvae eventually still engage in long-distance flights, thereby facilitating parasitoid dispersal. This may have a number of important implications for biological control of aphids by parasitoids. In this study, we determined the effect of parasitism by Aphelinus varipes (Hymenoptera: Aphelinidae) on wing development and flight of the soybean aphid, Aphis glycines (Hemiptera: Aphididae). We also quantified the influence of aphid flight distance on subsequent A. varipes development. Parasitism by A. varipes was allowed at different A. glycines developmental stages (i.e., alatoid 3rd and 4th-instar nymphs, alates) and subsequent aphid flight was measured using a computer-monitored flight mill. Only 35% of aphids parasitized as L3 alatoid nymphs produced normal winged adults compared to 100% of L4 alatoids. Flight performance of aphids parasitized as 4th-instar alatoid nymphs 24 or 48 h prior to testing was similar to that of un-parasitized alates of identical age, but declined sharply for alates that had been parasitized as 4th-instar alatoid nymphs 72 and 96 h prior to testing. Flight performance of aphids parasitized as alate adults for 24 h was not significantly different from un-parasitized alates of comparable ages. Flight distance did not affect parasitoid larval or pupal development times, or the percent mummification of parasitized aphids. Our results have implications for natural biological control of A. glycines in Asia and classical biological control of the soybean aphid in North America.     

瓜实蝇2种蛹寄生蜂生防潜能比较

[目的] 比较研究蝇蛹俑小蜂与蝇蛹金小蜂对瓜实蝇的控制潜能。[方法] 采用非选择性实验测定蝇蛹俑小蜂与蝇蛹金小蜂对瓜实蝇蛹的寄生效能、繁殖能力,并研究覆土厚度对2种寄生蜂寄生效能的影响。[结果] 2种寄生蜂对瓜实蝇的寄生率无显著性差异,但蝇蛹金小蜂的平均单雌产后代数比蝇蛹俑小蜂的多,分别为30和23头。土壤厚度显著影响2种寄生蜂的寄生效能,随着土壤厚度的增加,2种寄生蜂对瓜实蝇蛹的寄生率均显著下降,但蝇蛹金小蜂的寄生率下降更为迅速;蝇蛹俑小蜂最深可寄生8 cm土壤下的瓜实蝇蛹,而蝇蛹金小蜂在土壤厚度达到3 cm时就不能完成寄生。[结论] 蝇蛹俑小蜂较蝇蛹金小蜂更适合应用于瓜实蝇蛹的生物防治。     

Non-target parasitism of the endemic New Zealand red admiral butterfly (Bassaris gonerilla) by the introduced biological control agent Pteromalus puparum

The New Zealand red admiral butterfly, Bassaris gonerilla (F.) (Lepidoptera: Nymphalidae), has been known as a non-target host for the introduced biological control agent Pteromalus puparum (L.) (Hymenoptera: Pteromalidae) for at least 35 years, but the level of parasitism has never been quantified. Pre-imaginal mortality in B. gonerilla was assessed over the southern summer of 2000/01 at six field sites in the Christchurch area of the South Island, New Zealand. Individual eggs and larvae were identified by tagging the stem of the Urtica ferox Forst.f. plant on which they were found and the fate of these individuals was checked weekly. These data were used to construct a partial life table for B. gonerilla. Egg mortality was very high (95%), with parasitism by an unidentified Telenomus sp. Haliday (Hymenoptera: Scelionidae) causing 57% mortality. Mortality in the larval and pupal stages increased at a constant rate with age and the major mortality factor was disappearance, which was assumed to be a result of predation and dispersal of larvae. The introduced biological control agent P. puparum parasitized 14% of B. gonerilla pupae sampled. However, parasitism by another exotic parasitoid, the self-introduced Echthromorpha intricatoria (F.) (Hymenoptera: Ichneumonidae), was even higher at 26%. A survey of pupal parasitism in three regions of New Zealand (Wellington, Christchurch, and Dunedin) revealed overall parasitism levels of 67% by E. intricatoria and 8% by P. puparum, but due to the difference in emergence times of B. gonerilla and its parasitoids, these are likely to be overestimates of percent parasitism. It is concluded that P. puparum has permanently enhanced mortality in B. gonerilla, but the level of mortality is low relative to egg parasitism by Telenomus sp., larval disappearance mortality, and pupal mortality due to E. intricatoria parasitism. To determine if this level of pupal parasitism has had population effects will require more data and the development of a population model for B. gonerilla.     

Changes in abundance of native and introduced parasites (Hymenoptera: Braconidae), and of the target and non-target plant bug species (Hemiptera: Miridae), during two classical biological control programs in alfalfa.

High numbers of tarnished plant bugs [Lygus lineolaris (Palisot)], were once common in alfalfa, as was a low level of parasitism (9%) by the native Peristenus pallipes (Curtis). After the bivoltine European parasite Peristenus digoneutis Loan became well established, average parasitism of the first and second generations increased to 64%, and tarnished plant bug numbers dropped by 65%. This reduced host density eventually caused a decline in total parasitism by both parasite species to 22%. A few P. digoneutis also attacked the alfalfa plant bug, Adelphocoris lineolatus (Goeze), but did not reduce this pest or increase its parasitism rate. At another location, where P. digoneutis is not established, parasitism of first generation alfalfa plant bugs, an adventive (accidently introduced) pest, was increased to 21% by the introduced univoltine parasite, Peristenus conradi Marsh, and a slight reduction in the pest may have resulted. P. digoneutis did not parasitize the meadow plant bug, Leptopterna dolabrata (L.), an adventive pest of forage grasses, so did not affect this mirid or its parasite. Neither introduced parasite eliminated the native parasites of the tarnished or alfalfa plant bugs. The narrow host ranges of the braconid parasites of mirid nymphs are contrasted with the broad host range of the native tachinid parasite [Phasia robertsoni (Towns.)] of adult mirids. The major changes in mirid abundance and their mortality by parasites that slowly occurred during this 19-year study demonstrate the need for long-term field research, to adequately document and understand these complex interactions.     

Natural establishment of a parasitoid complex on Bactrocera latifrons (Diptera: Tephritidae) in Hawaii

Bactrocera latifrons (Hendel) (Diptera: Tephritidae) is the most recent of four tephritid fruit fly species accidentally introduced into Hawaii. Although parasitoids have been released against other tephritid fruit fly species and have shown partial success in Hawaii, no parasitoids were released until 2004 to suppress populations of B. latifrons. The present study was conducted to document the parasitoid complex that has naturally established against B. latifrons in Hawaii and to assess whether there is a need for improving the biological control of this species. Based on ripe turkeyberry (Solanum torvum Sw) fruit collections over three consecutive years B. latifrons was the dominant tephritid fruit fly infestating turkeyberry at all four sites surveyed, across three major islands in Hawaii. The overall percentage parasitism of B. latifrons ranged from a low of 0.8% (Hana, Maui) to a high of 8.8% (Kahaluu, Oahu). Five primary parasitoid species were recovered from individually held B. latifrons puparia: Fopius arisanus (Sonan), Psyttalia incisi (Silvestri), Diachasmimorpha longicaudata (Ashmead), D. tryoni (Cameron), and Tetrastichus giffardianus Silvestri. F. arisanus was the predominant parasitoid at three of the four sites. Low levels of parasitism suggest that there is a need to improve biological control of B. latifrons, to minimize chances of this species causing economic impacts on crop production in Hawaii. We discuss the possibility of improving biological control of B. latifrons through augmentative releases of F. arisanus or introduction and release of specific and efficient new parasitoid species.     

Biological control of Lygus lineolaris by Peristenus spp. in strawberry

Peristenus digoneutis Loan (Hymenoptera: Braconidae) was introduced to the US for biological control of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois) (Hemiptera: Miridae), and has since spread through much of the northeast. The purpose of this study was to determine if P. digoneutis and a native congener, Peristenus pallipes (Curtis), parasitize L. lineolaris in strawberry (where it is a key pest), and what factors relate to parasitism levels. During 1997–1999 we monitored parasitism on 17 strawberry farms in 14 counties in eastern and western New York State. We found that in eastern NY (where P. digoneutis has been established since the early 1990s), overall mean parasitism was 19.7% (ranging from 0 to 70%), mostly by P. digoneutis. Mean parasitism was significantly lower (12.3%, ranging from 0 to 58%) in western NY (where P. digoneutis was first recorded in 1999), and was mostly by P. pallipes. P. pallipes parasitism was significantly lower in eastern than western NY, suggesting the potential for competitive interaction with P. digoneutis. The insecticide regime of a farm was an important factor influencing parasitism rate, which was 5- to 6.5-fold higher on organic or casually sprayed farms than on intensely treated farms, though pest density under these three regimes was not significantly different. L. lineolaris density, and parasitism rate in nearby alfalfa and abandoned fields were also significant factors for parasitism in strawberry.     

Candidates for biocontrol of Macfadyena unguis-cati in South Africa: biology, host ranges and potential impact of Carvalhotingis visenda and Carvalhotingis hollandi under quarantine conditions

Macfadyena unguis-cati (L.) Gentry (Bignoniaceae) was introduced as an ornamental in South Africa, but is fast becoming an important invasive plant in many areas. It is difficult to control the plant chemically and mechanically. The first biocontrol agent, the chrysomelid Charidotis auroguttata (Boheman), has been released. It established at some release sites, but numbers have so far remained low. Additional biocontrol agents were sought to augment C. auroguttata. The potential host ranges of two foliage feeding lace bugs, Carvalhotingis visenda (Drake and Hambleton) and C. hollandi (Drake) (Hemiptera: Tingidae) were evaluated on the basis of nymphal no-choice and adult multi-choice tests involving 23 plant species in 11 families. In no-choice tests, nymphs of both species were able to survive and complete development on M. unguis-cati only, and adults of both species only fed and oviposited on M. unguis-cati during the adult multi-choice tests. Host specificity tests thus confirm that the tingids are highly host specific biocontrol agents, and will not pose risk to any non-target plants in South Africa. A study to determine the potential impact of C. hollandi nymphal feeding on M. unguis-cati showed a significant decrease in the chlorophyll contents of leaves when compared to those of control plants. These studies indicate that, once released, the two lace bug species could contribute significantly to the biological control of M. unguis-cati in South Africa.     

When good bugs go bad: intraguild predation by Jalysus wickhami on the parasitoid, Cotesia congregata

We report a case of direct intraguild predation involving an insect predator and parasitoid in an agricultural system. The spined stilt bug, Jalysus wickhami Van Duzee, feeds on eggs of the tobacco hornworm, Manduca sexta L., and also on prepupal and pupal stages of the gregarious hornworm parasitoid, Cotesia congregata (Say). In two separate trials, mean mortality of attached parasitoids was significantly lower (66%, 73%) than that of their detached siblings (97%, 96%) after a 3 day exposure to stilt bugs, demonstrating that attachment to the host offered some protection against predation. In no-choice experiments, prepupal parasitoids suffered greater mortality (0 day-old=61%, 1 day-old=65%) than pupal parasitoids (2 day-old=50%, 3 day-old=14%). When offered in combination with 0 or 2 day-old hornworm eggs, respective mortality of 0, 1, 2 and 3-day-old pupal parasitoids showed a similar pattern (67%, 63%, 33% and 23%). In another experiment, mortality of 0-day-old pupal parasitoids (64%) was greater than that of 3 day-old pupal parasitoids (38%). Mortality of pupal parasitoids was not affected by the availability of hornworm eggs, a highly acceptable food. Younger pupal parasitoids (=prepupae) probably suffered greater mortality because they were more easily fed on by stily bugs than older (pupated) ones. Because C. congregata overwinters in the prepupal stage, it may be particularly vulnerable to attack late in the season when stilt bug populations are large and hornworm eggs are relatively uncommon.     

Mass rearing and augmentative releases of the native parasitoid Chouioia cunea for biological control of the introduced fall webworm Hyphantria cunea in China

The fall webworm, Hyphantria cunea Drury, is an economically important introduced pest in China. A native pupal endoparasitoid, Chouioia cunea Yang (Hymenoptera: Eulophidae, Tetrastichinae) causes considerable mortality of H. cunea pupae in some areas of China. Alternative hosts of C. cunea were tested to find potential substitute hosts for mass rearing the parasitoid. Antheraea pernyi Guerin-Meneville (Lepidoptera: Saturniidae) was chosen as a surrogate host as a single pupa produced an average of 6552 wasps and a maximum of 11,256 per pupa. A mass-rearing technique and successful release program of this parasitoid is described. Parasitism by C. cunea in the areas where it was released averaged 67.74% and was usually over 80%. Other native parasitoids, such as Coccygomimus disparis (Viereck), C. parnasae (Viereck) (Hymenoptera: Ichneumonidae) and Exorita japonica Townes (Diptera: Tachnidae), caused the total average parasitism to exceed 90% on average, reaching a maximum of 96.28% in the release areas. These successful releases indicate that an introduced pest species can also be controlled by the mass-rearing and release of native parasitoids in the country of introduction.     

The parasitoid complex affecting Choristoneura rosaceana and Pandemis limitata in organically managed apple orchards

Parasitism and density of obliquebanded leafroller,Choristoneura rosaceana (Harris), and thethree-lined leafroller, Pandemis limitata(Robinson) (Lepidoptera: Tortricidae), were studied in 1998–2001 in organically managed apple orchards in the southern interior of British Columbia, Canada. The density of the overwintered generation of leafrollers in spring was significantly higher than thedensity of the first generation in summer. There was a significant inverse correlation between the density of leafrollers and the percent parasitism within generation and therefore parasitoids may play a role in controlling leafroller density. Parasitism of the overwintered generation (means between 5.5 and 24.7%) was significantly lower than parasitism documented in the summer generation (means up to 67.9%). The parasitoid complex recorded as emerging from these leafrollerpopulations in 1998–1999 included 30 species, of which eight have not been previously recorded in the literature as parasitoids of either leafroller species. The most frequently occurring parasitoids included Apophuasimplicipes (Cresson) and two species of Glypta(Hymenoptera: Ichneumonidae), Macrocentrus linearis (Nees), Meteorus trachynotus Viereck, Apantelespolychrosidis Viereck, Apanteles atra (Ratzeburg) and Microgaster epagoges Gahan (Hymenoptera: Braconidae), Colpoclypeus florus (Walker) and one Sympiesisspecies (Hymenoptera: Eulophidae). All of these more common parasitoids, except Glypta sp. 1 and M. epagoges, were recorded parasitizing leafroller hosts in both leafroller generations. The number of days to adult parasitoid emergence, when field collected parasitized hosts were held at 20°C, was recorded for four of the parasitoid species. Meteorus trachynotus was found to emerge early enough in spring toparasitize the few remaining overwintered early instar leafrollers. Glypta sp. 1, A. simplicipes and one Apanteles species emerged to coincide with the first generationin the summer.     

Phenology and parasitism rates in introduced populations of <Emphasis Type="Italic">Pseudacteon tricuspis</Emphasis>, a parasitoid of <Emphasis Type="Italic">Solenopsis invicta</Emphasis>

We documented patterns of seasonal abundance and rates of parasitism in introduced populations of Pseudacteon tricuspis Borgmeier, a phorid parasitoid of the red imported fire ant, Solenopsis invicta Buren. Adult P. tricuspis populations were censused at monthly intervals for 1 year at three sites in northern Florida. Censuses were conducted by aspirating phorids attracted to disturbed S. invicta mounds. Pseudacteon tricuspis adults were present in every month at all sites, although abundances varied greatly among sites and over time. The highest densities of flies (up to 453 censused at 10 disturbed S. invicta mounds in 30 min) were observed in November, and changes in abundance over time were positively correlated among sites. Sex ratios were usually male biased. Parasitism rates were evaluated by collecting workers from field colonies and monitoring them in the laboratory for evidence of parasitism. Parasitism rates were very low – always less than 1%. The average parasitism rate per colony over 16 colonies and 2 years was 0.058%. No pupariation occurred within the first 8 days of collection, suggesting parasitism by P. tricuspis induced behavioral changes in parasitized workers that precluded such workers from our collections. If so, true field parasitism rates may be several times higher than measured here, yet still low in an absolute sense. These low parasitism rates can be reconciled with observed adult phorid densities by considering the large number of host ants present at the study sites.     

Parasitoids acquired byColeophora parthenica [Lepidoptera: Coleophoridae] ten years after its introduction into southern California for the biological control of Russian thistle

Parasitism of the stem and branch-boring mothColeophora parthenica Meyrick [Lepidoptera: Coleophoridae], introduced into California for the biological control of Russian thistle,Salsola australis R. Brown [Chenopodiaceae] was studied in the Coachella Valley of southern California during 1985–1986. Eight parasitoid species were reared from overwintering larvae ofC. parthenica, but none from the F₁ larvae, and just 2 individuals of one species from the F₂ summer generation. The level of parasitism of overwintering larvae was positively correlated with branch diameter, and ranged from 2% in the primary (main) branches to 45% in the tertiary branches in the spring 1985 sample, and from 2% to 19% in the spring 1986 sample, respectively. Rates of parasitism>20% were only found at sites with higher plant cover and chenopod diversity, but no other plant source or alternate hosts of the parasitoids ofC. parthenica were found. The 2 dominant species, the solitary, hymenopterous ectoparasitoids,Norbanus perplexus (Ashmead) [Pteromalidae] andEurytoma strigosa Bugbee [Eurytomidae], are both congeners of native parasitoids ofC. parthenica in Pakistan. The 2 other species of parasitoids ofC. parthenica in southern California for which other hosts are known are polyphagous and external on the larvae. No specialized endoparasitoid Braconidae, like those which dominate the native parasitoid complex in Pakistan and the U.S.S.R., have transferred toC. parthenica during its first 10 years in southern California.      

Is one parasitoid enough? A test comparing one with a pair of parasitoid species in the biological control of arrowhead scales

In classic biological control using natural enemies, the question of whether a single species or multiple species should be introduced has been a matter of debate. The introduction of two parasitoids, Aphytis yanonensis and Coccobius fulvus (Hymenoptera: Aphelinidae), to control the arrowhead scale, Unaspis yanonensis (Hemoptera: Diaspididae), which is a serious pest in Japanese citrus orchards, has been one of the most successful biological control projects in Japan. The success of this program may be explained by two alternative hypotheses: (1) the parasitoid species work complementarily, or (2) only one of them plays a major role. To test which hypothesis is applicable to this host-parasitoid system, we conducted caging experiments and observed temporal changes in the proportion of the parasitisms and the densities of arrowhead scales enclosed with one of the following combinations of parasitoids: (1) A. yanonensis and C. fulvus together, (2) A. yanonensis alone, (3) C. fulvus alone, or (4) neither parasitoid. Parasitisms in the cohorts with A. yanonensis and C. fulvus together and C. fulvus alone rapidly increased to approximately 70%; parasitism with A. yanonensis alone also increased slightly, although it remained consistently lower that those with A. yanonensis and C. fulvus together and C. fulvus. At the end of the experiment, parasitisms with A. yanonensis and C. fulvus together and C. fulvus alone were significantly higher than that with A. yanonensis alone. Parasitism by C. fulvus constituted most of (74%) the parasitism in the cohort with A. yanonensis and C. fulvus together. Further, only C. fulvus suppressed the population growth rates of scales significantly. These results suggest that C. fulvus alone successfully suppresses scale populations as efficiently as both species together do.     

What makes a successful biocontrol agent? A meta-analysis of biological control agent performance

We qualitatively reviewed the biocontrol literature in two major journals, Biological Control and Environmental Entomology, over the past 10 years by scoring 878 studies into 11 biocontrol-oriented questions. Quantitative meta-analyses were then used on data from 145 studies to examine the effects of different types of biocontrol agents (parasitoids, predators, and pathogens) on several attributes of weed and pest populations. Results for our qualitative review showed that most biocontrol studies were focused on lepidopteran pests, and that parasitoids were the most common biocontrol agents used. Our quantitative review showed that, for weeds, biocontrol agents significantly reduced weed biomass (−82.0%), flower (−98.9%), and seed production (−89.4%). For pests, our quantitative review showed that biocontrol agents significantly reduced pest abundance by 130% compared to control groups, increased parasitism (+139.0%) and increased overall pest mortality (+159.0%) compared to targets not exposed to biocontrol agents. Effects on pest mortality tended to be stronger for parasitoids than predators, although reductions caused in pest abundance were much stronger when predators were used as biocontrol agents. Addition of two or more biocontrol agents increased mortality by 12.97% and decreased pest abundance by 27.17% compared to single releases. Separate sets of meta-analyses demonstrated that the negative impacts of biocontrol on non-target species were much smaller than those for target species, although adverse effects of biocontrol on non-target organisms are based on small sample sizes and should be interpreted with caution. Our results also showed that biocontrol efficacy tended to be higher when agents were generalists than when they were specialists. Large fail–safe numbers found for most of the estimated effects indicate the robustness of the results found for the efficacy of biological control programs.     

Flower color affects tri-trophic-level biocontrol interactions

The adults of many parasitoid species require nectar for optimal fitness, but very little is known of flower recognition. Flight cage experiments showed that the adults of an egg parasitoid (Trichogramma carverae Oatman and Pinto) benefited from alyssum (Lobularia maritima L.) bearing white flowers to a greater extent than was the case for light pink, dark pink or purple flowered cultivars, despite all cultivars producing nectar. Survival and realised parasitism on all non-white flowers were no greater than when the parasitoids were caged on alyssum shoots from which flowers had been removed. The possibility that differences between alyssum cultivars were due to factors other than flower color, such as nectar quality, was excluded by dying white alyssum flowers by placing the roots of the plants in 5% food dye (blue or pink) solution. Survival of T. carverae was lower on dyed alyssum flowers than on undyed white flowers. Mixing the same dyes with honey in a third experiment conducted in the dark showed that the low level of feeding on dyed flowers was unlikely to be the result of olfactory or gustatory cues. Flower color appears, therefore, to be a critical factor in the choice of plants used to enhance biocontrol, and is likely also to be a factor in the role parasitoids play in structuring invertebrate communities.