Parasitoid Ratio Research Articles

Superparasitism in mass reared Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae), a parasitoid of fruit flies (Diptera: Tephritidae)

Abstract Superparasitism frequency and its effects on the quality of mass-reared Diachasmimorpha longicaudata (Ashmead) parasitoids were investigated under laboratory conditions. Percentage of adult emergence, sex ratio, survival, fecundity and flight ability of adult parasitoids that emerged from Anastrepha ludens (Loew) pupae with different levels of superparasitism were determined. A high prevalence of superparasitism was observed. The number of scars per pupa, produced by insertion of the parasitoid ovipositor, ranged from 1 to 30, with an average (±SD) of 8.3 ± 6.2. Adult parasitoid emergence decreased as the level of superparasitism increased. However, the fraction of females rose with increasing superparasitism and the flight ability was lower in adults emerging from pupae with only one scar, compared with adults emerging from superparasitized hosts. Female longevity and fecundity were not affected by superparasitism. Our results support the hypothesis that superparasitism in D. longicaudata might be adaptive, since adults emerging from hosts with moderate levels of superparasitism showed the highest percentage of emergence and there were no significant differences in the other quality control parameters tested. Our findings are relevant to the mass rearing process, where the ratio of hosts to parasitoids can be optimized as well as the distribution of eggs deposited in host larvae. This contributes to efficient mass rearing methods for augmentative biological control programs.

Functional response and mutual interference of <i>Diaeretiella rapae</i> (Hymenoptera: Aphidiidae) on Brevicoryne brassicae (Homoptera: Aphididae)

Functional response and mutual interference are the most important behavioural characteristics that reveal different aspects of host–parasitoid interactions. In this study, functional response and mutual interference of the parasitoid wasp, Diaeretiella rapae (M’Intosh) attacking the cabbage aphid, Brevicoryne brassicae (Linnaeus) were investigated. Logistic regression was used to distinguish the shape of the functional response (type II or III). Nonlinear least-square regression was used to estimate the attack rate (a) and handling time (Th). Nicholson’s model and linear regression were used to determine per capita searching efficiency and interference coefficient, respectively. Logistic regression suggested a type II response on B. brassicae nymphs. The per capita parasitism decreased significantly from 80.80 (67.33%) to 11.85 (9.88%) as parasitoid densities increased from 1 to 8 females. Consequently, the per capita searching efficiency decreased significantly from 1.173 to 0.205 as parasitoid densities increased from 1 to 8. The rate of parasitism increased as the host density increased from 2 to 50, and subsequently the parasitoid density decreased from 8 to 1. Therefore, different host–parasitoid ratios could affect the efficacy of D. rapae.

Consequences of Acclimation on Survival and Reproductive Capacities of Cold-Stored Mummies of <I>Aphidius rhopalosiphi</I> (Hymenoptera: Aphidiinae)

The cold storage of parasitoid mummies is a crucial point during mass production of parasitoids for aphid control in wheat, Triticum spp. In this study, the effect of acclimation to cold before storage of mummies containing Aphidius rhopalosiphi DeStefani-Peres (Hymenoptera: Braconidae: Aphidiinae) was evaluated on survival, sterility, and fecundity of parasitoids. Groups of 1-d-old and 3-d-old mummies were stored at -5 degrees C for 10 d without acclimation or after one of five different acclimation treatments. One-day-old and 3-d-old mummies contain prepupa and postmetamorphosis but not yet sclerotinized adult parasitoids, respectively. The offspring and sex ratio of stored parasitoids were compared with a control that was left at rearing temperatures (20 degrees C). Without acclimation, two-thirds of the parasitoids died during storage, for both ages of mummies tested. For stored 1-d-old mummies, subsequent survival increased under progressive exposure to low temperatures and reached 67.3% after 480 min of acclimation. In the same way, percentage of male sterility decreased with acclimation duration, whereas female fecundity increased. For stored 3-d-old mummies, subsequent survival improved with the duration of the acclimation treatment, but not male sterility and female fecundity.

Complementary biocontrol of aphids by the ladybird beetle Harmonia axyridis and the parasitoid Aphelinus asychis on greenhouse roses

We investigated whether two biocontrol agents, the parasitoid wasp Aphelinus asychis Walker and the predatory ladybird beetle Harmonia axyridis Pallas, would act in concert to reduce densities of an aphid ( Macrosiphum euphorbiae Thomas) that attacks greenhouse-grown roses ( Rosa hybrida L.). We conducted three types of experiments: feeding trials in microcosms that examine predator preference for mummies versus aphids; a longer-term pairing of the two natural enemies and aphids on whole plants in large cages; and a trial release of ladybird beetles into a rose greenhouse infested with the aphid and parasitoid. In the microcosm feeding trials both larvae and adults of H. axyridis fed on aphids, but also on parasitoid mummies, raising the possibility that intraguild predation of parasitoids by H. axyridis could disrupt aphid control. In cages, ladybird beetles dampened peak aphid densities during an outbreak without altering densities of parasitoid pupae or the ratio of parasitoids to aphids. In our whole-greenhouse release, we saw no evidence that H. axyridis disrupted aphid control by resident A. asychis . Together, these results suggest that H. axyridis can complement aphid biocontrol by the parasitoid A. asychis , rather than disrupting control through intraguild predation.

INTERACTIONS BETWEEN SPECIALIST AND GENERALIST NATURAL ENEMIES: PARASITOIDS, PREDATORS, AND PEA APHID BIOCONTROL

Most biological control systems involve a diverse community of natural enemies. We investigated how specialist and generalist natural enemies differ as biological control agents of pea aphids (Acyrthosiphon pisum), and how interactions among natural enemies affect successful control. In alfalfa, pea aphids are attacked by a specialist parasitoid wasp, Aphidius ervi, and a guild of generalist predators primarily made up of Nabis and Orius bugs, coccinellid and carabid beetles, and web-building spiders. In three field experiments, we manipulated the parasitoid, then the generalist predator guild, and finally both classes of natural enemy, and recorded resulting impacts on pea aphid population control. The parasitoid caused little immediate reduction in aphid population growth but caused a large decline after a delay corresponding to the generation time of the parasitoid. In contrast, the generalist guild caused an immediate decline in the aphid population growth rate. However, the generalists did not exert density-dependent control, so aphid densities continued to increase throughout the experiment. The third field experiment in which we simultaneously manipulated parasitoids and predators investigated the possibility of “nonadditive effects” on aphid control. Densities of parasitoid pupae were 50% lower in the presence of generalist predators, indicating intraguild predation. Nonetheless, the ratio of parasitoids to aphids was not changed, and the impact of the two types of natural enemies was additive. We constructed a stage-structured model of aphid, parasitoid, and predator dynamics and fit the model to data from our field experiments. The model supports the additivity of parasitoid and predator effects on aphid suppression but suggests that longer-term experiments (32 d rather than 20 d) would likely reveal nonadditive effects as predation removes parasitoids whose response to aphid densities occurs with a delay. The model allowed us to explore additional factors that could influence the additivity of parasitoid and predator effects. Aphid density-dependent population growth and predator immigration in response to aphid density would likely have little influence on the additivity between parasitism and predation. However, if a parasitoid were to show a strong Type II functional response, in contrast to A. ervi whose functional response is nearly Type I, interactions with predators would likely be synergistic. These analyses reveal factors that should be investigated in other systems to address whether parasitism and predation act additively on host densities. Corresponding Editor: E. Evans.

Influence of Feeding Treatment, Host Density, Temperature, and Cool Storage on Attack Rates ofTachinaephagus zealandicus(Hymenoptera: Encyrtidae)

Tachinaephagus zealandicus As hmead isa gregariousendoparas itoid that attacksthird instars of muscoid sies, including house sies, Musca domestica L. A colony of thisparas itoid was established from samples collected from a poultry farm in Santa Cruz da Conceicao, Sao Paulo, Brazil. The objective of this study was to evaluate the insuence of feeding treatment, host density and temperature on attack rateson T. zealandicus. Paras itoidsthat were given honey asadultsattacked two to three times as many house sy larvae (25 host attacks/female/d) as parasitoids that were given only water or nothing. Host attacks and progeny production by T. zealandicus on house sy and Chrysomyia putoria increased over the range of host:parasitoid ratios tested, reaching a maximum of 21Ð22 hosts killed and 13 progeny produced/female/d at the highest host density of 32 larvae/female. Host attacks were higher at 22C than at the other temperaturesstudied (20 Ð29 C), but differences in attack rateswere small over the range of 20 Ð27 C (10 Ð13 host attacks/female). Comparatively few hosts (6.3) were attacked at 29C. Higher rates of progeny production also were observed among parasitoids tested at lower temperatures (9 Ð11 progeny produced/female at 20 Ð22C) than at 29C (1.8 progeny/female). Femalesof T. zealandicus that were stored at 15C after emergence had highest rates of host attacks (58 Ð 62 hosts killed per group of Þve female parasitoids) and progeny production (174 Ð261 progeny) after 6 Ð12 d of storage at this temperature; relatively few hosts were attacked or parasitized (6 Ð9 host attacks and progeny/group) after 0 o r1da t 15C.

Ratio‐dependent parasitism with Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) on egg rafts of Nezara viridula (Linnaeus) (Hemiptera: Pentatomidae): effect of experimental variables and compatibility of ‘ratio’ and ‘Holling’ models

Abstract Egg rafts of Nezara viridula were exposed to the parasitoid wasp Trissolcus basalis in experimental arenas to establish the relationship of the rates of attack and parasitism to various combinations of arena size, parasitoid density, host density and parasitoid‐to‐host ratio. Arena sizes were varied in the ratio 1:9:63, with the largest having a search area of 1.44 m2. Parasitoid and host densities were varied over a 27‐fold range. The parasitoid‐to‐host ratios used were 1:1, 3:1 and 6:1. Finding time was related to a constant factor (flight propensity), rather than to the difficulty of finding (density of hosts). Initial attack rates were therefore related only to parasitoid numbers (or density), even at the lower densities and ratios. Parasitism rates (a function of attack rate per host) were thus also strongly related to parasitoid to host ratio, regardless of densities used and arena sizes. Even reducing host density, while keeping time and parasitoid density constant, increased the parasitism rate. A ratio model for parasitism rate was therefore compatible with the data but the more explicit Holling ‘disc’ equation was also compatible because handling time was sufficiently large to make it sensitive to the ratio of parasitoids to hosts for the densities used. We conclude that the two models would predict different results if the density of host egg rafts was in a range below one per square metre.

Effects of Snowdrop Lectin (<I>Galanthus nivalis</I> Agglutinin) Expressed in Transgenic Sugarcane on Fitness of <I>Cotesia flavipes</I> (Hymenoptera: Braconidae), a Parasitoid of the Nontarget Pest <I>Diatraea saccharalis</I> (Lepidoptera: Crambidae)

Cotesia flavipes (Cameron) is a parasitoid responsible for maintaining populations of sugarcane borer, Diatraea saccharalis (F.), below economic levels in south Texas sugarcane Þelds. Transgenic sugarcane expressing the snowdrop lectin (Galanthus nivalis agglutinin, GNA) was developed against the Mexican rice borer, Eoreuma loftini (Dyar), the primary pest of south Texas sugarcane. The potential impact of GNA-expressing sugarcane on various biological and Þtness parameters of Cotesia flavipes (Cameron) was studied in the laboratory to gain insight on likely effects of the transgenic sugarcane on biological control of sugarcane borer by C. flavipes. Females of C. flavipes were offered sugarcane borer larvae fed one of two diet treatments for oviposition for two successive generations: (1) artiÞcial diet containing transgenic sugarcane tissue or (2) artiÞcial diet containing nontransgenic sugarcane tissue. Small to marginal negative effects of artiÞcial diet con- taining transgenic sugarcane tissue were evident in the rate of host suitability, number of cocoons and adult parasitoids emerging per host, percentage cocoons yielding parasitoids, and sex ratio and adult lifespan of parasitoids. These effects were variable between the two parasitoid generations examined. In contrast, differences were not detected between diet treatments in rates of host acceptance, egg load of females, and egg to adult developmental periods. The negative effects of transgenic sugarcane on C. flavipes detected in this study are important because GNA levels in the diet (0.49% of total protein content) containing transgenic sugarcane tissue were 50% of the level expressed in trans- genic sugarcane plants. Results are discussed in relation to potential impacts of the transgenic cultivar on biological control of sugarcane borer by C. flavipes.

Ovipositional studies of Bracon hebetor Say, on two host insects under laboratory conditions

A host (Corcyra cephalonica / Maruca testulalis) parasitoid (Bracon hebetor) ratio of 2:1 was found to result in higher parasitisation, though the production of females was equal in all the ratios tried. Parasitisation was found to be higher in fourth instar larvae in both the host insects. Three mated female parasitoids per ten host larvae resulted in the best utilisation of the larvae supplied as became evident from the relatively high parasitisation and production of female parasitoid.

Impacts of Developing on Bt Maize-Intoxicated Hosts on Fitness Parameters of a Stem Borer Parasitoid

Transgenic Bt maize cultivars are widely used in U.S. agriculture. Control of target pests by these cultivars is not complete, and non-target pests are usually tolerant of the toxic activity of these cultivars. Sublethally intoxicated pest individuals may become hosts for parasitoids, but their quality as hosts may be affected as a result of intoxication. This study addresses the effects on various fitness parameters in parasitoids that develop on intoxicated hosts. The parasitoid used in this study was Parallorhogas pyralophagus (Marsh), a gregarious, external idiobiont, and the host was Eoreuma loftini Dyar, a subtropical stemborer. Results showed that ingestion of Bt maize tissue by E. loftini larvae negatively affected some fitness components in P. pyralophagus, whereas other components were not affected. Specifically, immature stage developmental mortality was greater, adult longevity was approximately 1 d shorter in females, and developmental times were approximately 2 d longer in both males and females when P. pyralophagus developed on hosts fed Bt maize tissue relative to hosts fed non-Bt maize tissue. Moreover, parasitoid brood size was positively correlated with host size when hosts were fed non-Bt maize tissue, while this relationship was absent when hosts were fed Bt maize tissue. However, feeding on Bt maize tissue did not affect egg loads, adult size-egg load and adult size-longevity relationships, adult sizes of females and males, and sex ratio of parasitoids whose hosts were fed Bt maize tissue. Results are discussed in relation to the importance of sublethal effects of Bt maize cultivars on parasitoids and potential impacts on naturally-occurring biological control.

Oviposition response and development of the egg‐pupal parasitoid Fopius arisanus on Bactrocera oleae, a tephritid fruit fly pest of olive in the Mediterranean basin

AbstractTo date, information is wanting with regard to the use of new exotic parasitoids against olive fruit fly, Bactrocera (=Dacus) oleae (Gmelin) (Diptera: Tephritidae), a serious pest of olives Olea europaea L., in the Mediterranean basin. We investigated the oviposition response and developmental biology on B. oleae of Fopius (=Biosteres) arisanus (Sonan) (=Opius oophilus Fullaway) (Hymenoptera: Braconidae), an egg‐pupal parasitoid of tephritid fruit flies, never tested before as a potential parasitoid of this host. Our results showed that olive fruits infested with B. oleae eggs exerted a relevant attraction to gravid F. arisanus and represented a stimulus for oviposition. Nevertheless they were not as attractive to female parasitoids as the Mediterranean fruit fly, Ceratitis capitata Wiedemann (Diptera: Tephritidae), eggs infested papaya fruits (Carica papaya L.). In our experimental conditions, F. arisanus completed development in B. oleae within 33 ± 1.7 days (males) and 35 ± 1.6 (females). Increases in host egg to female parasitoid ratios of 1:1, 5:1, 10:1 and 20:1 corresponded with decreases in the percentage of B. oleae parasitisation and host killing but corresponded also with increases in absolute parasitisation. Our findings are discussed in light of possibilities of utilising F. arisanus for biological control of olive fruit fly.

Factors Affecting Parasitization of Spodoptera exigua (Lepidoptera: Noctuidae) and Sex Ratio of the Parasitoid Cotesia marginiventris (Hymenoptera: Braconidae)

Biological factors hypothesized to affect parasitization by Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), an endoparasitoid of larvae of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), as well as its sex ratio, were examined in the laboratory. Highest parasitization occurred when (1) adult female parasitoids were closely associated with hosts, (2) adult female parasitoids were 1 day old, (3) a host:female parasitoid ratio of between 10:1 and 30:1 was maintained, (4) second-instar beet armyworms were used as hosts, and (5) adult female parasitoids were exposed to hosts for 24 h. Percentage of female progeny was higher when females were closely associated with second instars.

A two-component model of host–parasitoid interactions: determination of the size of inundative releases of parasitoids in biological pest control

A two-component differential equation model is formulated for a host–parasitoid interaction. Transient dynamics and population crashes of this system are analysed using differential inequalities. Two different cases can be distinguished: either the intrinsic growth rate of the host population is smaller than the maximum growth rate of the parasitoid or vice versa. In the latter case, the initial ratio of parasitoids to hosts should exceed a given threshold, in order to (temporarily) halt the growth of the host population. When not only oviposition but also host-feeding occurs the dynamics do not change qualitatively. In the case that the maximum growth rate of the parasitoid population is smaller than the intrinsic growth rate of the host, a threshold still exists for the number of parasitoids in an inundative release in order to limit the growth of the host population. The size of an inundative release of parasitoids, which is necessary to keep the host population below a certain level, can be determined from the two-component model. When parameter values for hosts and parasitoids are known, an effective control of pests can be found. First it is determined whether the parasitoids are able to suppress their hosts fully. Moreover, using our simple rule of thumb it can be assessed whether suppression is also possible when the relative growth rate of the host population exceeds that of the parasitoid population. With a numerical investigation of our simple system the design of parasitoid release strategies for specific situations can be computed.

Host age selection by the host-feeding pupal parasitoid Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae).

Laboratory experiments were conducted to determine the effects of host age on oviposition and host-feeding by Diadromus subtilicornis, a pupal endoparasitoid of the diamondback moth Plutella xylostella. When given 6 hosts of single ages, females oviposited in hosts of prepupae, 1-d-old green and 2-d-old pupae 66-75% of the time. However, females oviposited in only 43% of 1-d-beige pupae, 31% of 3-d-old pupae and none of 4-d-old pupae. They often rejected those hosts after antennal examination or drilling. Total time taken for oviposition increased with increasing host age. When given 36 hosts of 6 different ages, females preferred hosts of prepupae, 1-d-old green and 2-d-old pupae for oviposition and hosts of 1-d-old beige, 2-d-old and 3-d-old pupae for host-feeding, suggesting host-age selective oviposition and host-feeding. Although 93% of parasitized prepupal hosts yielded emerged adult parasitoids, successful emergence rate decreased with increasing age. No parasitized 3-d-old pupal hosts yielded adult parasitoids. Sex ratio (% females) of parasitoids emerged from hosts ranged from 28.6% for 1-d-old beige to 44.4% for prepupae. Host ages did not affect developmental time and size of emerged adults. We suspected that D. subtilicornis has mixed host-feeding strategies of non-concurrent and concurrent types according to host availability.

Host Location by House Fly (Diptera: Muscidae) Parasitoids in Poultry Manure at Different Moisture Levels and Host Densities

Manure moisture varies widely under field conditions, but little is known about the impact of this environmental factor on natural enemies of house flies. Five species of parasitoids [Muscidifurax raptor Girault & Sanders, Spalangia cameroni Perkins, Spalangia endius Walker, Spalangia gemina Boucek, and Dirhinus himalayanus (Masi)] were evaluated for their ability to locate house fly, Musca domestica L., pupae in manure containing 45, 55, 65, 75, or 85% moisture at low (2.5:1) and high (50:1) host:parasitoid ratios. M. raptor preferred manure with 75% or less moisture, and responded most strongly to manure containing 45% moisture, with little change in search patterns caused by host density. The 3 Spalangia spp. showed a strong preference for manure containing 45–65% moisture, but attacked pupae in wetter manure when hosts were in short supply. S. endius showed the strongest preference among the Spalangia spp. for dry manure, attacking twice as many hosts in the 45% moisture treatments as in the 55 or 65% treatments. D. himalayanus was similar to M. raptor and preferred dry manure, although this species had low overall rates of host attacks and reproduction compared with the other species. The results suggest that releases of combinations of species with different manure moisture preferences may be more effective than single-species releases in poultry houses that are heterogeneous with respect to moisture conditions.

The effect of varying Bemisia argentifolii and Eretmocerus mundus ratios on parasitism

We investigated the effects of different host: parasitoid ratios on the efficacy of the parasitoid Eretmocerus mundus attacking the silverleaf whitefly, Bemisia argentifolii. When host density was held constant (100 second instars) and parasitoid density was decreased from 15 to 1 females, the percentage of total host mortality was significantly lower at low parasitoid densities. The number of host nymphs killed, and the number of female parasitoid progeny per female, increased 3.6 and 20.4 times, respectively. The emergence rate, sex ratio, longevity, and body lengths of progeny were significantly larger at the lowest parasitoid density while developmental time was significantly shorter. When the number of hosts was increased from 5 to 250 and parasitoid density was held constant (5 females), the percentage of nymphal mortality decreased 1.6 times. The percentage of desiccated nymphs was significantly highest (65.7%) at the lowest host density, while percentage parasitism (34.3%) was significantly lowest at the lowest host density. The data could be described using a Type I functional response curve. We propose a generalized index of efficacy (GIE) to summarize and compare the total effects of parasitoid--host ratios. This index showed that the most efficient ratio was one parasitoid female per ten second instar host nymphs.

Preferência de Palmistichus elaeisis por pupas de diferentes lepidópteros praga

Palmistichus elaeisis Delvare & LaSalle, 1993 (Hym., Eulophidae), uma espécie de endoparasitóide foi coletado em pupas de Sabulodes sp. (Lep., Geometridae), em área de eucalipto. Foram testadas quatro espécies de lepidópteros-praga: Diatraea saccharalis (Fabricius, 1794) (Crambidae), Anticarsia gemmatalis Hübner, 1818, Heliothis virescens (Fabricius, 1781) e Spodoptera frugiperda (J.E. Smith, 1797) (Noctuidae), com o objetivo de estudar a preferência para oviposição, visando a criação massal do parasitóide. Os resultados mostraram que a porcentagem de parasitismo variou de 90% (H. virescens) a 100%. Em D. saccharalis, o parasitóide P. elaeisis apresentou a maior razão sexual (0,98) e em H. virescens a menor (0,69). A espécie de hospedeiro influenciou significativamente o tamanho dos machos de P. elaeisis (1,64 mm em S. frugiperda e 1,49 mm em H. virescens), mas não o das fêmeas. O sexo do hospedeiro é importante no total de parasitóides produzidos, enquanto que as espécies do hospedeiro não interferiram no total de parasitóides produzidos.

Biology and rearing of the fruit fly parasitoid Biosteres arisanus: clues to insectary propagation

AbstractAspects of Biosteres arisanus (Sonan) (=Opius oophilus Fullaway) (Hymenoptera: Braconidae) development on the oriental fruit fly, Bactrocera (=Dacus) dorsalis (Hendel), were investigated to facilitate mass production in the insectary. Life table statistics were generated for cohorts of B. arisanus females. Overlap in the emergence of fruit flies and parasitoids necessitated a procedure for segregation, preferably before adult eclosion. Rate of parasitization by B. arisanus increased with host clutch size reaching a plateau at 20:1 host egg to female parasitoid ratio. Duration of the oviposition period influenced the level of host parasitization; host eggs were exposed to parasitoids for 24 h with minimal superparasitism. Females were highly productive within 3 weeks after emergence producing 40–70% females in the progeny. Adult males were shorter lived than females by ≈5 days. Based on a net reproductive rate (R0) of >16 daughters per female parent, a population increase of 10% was predicted each day. Handling procedures that could facilitate efficient production of parasitoids are discussed.

Biological Control of Bruchids in Cowpea Stores by Release of Dinarmus basalis (Hymenoptera: Pteromalidae) Adults

Bruchids cause major losses during cowpea storage in West Africa, Two species, Callosobruchus maculatus (F.) and Bruchidius atrolineatus (Pic), and one endemic pteromalid, Dinarmus basalis (Rondani), are present in the Bobo Dioulasso region of Burkina Faso where this study was carried out. B. atrolineatus adults emerging in the stores from December to January are in reproductive diapause and the C. maculatus population is small. Numbers of C. maculatus increase substantially from February onward when temperatures and humidity rise. The number of D. basalis is low at the beginning of cowpea storage in December and the parasitoid cannot control the C. maculatus population, which therefore causes important seed weight losses. When adults of D. basalis are introduced into the stores at regular intervals, either during the first 2 mo of storage or during the entire storage period, the parasitoids reduce the increase in C. maculatus numbers and the seed weight losses are <100 g/kg of seeds, Releases of D. basalis adults were made in stores containing varieties of cowpea which had been infested naturally in the fields before harvest or were infested artificially. The introduction of high numbers early in the storage period limited the buildup of the bruchid population even when the climatic conditions become favorable for the rapid multiplication of C. maculatus. Studies in the laboratory under controlled conditions confirmed the results obtained under natural conditions in Burkina Faso. A high ratio of parasitoids to host larvae and pupae was critical to the successful biological control. It is only when this ratio is high that C. maculatus populations can be controlled. Biological control of bruchids, using inoculation by D. basalis adults in cowpea stores at the beginning of the storage time, is possible and limits weight losses of stored seeds to <10% after 6 or 7 mo.

Tritrophic Studies of Russian Wheat Aphid, a Parasitoid, and Resistant and Susceptible Wheat over Three Parasitoid Generations

Parasitism of Russian wheat aphid,Diuraphis noxia(Mordvilko), byDiaeretiella rapae(McIntosh) was studied over three parasitoid generations on resistant and susceptible wheat,Triticum aestivumL. Rate of increase forD. noxiawas lower when plants were initially infested with 20 aphids than 10 aphids per plant. Plant damage was similar between the two aphid densities (10 and 20 aphids per plant). Rate of population increase forD. noxiawas lower on the resistant wheat line (PI 137739) in all three generations compared with the susceptible wheat cultivar (‘Stephens’). Plant biomass was greater in infested plants of both wheat entries exposed to parasitoids than infested plants without parasitoids. Percentage emergence, sex ratio, adult longevity, and head width of parasitoids were not changed by plant resistance. Longevity of female parasitoids was greater when parasitoids emerged from aphids reared on the resistant line. The resistance in PI 137739 was compatible withD. rapaegrowth and development.