Relative preference of Lygus hesperus (Hemiptera: Miridae) to selected host plants in the field

Abstract Information on host plant preference of agriculturally important insect pests, such as Lygus hesperus (Knight), can be helpful in predicting its occurrence and future movement among crop and non-crop host plants. A field study was conducted during 2005 and 2006 to evaluate the host preference of Lygus to cotton and four other host plants in the Texas High Plains, including alfalfa (Medicago sativa L.), wild sunflower (Helianthus annuus L.), Russian thistle (Salsola iberica L.) and pigweed (Amaranthus palmeri L.). Sampling for both nymphs and adults during 2005 (July to November) and 2006 (June to November) showed that alfalfa and Russian thistle were the two most preferred hosts out of the five hosts evaluated. Abundance of nymphs (numbers per 50 sweeps per host plant) during the sampling period also indicated the superior reproductive suitability of alfalfa and Russian thistle. Cotton appeared to be the least attractive host plant for Lygus when Russian thistle and alfalfa were available in the host mosaic. Seasonal abundance of Lygus was found to be lower during 2006 compared to 2005, which may be explained by the difference in rainfall patterns during these two years. In terms of species dominance, L. hesperus was the most dominant species in the sampled population followed by an inconsiderable fraction of L. elisus and L. lineolaris.     

Being a parasitoid of parasites: host finding in the tick wasp Ixodiphagus hookeri by odours from mammals

The parasitic wasp Ixodiphagus hookeri Howard (Hymenoptera: Encyrtidae) parasitizes larvae and nymphs of a number of tick species worldwide. Ticks themselves are parasitic on vertebrate hosts. To study the specificity and reliability of vertebrate odours used by I. hookeri for host location, we conducted bioassays in a four‐chamber olfactometer. Wasps were arrested by carbon dioxide and by odours from roe deer faeces and odours from hair of roe deer and wild boar. Odours from faeces of cattle, rabbit, and field mouse as well as odours from hair of cattle and field mouse had no effect. Odours from faeces of the host tick species Ixodes ricinus L. (Acari: Ixodidae) were attractive only up to a distance of 1 cm. Thus, I. hookeri reacts to general (carbon dioxide) and specific vertebrate odours from wild boar and deer. Examination of freshly shot specimens demonstrates that deer and wild boars are infested with a sufficient number of tick nymphs to tap the full reproductive potential of an I. hookeri female, which makes cues from these mammal species reliable. These results indicate that I. hookeri locates its hosts using specific and reliable mammal odours and that ticks are parasitized on their vertebrate hosts. The implications of this host‐finding strategy and its benefits for the parasitoid are discussed.     

Reproductive strategy of winged and wingless morphs of the aphids Sitobion avenae and Metopolophium dirhodum

The reproduction of apterous and alate morphs of the aphids Sitobion avenae and Metopolophium dirhodum is compared on the basis of fecundity in 5- and 10-day periods of adult life. Apterae of both species are consistently more fecund than alatae of comparable weight, producing about three more nymphs on average in any 5-day period. The reproductive differences are related to the number and quality of embryos at eclosion and to ovulation rates, both of which in turn appear to be linked to wing-muscle maintenance. These relationships between weight, embryos and reproduction may be used to predict a newly moulted adult aphid's fecundity, a method which may facilitate the assessment of resistance to aphids in new cereal varieties, by obviating lengthy recording of reproduction. The strategies by which alatae of these and other aphid species minimize the difference between their fecundity and that of apterae are discussed.     

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different resistance genes

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different A. idaei resistance genes were studied under infestation tunnel, glasshouse and laboratory conditions. Each test consistently ranked raspberry cultivars, with increasing levels of resistance in the order, non-resistant (cv. Mailing Jewel), resistant cultivars containing minor genes, major gene A₁, gene A₁₀ (red raspberry) and gene A₁₀ (black raspberry) respectively. Resistance was expressed in three different ways; decreased alatae settling and feeding, decreased apterae settling and decreased aphid fecundity and rate of nymphal development. Following exposure to a large population of alatae, significantly fewer aphids settled on A₁, A₁₀ and to a lesser extent, minor gene-containing cultivars, compared to cv. Mailing Jewel. More alatae settled on the top than the bottom zone of non-resistant, minor gene resistant and two of four A₁-containing cultivars; alatae settling was low on all zones of A₁₀-containing raspberries. Aphid fecundity and nymph development patterns on different cultivars and resistance classes were similar to those found for alatae settling. After 7 days reproduction more than 30% of the nymphs developed to third or fourth instar on cv. Mailing Jewel, whilst on minor gene and major gene-containing cultivars the total number of nymphs and the proportion of later instars decreased. On resistant cultivars the nymphs were found mainly on the middle and bottom leaf zones, compared to cv. Mailing Jewel. A rapid (48 h) screening test using floating leaflets was developed and, on the basis of apterae settling, ranked cultivars reliably; it was particularly effective in distinguishing moderate and strong resistance to strain 1 A. idaei. Gene A₁-containing cultivars bred in England were much more resistant to strain 1 A. idaei than those bred in Scotland. Possible reasons for this difference are discussed, in relation to modified screening procedures and to the control of the viruses transmitted by this aphid vector.     

Aspects of life-table studies and functional response of Lysiphlebia mirzai

The fecundity, reproductive rate, and survival of Lysiphlebia mirzai parasitising third instar nymphs of the cereal aphid Rhopalosiphum maidis were measured at six different host densities under constant laboratory conditions. The survival rate (l_x) of the female parasitoids was unaffected by host density, with an average adult life-span of 5–6 days at all densities. The age-specific fecundity rate (m_x) was host density-dependent. The value of m_x decreased rapidly from the first day of parasitisation. The number of hosts available determined the maximum possible number of mummies. At 200 hosts available per day, the average fecundity was 184.6 mummies/female; the maximum number of mummies yielded by any female was 200. The relationship between host density and the number of aphids parasitised per female was linear at 50 aphids/cage/day, but at higher host densities (100 aphids/cage/day) a significant curvilinear regression was observed. The intrinsic rate of natural increase (r_m) increased with increasing host density. Maximum value of r_m (0.262) was obtained at a host density of 200. The response of r_m to changes in host density and parasitoid sex ratio is shown. A typical type II functional response was observed for L. mirzai. The curve was described by a logistic curve, N_p=200/[1+exp(5.65–1.60 ln N_o)]. The search rate of the parasitoid was inverse host density-dependent. No significant variation in the sex ratio of F₁ offspring was observed at different initial host densities. Sex ratio values exceeded 0.5 at all host densities. The results evaluated the reproductive potential of L. mirzai as a promising biological control agent.     

Observations on the morphs of Macrosiphum avenae and Metopolophium dirhodum on cereals during the summer and autumn

From June to early August 1970, populations of Macrosiphum avenae and Metopolophium dirhodum on marked tillers of field barley were compared with the numbers of alatae trapped at crop height and at 12.2 m. There were always more M. dirhodum than M. avenae on the tillers. Only apterae were produced until mid-June when both alatae and apterae occurred; after mid-July only alate M. avenae were found. Until mid-June most of the flying alatae were caught at 12.2 m as they migrated from spring hosts to cereals. Thereafter, more alate M. avenae were trapped at 12.2 m than at crop level, whereas numbers of alate M. dirhodum were usually comparable at both heights. Although crop and flying populations occasionally showed temporal similarities, insufficient is known about their field distribution and the factors affecting their alate production and flight activity to interpret this relationship. In the autumn, two consecutive reproductive phases of M. dirhodum occurred on winter wheat grown in pots outdoors. Initially, apterous virginoparae and alatae, probably sexuparae, were produced, whereas only alate males appeared during the second phase. In contrast, M. avenae deposited mainly apterous virginoparae although some oviparae developed in October to lay scattered, probably infertile eggs on the tillers.     

Aphid colonization of spring cereals

In 1970-1, Metopolophium dirhodum, Rhopalosiphum padi and Sitobion avenae were the commonest alatae trapped from April/May to August, with most in July and early August. The first alatae appeared in the Rothamsted survey suction trap 0–34 days before aphids were found on the cereals, but during May and June no relationship was found between the numbers trapped and the number on the crop. Most species occurred first near the sheltered edge of the crop, but M. dirhodum was widespread over the field. Most infestations were quickly dispersed by the movements of older morphs; adults only stayed in one place for about 2 days. Alate M. dirhodum moved more often than apterae, but both morphs of S. avenae moved equally often and more frequently between larvipositions than did those of M. dirhodum. Apterae deposited more nymphs in a ‘group’ than alatae, and M. dirhodum deposited more than S. avenae. Few ‘groups’ persisted for more than a week. Although M. dirhodum occupied the crop area faster than S. avenae, all 0–3 m lengths of row sampled being infested within 2–5 wk of their first appearance, most or all of the tillers were colonized only in late July 1970.     

Life cycle and host specificity of Amblyomma triste (Acari: Ixodidae) under laboratory conditions

We report biological data of two generations of Amblyomma triste in laboratory and compared the suitability of different host species. Infestations by larval and nymphal stages were performed on guinea pigs (Cavia porcellus), chickens (Gallus gallus), rats (Rattus norvegicus), rabbits (Oryctolagus cuniculus), wild mice (Calomys callosus), dogs (Canis familiaris) and capybaras (Hydrochaeris hydrochaeris). Infestations by adult ticks were performed on dogs, capybaras and rabbits. Tick developmental periods were observed in an incubator at 27 °C and RH 90%. Guinea pigs were the most suitable hosts for larvae and nymphs, followed by chickens. The remaining host species were less suitable for immature ticks as fewer engorged ticks were recovered from them. Mean larval feeding periods varied from 3.8 to 4.7 d between different host species. Mean larval premolt periods ranged from 8.9 to 10.4 d. Nymphal mean feeding periods varied from 4.2 to 6.2 d for ticks fed on different host species. Premolt period of male nymphs (mean: 15.4 d) was significantly longer than that of female nymphs (14.7 d). Female nymphs were significantly heavier than male nymphs. The overall sex ratio of the adult ticks emerged from nymphs was 0.9:1 (M:F). Capybaras were the most suitable host for the tick adult stage as significantly more engorged females were recovered from them and these females were significantly heavier than those recovered from dogs or rabbits. The life cycle of A. triste in laboratory could be completed in an average period of 155 d. The potential role of guinea pigs, birds and capybaras, as hosts for A. triste in nature, is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oviposition behaviour, influence of experience on host size selection, and niche overlap of the solitary Leptomastix epona and the gregarious Pseudaphycus flavidulus, two endoparasitoids of the mealybug Pseudococcus viburni

Oviposition behaviour and host size selection of the solitary parasitoid Leptomastix epona(Walker) and the gregarious Pseudaphycus flavidulus(Brèthes) [both Hymenoptera: Encyrtidae] were examined on five size classes of the mealybug Pseudococcus viburni(Signoret) [Hemiptera: Pseudococcidae]. The host size classes mostly consisted of one stage (first, second, third instar nymph, young adult and preovipositing adult) and were presented together to wasps of either parasitoid species. Both parasitoid species locate the host by drumming the surface of the patch with the antennae. Leptomastix eponaseems to use mainly the antennae to examine the host but P. flavidulusmay accept or reject a host for oviposition after antennation or insertion of the ovipositor. Leptomastix eponaattempts oviposition in all the host stages from second instar nymphs but P. flavidulusincludes first instar. Both parasitoid species select mainly larger hosts (>1 mm, third instar nymphs) to oviposit but P. flavidulusis able to parasitize more second instar nymphs compared to L. epona. Female wasps of L. eponamay host feed on small mealybugs (second and third instar nymphs) that they do not use for oviposition. Oviposition experience of either parasitoid species for 24 hours does not influence host size selection on patches with hosts of similar mixed sizes. Oviposition decisions are independent of the host sizes of the preceding ovipositions. Implications about stability of a single parasitoid – host system and the success of biological control of the mealybug were discussed in respect of the developmental refugia of the two parasitoid species. Niche overlap of the two parasitoid species was discussed with a view to giving an insight into a single or multiple introduction.     

Differential colonization of wheat cultivars by two biotypes of Russian wheat aphid （Homoptera： Aphididae）

Susceptible and resistance wheat cultivars, Triticum aestivum L, were presented to two biotypes of Russian wheat aphid, Diuraphis noxia （Mordvilko）, in multiple choice tests to assay their relative acceptability as host plants. Both apterae （third and fourth instars） and alate adults were offered plants at the two-leaf stage in different cultivar combinations at 22±1℃ and 16：8 （L： D） hour photoperiod. Apterae were released from Petri dishes in the center of a circle of test plants, whereas alatae dispersed from a mature aphid colony to settle on plants arranged in rows. Both alatae and apterous nymphs of both biotypes readily colonized all cultivars tested：‘2137＇, ‘Akron＇,‘Ankor’,‘ Halt’ ,‘ Jagger’ ,‘ Prairie Red’ , ‘Stanton＇,‘TAM 107＇,‘TAM 110＇,‘Trego＇, ‘ Yuma＇, and ‘Yumar＇. Fewer biotype I apterae responded （settled and fed） in the combination containing more resistant （Dn4- and Dny-expressing） cultivars, compared to the combinations that had fewer. The reverse was true for biotype 2 apterae; more aphids responded in the combination containing the largest number of Dn4 expressing cultivars. Differential colonization of cultivars was observed in only one combination, in which biotype 2 apterae colonized Akron and Yumar in larger numbers than they did Stanton and Yuma. A separate experiment confirmed that, 48 hours after infestation, more biotype 2 apterae abandoned plants of Yuma than plants of Yumar. This differential response was likely due to genetic differences between the two ＇ near isogenic＇ lines that include the lack of Dn4 expression in Yuma. Choice tests with alatae did not result in differential rates of cultivar colonization by either biotype in any combination tested. These results suggest that young wheat plants appear to lack any meaningful antixenosis toward D. noxia, even though the aphids appear to perceive, and sometimes respond to, certain differences in cultivar suitability.     

Chemical aspects of host‐acceptance behaviour in the bird cherry‐oat aphid Rhopalosiphum padi: host‐acceptance signals used by different morphs with the same genotype

Host acceptance by gynoparae and winged virginoparae of the bird cherry‐oat aphid Rhopalosiphum padi (L.) is investigated utilizing leaves and aqueous extracts of the primary and secondary hosts, as well as nonhost plants. Gynoparae are specialized to reproduce on bird cherry Prunus padus L., whereas virginoparae reproduce and feed on various grasses. Host acceptance is assessed using levels of reproduction and survival for adults, as well as survival for nymphs. Little is known of host acceptance by nymphs. The data show that gynoparae and winged virginoparae produce nymphs almost exclusively on their host plants, bird cherry and barley leaves, respectively, over 72 h. When tested with aqueous plant extracts, however, gynoparae produce nymphs almost exclusively on bird cherry extract and progeny numbers are found to be similar to those on plant leaves. Few nymphs are produced on artificial diet. By contrast, winged virginoparae produce nymphs on aqueous extracts of barley, bird cherry and bean, as well as on artificial diet. The numbers of nymphs deposited by gynoparae are similar on aqueous extracts of bird cherry leaves collected at different times during the growing season. When extracts from leaves of various Prunus species are tested, only leaves of P. padus and Prunus virginiana stimulate parturition. Oviparae, the sexual female nymphs of gynoparae, survive well for 96 h on both bird cherry and barley leaves but not on bean seedlings, whereas nymphs of winged virginoparae survive well only on barley leaves. They do not survive for 96 h on any plant‐leaf extracts, although they do survive on artificial diet.     

Effects of host age and host availability on developmental period,adult size,sex ratio,longevity and fecundity inLariophagus distinguendus Förster (Hymenoptera: Pteromalidae)

The effects of host age on parasitoid reproductive capacity are studied using the pteromalid parasitoid Lariophagus distinguendusFörster and its bruchid hosts, Callosobruchus chinensis (L.) and C. maculatus (F.). A series of experiments were performed to investigate relationships between age and size of host parasitized and the developmental period of pre-imaginal progeny, sex ratio, female size, longevity, fecundity and oviposition rate. There was no effect of host size on preimaginal parasitoid developmental period. Sex ratio varied from less than 5% females from young (small) hosts to 60% females from mature (large) hosts. Adult size, female longevity, fecundity, and oviposition rate were also positively related to host age. Females provided mature hosts lived longer than those provided either young hosts or no hosts, possibly because of an increased ability to host-feed from the larger hosts. The implications of these findings to parasitoid population reproductive capacity and host-parasitoid synchrony are discussed.     

Convergent development of a parasitoid wasp on three host species with differing mass and growth potential

Koinobiont parasitoids develop in hosts that continue feeding and growing during the course of parasitism. Here, we compared development of a solitary koinobiont endoparasitoid, Meteorus pulchricornis Westmael (Hymenoptera: Braconidae), in second (L2) and fourth (L4) instars of three host species that are closely related (Lepidoptera: Noctuidae) but which exhibit large variation in growth potential. Two hosts, Mamestra brassicae L. and Spodoptera littoralis Boisduval, may reach 1 g or more when the caterpillars are fully mature, whereas Spodoptera exigua Hübner is much smaller with mature caterpillars rarely exceeding 200 mg. Parasitoid survival (to pupation) in the two host instars was much higher on the larger hosts than on S. exigua. However, other fitness correlates in M. pulchricornis were very similar in the three host species. Development time was fairly uniform in L2 and L4 hosts of the three host species, whereas wasps were larger in L4 than in L2 hosts. However, M. pulchricornis developmentally arrested each of the hosts differently. The mass of dying L2 and L4 hosts after parasitoid larval egression (i.e., when they emerge from the dying caterpillar) varied significantly, with S. littoralis being by far the largest and S. exigua the smallest. These results reveal that M. pulchricornis is able to adjust its own development in response to species‐specific differences in host resources.     

Pear rust mite,Epitrimerus pyri (Acari: Eriophyidae) oviposition and nymphal development on Pyrus and non-Pyrus hosts

The ovipositional response of deutogynes of the pear rust mite, Epitrimerus pyri Nalepa, and the rate of development of E. pyri nymphs differed according to the host plant to which mites were exposed. Among the fourteen Pyrus hosts examined, leaves from the Clapp's Favorite cultivar of P. communis elicited the strongest egg-laying response and the fastest rate of nymphal development, whereas egg-laying was least and nymphal development slowest on leaves from P. calleryana. Among the three non-Pyrus hosts, E. pyri oviposited and completed nymphal development on apple and quince leaves, but would not deposit eggs on apricot. Antixenosis appears to be the primary mechanism of resistance, although antibiosis may be operating to a lesser extent. These results are discussed in relation to the resistance of Pyrus to other arthropod pests.     

Mating and host density affect host feeding and parasitism in two species of whitefly parasitoids

Abstract The parasitoids in the genera of Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies, and some of them not only parasitize hosts but also kill them with strong host‐feeding capacity. Two whitefly parasitoid species, Encarsia sophia and Eretmocerus melanoscutus were examined to determine if mating and host density affected their host feeding and parasitism. The whitefly host, Bemisia tabaci, was presented to these two wasp species in densities of 10, 20, 30, 40, 50 and 60 third‐instar nymphs per clip cage. Mated whitefly parasitoid females fed on more hosts than unmated females under a range of host densities (under all six host densities for En. sophia; under the densities of 40 nymphs or more for Er. melanoscutus). Meanwhile, mated females parasitized more whitefly nymphs than unmated females under all host densities for both species. With increase of host density, mated or unmated Er. melanoscutus females killed more hosts by host feeding and parasitism. Mated En. sophia females killed more hosts by host feeding with increase of host density, whereas unmated females did not parasitze whitefly nymphs at all. Our results suggest that only mated female parasitoids with host‐feeding behavior should be released in crop systems to increase their bio‐control efficiency.     

Behavioural responses of Ixodes ricinus nymphs to carbon dioxide and rodent odour

Many haematophagous ectoparasites use carbon dioxide (CO₂) and host odour to detect and locate their hosts. The tick Ixodes ricinus (Linnaeus) (Ixodida: Ixodidae) walks only small distances and quests in vegetation until it encounters a host. The differential effects of CO₂ and host odour on the host‐finding behaviour of I. ricinus have, however, never been clarified and hence represent the subject of this study. The effects of CO₂ and odour from bank voles on the activation and attraction of I. ricinus nymphs were analysed in a Y‐tube olfactometer. Carbon dioxide evoked a response in the absence and presence of host odour, but did not attract nymphs. Host odour, however, did not evoke a response but did attract nymphs in the absence and presence of CO₂. The current results show that CO₂ is an activator, but not an attractant, and that host odour is an attractant, but not an activator, of I. ricinus nymphs, and provide ecological insights into the host‐finding behaviour of I. ricinus.     

Using superparasitism by a stem borer parasitoid to infer a host refuge

Abstract. 1. Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae), a parasitoid of the European corn borer, Ostrinia nubilalis, and the Asian corn borer, O. furnacalis (Lepidoptera: Pyralidae), has high fecundity but has been reported to parasitize a low proportion of host larvae. This was corroborated in field collections: in Hebei (China) and Delaware (U.S.A.), M. cingulum parasitized only 15 and 25%, respectively, of hosts collected. 2. Because M. cingulum females cannot oviposit through plant tissue, they must parasitize hosts either before they have bored into stalks or while they are near entrance holes, so that at any one time, many Ostrinia larvae may be unavailable to M. cingulum. This refuge, together with fluctuations in abundance of foraging M. cingulum females, may explain why M. cingulum parasitizes relatively few Ostrinia larvae. 3. To test this hypothesis, levels of superparasitism were measured in the field. Low parasitism resulting from a refuge for host larvae should cause high rates of superparasitism in hosts outside the refuge. 4. Because M. cingulum is polyembryonic, the number of parasitoids per host does not indicate the level of superparasitism. Random amplified polymorphic DNA markers were used to determine the number of different genotypes emerging from each host. The resulting frequency distributions were fitted to those expected under random oviposition to estimate the proportion of Ostrinia larvae unavailable to M. cingulum. 5. In the samples from Hebei and Delaware, the level of superparasitism was much higher than expected by chance if all hosts were available. Fitting the frequencies of genotypes per host to a Poisson distribution, the authors estimated that 74–82% and 69–74% of host larvae were unavailable to M. cingulum in these collections, respectively. This means that M. cingulum parasitized 60–84% and 82–95% of available hosts in these collections, respectively. These levels of parasitism contrast strongly with the 15–25% found when all hosts were assumed available for oviposition. 6. Genetic distances of M. cingulum within and between hosts did not differ, allowing rejection of the hypothesis that high levels of superparasitism resulted from a female laying several eggs in the same host. 7. The hypothesis that M. cingulum parasitizes few Ostrinia larvae because many larvae are in a refuge explains these data and previously published information better than other hypotheses that have been suggested.     

Life-cycle and host specificity of Amblyomma tigrinum (Acari: Ixodidae) under laboratory conditions

Biological data of three generations of Amblyomma tigrinum in the laboratory are reported and the suitability of different host species for immature ticks are compared. Grouping the three generations, infestations by both the larval and nymphal stages were performed on chickens (Gallus gallus), wistar rats (Rattus norvegicus), rabbits (Oryctolagus cuniculus),wild mice (Calomys callosus), dogs (Canis familiaris) and opossums (Didelphis albiventris). Only dogs were used for infestations by adult ticks. Tick developmental periods were observed in an incubator at 27°C and RH 90%. The proportion of engorged larvae recovered from chickens (21.7% of the exposed larvae) was significantly larger (p<0.001) than those from the five mammal species used in the infestations (maximum of 3.1%). A significant larger (p<0.01) proportion of engorged larvae successfully molted after being fed on chickens than on mammal hosts. The proportion of engorged nymphs recovered from chickens (28.8% of the exposed nymphs) was significantly larger (p<0.001) than those from mammal hosts (range: 0–2.1%). Larvae showed similar feeding periods on exposure to different host species, except for those larvae fed on C. callosus, which showed significantly longer (p<0.001) feeding periods. Engorged larvae detachment peaked on the 5^th feeding day, followed by the 6^th day, on all hosts except for C. callosus. Larval premolt periods were similar for engorged ticks exposed to different host species, except for larvae fed on dogs, which showed significantly longer (p<0.001) premolt periods. Host detachment of engorged nymphs peaked on the 6^th feeding day on chickens. Although nymphal detachment on rats peaked on the 8^th day, only 15 nymphs were recovered from this host species. In a sample of 144 F₃ nymphs fed on chickens no significant difference (p>0.10) was found between the feeding or premolt periods of 82 males and 62 females, but female nymphs were significantly heavier (p<0.005) than male nymphs. Sixteen engorged females (61.5% of the exposed ticks) were recovered after being fed on dogs, and all these females laid viable eggs. Chickens, the only avian host, were the most suitable host when compared with the five mammal species. Dogs were demonstrated to be a suitable host for adults of A. tigrinum, which is consistent with, several reports of adult A. tigrinum ticks parasitizing dogs in different areas of South America. Our results reinforce that in these same areas avian species are the major hosts for immature stages of this tick species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Functional response and mutual interference of Peristenus spretus (Hymenoptera: Braconidae), a parasitoid of Apolygus lucorum (Heteroptera: Miridae)

Peristenus spretus Chen & van Achterberg (Hymenoptera: Braconidae) is a solitary endoparasitoid, which is considered for augmentative biological control of Apolygus lucorum Meyer-Dür (Heteroptera: Miridae) in Chinese cotton fields. Since the association of P. spretus with A. lucorum was only recently discovered, the biology of the parasitoid remains unknown. In order to understand its reproductive biology, the mutual interference and functional response of P. spretus were investigated by altering either the parasitoid or the host density while keeping the other constant. In both experiments, the effects of parasitoid and host densities on parasitism, superparasitism, progeny production and sex ratio were assessed. P. spretus exhibited a Holling type II functional response to changing host densities, indicating that parasitism increases with increasing host density until the parasitoid reaches its maximum reproductive capacity. The model suggested that a single P. spretus female could parasitise a maximum of 88 nymphs per day or four nymphs per hour. Increasing the wasp-nymph ratio from 1:10 to 1:80 significantly increased the offspring production more than fivefold from ±5.8 to ±35.6; further increasing the host densities (above 80 nymphs) did not significantly increase offspring production. Strong mutual interference of foraging P. spretus females occurred only at high parasitoid densities. Parasitoids foraging alone produced an average progeny of 33.4, whereas parasitoids foraging in groups of 16 produced only 2.6. The optimal wasp-nymph ratio for mass-rearing P. spretus is 4:100, given that resources of parasitoids and nymphs are unlimited.     

Learning of host-infested plant volatiles in the larval parasitoid Cotesia kariyai

The ability to learn plant volatiles in Cotesia kariyai females was examined by wind tunnel bioassays. Searching experience on a host-infested corn plant increased subsequent flight responses of females to the infested plant. Females experiencing host by-products together with the volatiles extracted from infested leaves one time showed an increased response. However, such behavioral changes were not observed in females which experienced only the host by-products or the volatiles. Thus, the increased response is considered to be preference learning. Multiple experiences of C. kariyai with host by-products together with the volatiles did not increase their flight response to the volatiles. Furthermore, this learned response gradually decreased within 2 days. These behavioral modifications based on experience would be advantageous for C. kariyai to locate their polyphagous hosts efficiently.