Spatial Variation in the Strength of a Trophic Cascade Involving Ruellia nudiflora (Acanthaceae), an Insect Seed Predator and Associated Parasitoid Fauna in Mexico

Spatial variation in the strength of herbivore top-down control represents an important source of variation in plant fitness measures and community structure and function. By measuring seed predator (larvae of a Noctuid moth) and parasitoid impacts on Ruellia nudiflora across a broad spatial scale in Yucatan (Mexico), this study addressed the following: (1) to what extent does seed predator and parasitoid attack intensity associated with R. nudiflora vary spatially? (2) Does parasitoid attack result in a positive indirect effect on the plant, and does the intensity of this effect vary spatially? During the peak of fruit production (late June–early July) of 2005, we collected fruits from 21 R. nudiflora populations and grouped them into four regions: center, east, north and south. For each fruit we recorded: observed seed number, number of seeds eaten, seed predator presence, parasitoid presence and number of seeds ‘saved’ by parasitoids. Seed predators attacked ca 30 percent of fruits/plant on average, while parasitoids were found in 24 percent of seed predator-attacked fruits. Results indicated spatial variation in seed predator and parasitoid attack levels; interestingly, a contrasting spatial gradient of attack intensity was observed: populations/regions with greatest parasitoid attack levels usually had the lowest seed predator attack levels and vice versa, suggesting top-down control of parasitoids on seed predators. We observed a weak overall indirect impact of parasitoids on R. nudiflora (4% seeds ‘saved’ on average), which nonetheless varied strongly across populations (e.g., close to 14% seeds saved at one population). Findings indicate a geographical structuring of interaction strengths across populations, as well as spatial variation in the strength of parasitoid cascading effects on plant reproduction.     

Interspecific competition between the braconid endoparasitoids Glyptapanteles porthetriae and Glyptapanteles liparidis in Lymantria dispar larvae

The effect of interspecific competition between the solitary endoparasitoid Glyptapanteles porthetriae Muesebeck (Hymenoptera: Braconidae) and the gregarious Glyptapanteles liparidis Bouché (Hymenoptera: Braconidae), was investigated in larvae of Lymantria dispar L. (Lepidoptera: Lymantriidae). Host larvae were parasitized by both wasp species simultaneously in premolt to the 2^nd or the 3^rd host instar or in an additional approach with a 4‐day delay in parasitization by the second wasp species. Host acceptance experiments revealed that both wasp species do not discriminate between unparasitized host larvae and larvae parasitized previously by the same or the other species. In more than 90% female wasps parasitized the larva they encountered first. During the period of endoparasitic development, larvae of the competing parasitoid species never attacked the egg stage of the other species. When host larvae were parasitized simultaneously by both wasp species, the rate of successful development of both species depended on the age of the host larva at the time of its parasitization; G. liparidis emerged successfully from 44% of host larvae parasitized during the premolt to 2^nd instar, G. porthetriae from 28%, and in 20% of the hosts both parasitoid species were able to develop in one gypsy moth larva. However, when host larvae were parasitized simultaneously during premolt to the 3^rd instar, G. liparidis was successful in 90% of the hosts, compared to 8% from which only G. porthetriae emerged. In the experiments with delayed oviposition, generally the species that oviposited first succeeded in completing its larval development. Larvae of the species ovipositing with four days delay were frequently attacked and killed by larvae of the first parasitizing species or suffered reduced growth. As the secondary parasitoid species, G. porthetriae‐larvae were never able to complete their development, whereas G. liparidis developed successfully in at least 12,5% of the multiparasitized host larvae. Thus, multiparasitism of gypsy moth larvae by both Glyptapanteles species corresponds to the contest type; however, G. porthetriae is only able to develop successfully as the primary parasitoid of young host larvae.     

Predation by Podisus maculiventris (Hemiptera: Pentatomidae) on Plutella xylostella (Lepidoptera: Plutellidae) larvae parasitized by Cotesia plutellae (Hymenoptera: Braconidae) and its impact on cabbage

Biological control offers potentially effective suppression of the diamondback moth (DBM), Plutella xylostella, a serious pest of Brassica crops. Little is known of whether multiple natural enemies have additive, antagonistic, or synergistic effects on DBM populations. No-choice and choice tests were conducted to assess predation by Podisus maculiventris on DBM larvae parasitized by Cotesia plutellae and unparasitized larvae. In no-choice tests, P. maculiventris preyed on greater numbers of parasitized than unparasitized larvae and greater numbers of young larvae than old larvae. In choice tests with early third instar DBM, there was no difference in predation between parasitized or unparasitized larvae. However, in choice tests with older prey, P. maculiventris preyed on more parasitized than unparasitized larvae. Two field studies were conducted to test if this predator and parasitoid have additive, antagonistic or synergistic effects on DBM populations and plant damage in cabbage (Brassica oleracea var. capitata). In 2002, DBM populations were significantly lower in the presence of C. plutellae but not in the presence of P. maculiventris. There was not a significant interaction between the natural enemies. Plant damage was reduced only with C. plutellae. In 2003, DBM populations were significantly lower in the presence of C. plutellae and P. maculiventris, although the combination of natural enemies did not lead to a non-additive interaction. Plant damage was unaffected by the presence of either natural enemy. Because of its greater predation on parasitized larvae, P. maculiventris could be an intraguild predator of C. plutellae. Yet, their overall combined effect in the field was additive rather than antagonistic.     

Interspecific competition between Diadegma semiclausum Hellen and Diadegma mollipla (Holmgren), parasitoids of the diamondback moth, Plutella xylostella (L), feeding on a new host plant

Interspecific competition between an introduced parasitoid species aimed at controlling a herbivorous pest species and a native parasitoid parasitising the same host may influence the success of classical biological control programmes. In Kenya, interspecific competition between an introduced and a local parasitoid on two diamondback moth populations (DBM, Plutella xylostella) was investigated on two different host plants. We tested simultaneous and delayed competition of the local parasitoid Diadegma mollipla Holmgren and its exotic congenus D. semiclausum Hellen on a newly aquired DBM host plant (snowpea) in the laboratory. Under simultaneous competition, D. mollipla produced more progeny than D. semiclausum on snowpea. A head start of D. Mollipla, of four and eight hours before its congenus was introduced, resulted in a similar number of progeny of both species. In delayed competition (time intervals of 24 h, 48 h and 72 h), progeny production was similar for both parasitoids when the time interval was 24 h, irrespective of which species parasitized first. More progeny was produced by the species which attacked first, when the time interval was greater than 24 h, although it was only significant at 72 h. Competitive abilites of both parasitoids on the new host plant differed largely between laboratory and semi-field conditions. The influence of two host plants (snowpea and cabbage) on competition was studied in the greenhouse with different host and parasitoid densities. Parasitism levels of D. semiclausum were significantly higher than those of D. mollipla, regardless of host plant, host and parasitoid densities, but progeny production of D. mollipla on snowpea was still slightly higher than on cabbage. As compared to the confinement of parasitoids and larvae to small containers, D. mollipla parasitized very few larvae in the cages. Competitive ability of the two parasitoid species tested was influenced both by the density of the searching females and by parameters related to either the host plant and/or the herbivorous hosts.     

Parasitism and developmental parameters of the parasitoid <Emphasis Type="Italic">Diadegma</Emphasis><Emphasis Type="Italic">majale</Emphasis> (Gravenhorst) in control of <Emphasis Type="Italic">Plutella xylostella</Emphasis> (L.) on selected cultivars of canola

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most economically significant pests of canola, Brassica napus L., in Ardabil region, Iran. Use of host plant resistance integrated with biocontrol agents such as Diadegma majale (Gravenhorst) (Hymenoptera: Ichneumonidae) is an essential component of integrated management of P. xylostella. In this study, we investigated the parasitism by D. majale on six selected cultivars of canola under field conditions and preference and performance of the parasitoid on P. xylostella larvae under laboratory conditions. In field experiments, the highest larval density of P. xylostella was observed on Zarfam during 2008 and 2009. Larval densities were not significantly different among Opera, Hyola 401, Okapi, and Option 500 and Elite in 2008, but the lowest larval density was observed on Opera in 2009. No significant differences were observed among the rate of parasitized larvae on tested cultivars in 2008, while in 2009 the parasitism rate was significantly higher on Opera than on Zarfam. In free-choice situations, the percentage of parasitized larvae was significantly highest on Opera (88.7%) and lowest on Zarfam (62.95%). Developmental time from egg to adult, body mass, length of forewings and hindwings, length of hind femur and hind tibia of D. majale females reared on larvae of P. xylostella fed on Opera did not differ from other cultivars. Our results suggest that cultivation of Opera integrated with D. majale could provide effective and sustainable management of P. xylostella in the region.     

Is the insect or the plant the driving force in the cinnabar moth — Tansy ragwort system?

Summary The interactions between cinnabar moth, Tyria jacobaeae L. and its food plant, tansy ragwort, Senecio jacobaea L. were studied for 4 to 6 years at 9 sites in North America to assess if the herbivore drove the dynamics of the plants or if the plants determined the dynamics of the insects. Cinnabar moth larval density is not closely related to changes in the size and spacing of tansy ragwort plants although high densities of larvae were associated with a high proportion of rosettes the next year. Fluctuating moth populations live in areas where rosettes are small, closely spaced and numerous compared to flowering stem plants. This situation is also associated with greater larval dispersal which may lead to over exploitation of the food supply. The coefficient of variation of both the size of rosettes and the distance between clumps of plants is associated with the coefficient of variation in moth density. This suggests that the plants may be driving the dynamics of the insect populations. The size of the moth egg batches is correlated to the size of the flowering stem plants in the previous year, indicating an adjustment between moth fecundity and food plant size. The conclusion is that environmental factors such as weather and soil type determine to a large degree the characteristics and variation in the plant populations and this in turn controls the dynamics of the insects. The relation of this situation to the biological control of weeds is discussed.     

Interspecific competition between the braconid endoparasitoids Glyptapanteles porthetriae and Glyptapanteles liparidis in Lymantria dispar larvae

The effect of interspecific competition between the solitary endoparasitoid Glyptapanteles porthetriae Muesebeck (Hymenoptera: Braconidae) and the gregarious Glyptapanteles liparidis Bouché (Hymenoptera: Braconidae), was investigated in larvae of Lymantria dispar L. (Lepidoptera: Lymantriidae). Host larvae were parasitized by both wasp species simultaneously in premolt to the 2^nd or the 3^rd host instar or in an additional approach with a 4-day delay in parasitization by the second wasp species. Host acceptance experiments revealed that both wasp species do not discriminate between unparasitized host larvae and larvae parasitized previously by the same or the other species. In more than 90% female wasps parasitized the larva they encountered first. During the period of endoparasitic development, larvae of the competing parasitoid species never attacked the egg stage of the other species. When host larvae were parasitized simultaneously by both wasp species, the rate of successful development of both species depended on the age of the host larva at the time of its parasitization; G. liparidis emerged successfully from 44% of host larvae parasitized during the premolt to 2^nd instar, G. porthetriae from 28%, and in 20% of the hosts both parasitoid species were able to develop in one gypsy moth larva. However, when host larvae were parasitized simultaneously during premolt to the 3^rd instar, G. liparidis was successful in 90% of the hosts, compared to 8% from which only G. porthetriae emerged. In the experiments with delayed oviposition, generally the species that oviposited first succeeded in completing its larval development. Larvae of the species ovipositing with four days delay were frequently attacked and killed by larvae of the first parasitizing species or suffered reduced growth. As the secondary parasitoid species, G. porthetriae-larvae were never able to complete their development, whereas G. liparidis developed successfully in at least 12,5% of the multiparasitized host larvae. Thus, multiparasitism of gypsy moth larvae by both Glyptapanteles species corresponds to the contest type; however, G. porthetriae is only able to develop successfully as the primary parasitoid of young host larvae.     

Egg dispersion is more important than competition type for herbivores attacked by a parasitoid

Herbivore fitness can be altered by a combination of interacting organisms, such as its food plant, conspecifics, and predators/parasitoids. Here, we tested relative effects of plant species, herbivore intraspecific competition type, and spatial distribution of the herbivore among plant units on herbivore survival and whether parasitoids modified these effects. We used an endophagous bruchine seed predator Callosobruchus maculatus for the herbivore, and a braconid wasp Heterospilus prosopidis for the parasitoid. The survival rate of C. maculatus was measured for each of 16 combinations of two plants (bean species, Vigna unguiculata and V. radiata), two competition types of C. maculatus larvae (contest and scramble), two spatial distributions of hosts [sparse (1 C. maculatus larva per seed over 20 seeds) and dense (2 C. maculatus larvae per seed over ten seeds)], and with/without a parasitoid pair. In the absence of the parasitoid, C. maculatus survival rate was lower with V. radiata and in the contest type. With the parasitoid, the proportion parasitized hosts was independent of total host density. Neither the proportion of parasitized hosts nor host survival rate was affected by plant species or host strain, but they were affected by host spatial distribution. When host distribution was dense, a higher proportion of hosts were parasitized, and C. maculatus survival rate was lower. Here we discuss parasitoid potential as a selective agent for the sparse within-pod distribution of its hosts in the field.     

Does a Specialist Parasitoid Adapt to its Host on a New Host Plant?

The host plant expansion of a diamondback moth, Plutella xylostella (L.) (DBM) strain to snowpea (Pisum sativum L.) raised the question whether a specialist parasitoid Diadegma semiclausum (DS) could be conditioned to locate and parasitize its host on the new host plant. In a specialist parasitoid a behavioural change towards a plant outside the normal host plant range of its host due to developmental experience is not expected. The responsive behaviour, parasitism rates and fitness of three subsequent DS generations were investigated on the snowpea-strain of DBM. After three generations of DS on the pea 62.5% of females chose an DBM-infested pea plant over DBM infested cabbage. Only 16.4% of cabbage-reared DS was attracted to infested pea. Rearing of the parasitoid in host larvae on peas significantly increased the number of larvae parasitized on this host plant in the first generation; however, there was no further increase in generations 2 and 3. Larval mortality was similar for all parasitoid/DBM combinations on both host plants, but significantly higher mortality occurred in parasitoid pupae from peas. Development time of the parasitoid was slightly prolonged on the pea strain of DBM. The number of females produced by parasitoids reared on the pea strain of DBM was significantly reduced as compared to D. semiclausum reared on the cabbage strain on both host strains. Results show that DS has the potential to change its responsive behaviour in order to locate its host on a new host plant. According to the current view, a specialist parasitoid is not expected to change its reaction to a plant outside the normal host plant range of its host. Within 3 generations, responsive behaviour towards snowpea could be increased. However, fitness trade-offs, especially an extreme shift in sex ratio to males reduced reproductive success.     

菜蛾啮小蜂的生物学及温度对其 种群增长的影响

菜蛾啮小蜂Oomyzus sokolowskii (Kurdjumov) 是小菜蛾Plutella xylostella L. 的一种主要寄生天敌。观察表明,该蜂喜产卵于小菜蛾各龄幼虫,也可产卵于预蛹,进行幼虫至蛹期的跨期聚寄生。每头寄主蛹出蜂多为5～10头,平均7.8头,其中雌蜂占85%～90%。该蜂也可产卵于在小菜蛾幼虫体内寄生的菜蛾绒茧蜂高龄幼虫,故又是小菜蛾的兼性重寄生蜂。在杭州,每年该蜂在田间的活动期为4～10月,10月中下旬陆续以老熟幼虫或预蛹进入休眠越冬,第二年4月陆续羽化并开始产卵寄生。该蜂发育、存活和繁殖的适温范围为20～30℃,低于20℃或高于30℃对其存活不利,但在适温下发育羽化的雌蜂,短时间内在32～35℃高温下仍可大量产卵寄生。在20℃、25℃和30℃下,平均每雌一生可寄生小菜蛾幼虫数分别为3.1、13.2和6.8头,产子蜂数分别为20.5、92.1和504头,内禀增长率分别为0.082、0.240和0.263(雌/雌·天)。     

Predation potential of the earwig Euborellia annulipes on fruit fly larvae and trophic interactions with the parasitoid Diachasmimorpha longicaudata

The earwig Euborellia annulipes (Lucas) (Dermaptera: Anisolabididae), a generalist predator, has been observed in fruits infested with fruit fly larvae, which are frequently parasitized by parasitoid wasps. Neither the capacity of earwigs to predate on fruit flies nor intraguild interactions between earwigs and fruit fly parasitoids have been investigated. Here, we studied in laboratory conditions the predation on the fruit fly Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) by the earwig E. annulipes, and whether parasitism of fruit fly larvae by the parasitoid wasp Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae) influences predation by the earwig. We evaluated the predation capacity, functional response and prey preference of E. annulipes for parasitized and non-parasitized fruit fly larvae in choice and no-choice tests. We found that earwigs prey on second- and third-instar larvae and pupae of C. capitata and consumed larger numbers of second-instar larvae, followed by third-instar larvae and pupae. Females prey on larger numbers of fruit flies than did males, regardless of the prey developmental stage, but both sexes exhibited a type II functional response. Interestingly, males killed but did not consume fruit fly larvae more than did females. In no-choice tests, earwig females consumed equal numbers of parasitized and non-parasitized fruit fly larvae. However, in choice tests, the females avoided feeding on parasitized larvae. Subsequent tests with hexane-washed parasitized and non-parasitized larvae showed that putative chemical markings left on fruit flies by parasitoids did not drive the earwig preference towards non-parasitized larvae. These findings suggest that E. annulipes is a potential biological control agent for C. capitata, and that, because the earwig avoids consuming larvae parasitized by D. longicaudata, a combination of the two natural enemies could have an additive effect on pest mortality.     

Parasitism by Pteromalus cerealellae (Hymenoptera: Pteromalidae) on the Cowpea weevil, Callosbruchus maculatus (Coleoptera: Bruchidae): Host density, temperature effects, and host finding ability

Studies on interactions between a larval parasitoid, Pteromalus cerealellae (Boucek) and one of its hosts, Callosobruchus maculatus (F.) were carried out in the laboratory. The number of host larvae parasitized by P. cerealellae increased with host larvae at low densities and tended to a plateau at a density of 25 larvae per female parasitoid. Each parasitoid was able to parasitize more hosts and produced more offspring at 20 and 25 °C than at 30 °C. The number of non-infested seeds mixed with seeds infested with the last instar of C. maculatus did not preclude P. cerealellae from identifying infested seeds and attacking larvae inside them. When infested seeds were tightly packed, several host larvae escaped parasitism. P. cerealellae may be a useful biological control agent in newly harvested cowpea with low C. maculatus infestation, and lowering the temperature of the storage system may enhance the effectiveness of this parasitoid.     

Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens)

Interspecific interactions can vary within and among populations and geographic locations. This variation can subsequently influence the evolution and coevolution of species interactions. We investigated population and geographic variation in traits important to pollinating seed-consuming interactions between the senita cactus (Lophocereus schottii) and its obligate pollinating moth (Upiga virescens), both of which are geographically restricted to the Sonoran Desert. Female moths actively pollinate senita flowers and oviposit onto flowers. Their larvae consume developing seeds and fruit of flowers pollinated by females. Traits important to this interaction include fruit set from moth pollination, fruit survivorship, and costs of fruit consumption by larvae. We studied these traits for five populations at two widely separated geographic locations. On average, 37% of flowers set fruit, 22% of flowers produced mature fruit, and larvae consumed 25% of immature fruit pollinated by female senita moths. Senita cactus and senita moth interactions were strongly mutualistic in all populations that we studied. Although one population had statistically lower fruit set and fruit production than the other four, all five populations were qualitatively similar in fruit production, costs, and patterns of fruit survivorship. Hand-pollination experiments suggested that fruit set was resource-limited in all but this one population. Apparent pollen limitation in the one population explains the quantitative differences in fruit set and fruit survivorship among the populations. As predicted by theory and exemplified by the senita mutualism, specialized and/or obligate interactions vary little among populations and geographic locations. Received: 5 March 1999 / Accepted: 25 June 1999     

Entomological and functional role of floral strips in an organic apple orchard: Hymenopteran parasitoids as a case study

Habitat manipulation techniques improve the availability of resources required by natural enemies to increase their effectiveness. This study focused on the effects of floral strips on Hymenopteran parasitoid presence. The experiments were conducted during spring 2007 in one organic low-input apple orchard located in south-eastern France. The density and the diversity of parasitic wasps collected from sown floral strips were higher than those from mown plants. The family of parasitic wasps of Braconidae was strongly dominant, followed by Mymaridae and Pteromalidae. By studying 26 flowering species, the greatest diversity and density of parasitic wasps were collected from Potentilla reptans, Achillea millefolium, Trifolium repens and Torilis arvensis. In terms of the early flowering plants, the most important results were observed in Euphorbia helioscopia, Senecio vulgaris and Veronica persica. To give an idea of the functional role of these plants, we studied the parasitic wasps of the diapausing larvae (cocoon) of codling moth Cydia pomonella. We recorded three emerged species: Ascogaster quadridentata, Pristomerus vulnerator and the hyperparasite Perilampus fulvicornis. However, none of these species have been observed on the 26 studied plants. Hence, this result may be suggesting that the studied plants do not have a functional role concerning these parasitoids. These studies may be advantageous for biological control programs in order to select flowering plant species attracting parasitic wasps specific to fruit pests.     

Varying degree of physiological integration among host instars and their endoparasitoid affects stress‐induced mortality

In natural populations of insect herbivores, genetic differentiation is likely to occur due to variation in host plant utilization and selection by the local community of organisms with which they interact. In parasitoids, engaging in intimate associations with their host during immature development, local variation may exist in host quality for parasitoid development. We compared the development of a gregarious endoparasitoid, Cotesia glomerata L. (Hymenoptera: Braconidae), collected in The Netherlands, in three strains and three caterpillar instars (L1–L3) of its main host, Pieris brassicae L. (Lepidoptera: Pieridae). Hosts had been collected in The Netherlands and France, and were reared in the laboratory for one generation. We also used an established Dutch laboratory strain that had not been exposed to parasitoids for at least 24 generations. Parasitoid survival to adulthood was inversely correlated with host instar at parasitism. Adult parasitoid body mass was largest when hosts were parasitized as L1 and smallest when hosts were parasitized as L3, whereas egg‐to‐adult development time was quickest on L3 hosts and slowest on L1 hosts. Higher survival and faster development of C. glomerata on French L2 hosts also showed that there is variation in host‐instar‐related suitability. Many L2 and most L3 caterpillars that were parasitized exhibited signs of pathogen infection and perished within a few days of parasitism, whereas this never happened when hosts were parasitized as L1 or in non‐parasitized control caterpillars. Our results reveal that, irrespective of the host strain, L1 hosts are optimally synchronized with C. glomerata development. By contrast, the high precocious mortality of L3 larvae may be due to stress‐induced regulation by the parasitoid in order to ‘force’ its developmental program into synchrony with the developing parasitoid larvae. Our results underscore a potentially important role played by pathogens in mediating herbivore–parasitoid interactions that are host‐instar‐dependent in their expression.     

Enhancing biological control by an omnivorous lacewing: Floral resources reduce aphid numbers at low aphid densities

The availability of plant resources to omnivorous arthropod predators may have a positive, negative or negligible effect on their population densities and predation rates, depending on the availability of prey. At high prey densities, flowering buckwheat has been shown to negatively impact populations of the brown lacewing, an omnivorous predator, due to the probable increase in parasitism rate of lacewing larvae by their primary parasitoid, Anacharis zealandica. However, little is known about the effect of buckwheat flowers on this insect community at low prey densities. We used field cages to assess the effects of nectar provision by flowering buckwheat on the population dynamics of the pea aphid, the brown lacewing and its parasitoid A. zealandica in an alfalfa field, under low aphid densities in the New Zealand summer. The insects were sampled every 2 weeks with a suction device, then counted and released on each sampling date from 15 January to 15 March 2007. Buckwheat significantly increased lacewing populations and significantly decreased aphid numbers by 70% and 39%, respectively. The buckwheat had its greatest effect at the end of summer (February/March) for both these species. It had no effect on A. zealandica abundance.     

Parasitism rates and sex ratios of a parasitoid wasp: effects of herbivore and plant quality

Summary We studied interactions among collards, Brassica oleracea var. acephala, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae) by manipulating plant nitrogen (N) concentrations in field and laboratory experiments. Parasitoid abundance strongly reflected DBM abundance and was related to total leaf N. Parasitism rates were high (70.7%) and density-independent. Wasp sex ratios varied markedly (3–93% female) in response to the herbivores, the plants, or both. Higher proportions of female wasps emerged from DBM larvae on plants with high leaf N than on unfertilized plants. More female wasps also emerged from larvae parasitized as larger instars. We suggest that wasps have the potential to control DBM populations through long-term numerical responses mediated by variable sex ratios.     

Density effects of flowering phenology and mating potential in Nicotiana alata

Summary We investigated effects of plant density on floral phenology and potential mating in artificial populations of the outcrossing ornamental Nicotiana alata planted at three densities. Path analysis revealed that increasing plant density yielded significantly earlier peak flowering dates, significantly earlier last flowering dates, and significantly lower plant biomass. Direct effects of density on final flower number were not significant. Variation among replicate plots for first date of flowering was larger than variation among densities, indicating that factors other than density influence floral initiation.We did not record actual mating, but determined from phenological data the number and identity of potential mates. Increased density had several effects on potential mating patterns and on potential N_e, effective population number. At high density, fewer focal plants flowered for shorter durations. This led to less overlap in flowering time among plants, decreasing the number of potential parental combinations possible among the progeny. Two outcomes of high density, the lower total number of plants flowering and the lower number of plants flowering at most census dates, tended to reduce potential N_e. In contrast, it was low density, where variance in flower number was greatest, that was most likely to yield the greatest reduction in N_e due to variance in progeny number.At high density the potential for assortative mating among tall plants was much greater and occurred later than among large plants at low density. Much of the potential high density assortative mating occurred late in the phenology of individual plants, when there was likely to be lower fruit set.We discuss how ecological agents that alter flowering phenology can potentially alter the genetics of populations, the level and timing of assortative mating and, if genetic variation for response to such ecological agents exists, the potential selection regime.     

Releases of Trichogramma platneri (Hymenoptera: Trichogrammatidae) in Apple Orchards under a Sterile Codling Moth Release Program

Based on the premise that augmented host numbers may help multiply and support parasitoid populations, the egg parasitoid Trichogramma platneri Nargarkatti was released in apple orchards which were participating in a sterile codling moth, Cydia pomonella (L.), release program. Nonviable eggs resulting from matings involving at least one sterile codling moth partner can be successfully parasitized by T. platneri. Grain moth-reared, as well as codling moth-reared, T. platneri were released either in the spring or in the spring and summer oviposition period(s) of wild codling moth. Sentinel codling moth eggs were hung weekly, for 3-day periods, from May until September to determine fluctuations in T. platneri populations both during and between releases. Low numbers of wild or nonviable codling moth eggs or other susceptible host eggs resulted in sufficient eggs to maintain low spring- or summer-introduced T. platneri populations. T. platneri reduced codling moth damage in trees in which the Trichogramma were released.     

Effect of food plant switching by a herbivore on its parasitoid: Cotesia melanoscela development in Lymantria dispar exposed to reciprocal dietary crosses

1. Host plant switching by dispersing early instar lepidopterans could have implications for parasitoid performance, but this possibility has not been evaluated thoroughly. 2. The relative growth rates of Lymantria dispar parasitized by Cotesia melanoscela, and the weight of larvae at the time of parasitoid emergence, were affected most by the second larval food plant consumed. 3. The relative growth rates, pupal weights, weight of larva at the time of parasitoid emergence, and development times of L. dispar were affected significantly by the second larval food plant consumed. 4. Development time and size of Cotesia melanoscela were affected most by the second larval food plant consumed. 5. Parasitoid performance was affected most by the larval host’s relative growth rate and the final weight of the host larva at the time of parasitoid emergence. 6. Host plant switching affected the weight of L. dispar larvae at the time of parasitoid emergence, but the effect of switching per se was not a significant factor in C. melanoscela size or development. 7. Lymantria dispar larvae that fed on Populus as their second host outperformed larvae that fed ultimately on Acer. 8. Parasitoids yielded from L. dispar larvae that fed ultimately on Populus outperformed parasitoids yielded from larvae that fed ultimately on Acer. 9. Per cent mortality of L. dispar due to parasitism and percentage adult C. melanoscela emergence were highest in parasitized larvae fed Populus, poor in hosts fed Acer, and intermediate in switching larvae.