Interactions between the stem-mining weevils Ceutorhynchus napi Gyll. and Ceutorhynchus pallidactylus (Marsh.) (Coleoptera: Curculionidae) in oilseed rape

Abstract 1 The rape stem weevil Ceutorhynchus napi Gyll. and the cabbage stem weevil Ceutorhynchus pallidactylus (Marsh.) share the same habitat and food resource within the stems of oilseed rape, Brassica napus L. var. oleifera. Interactions occurring between these two sympatric species on this host were studied under both field and laboratory conditions. 2 The oviposition preference of C. pallidactylus and the within‐plant distribution of eggs and larvae were examined in field plots of oilseed rape. Female C. pallidactylus tended to lay their eggs in plants already infested by eggs and larvae of C. napi rather than in uninfested plants. The within‐plant distribution of the egg batches of C. pallidactylus did not differ significantly between uninfested plants and those preinfested by C. napi. Ovipositing females of C. napi and C. pallidactylus generally showed a significant preference for plants with larger stem diameter. 3 Laboratory choice tests provided further evidence for the oviposition preference of C. pallidactylus. Females laid significantly more eggs in leaves of plants that had been previously infested by C. napi than in leaves of previously uninfested plants. 4 Larvae of C. pallidactylus showed a significant shift of their feeding niche towards the stem base when feeding in individual plants attacked by both species. This possibly indicates ressource partitioning between C. pallidactylus and C. napi. The within‐plant distribution of C. napi larvae was not affected by the simultaneous attack of C. pallidactylus. 5 The size of the head capsule of full‐grown larvae of C. napi and C. pallidactylus was not significantly correlated with the diameter of the stem of their host plant or with the number of conspecific larvae within individual plants.     

Impact of the belowground herbivore and potential biological control agent, Ceutorhynchus scrobicollis, on Alliaria petiolata performance

We studied the influence of the root-crown weevil Ceutorhynchus scrobicollis on its host plant Alliaria petiolata, a European biennial herb that is currently invading much of temperate North America. Varying timing of attack and herbivore densities in a common garden allowed to assess seasonality of plant response, density-dependence of impact, and the effect of intraspecific competition on C. scrobicollis recruitment (number of F₁ generation adults emerged). Data collected in the common garden were compared with data collected at field sites. C. scrobicollis is a common weevil in Europe, frequently attaining high attack levels on its host plant. In the common garden, weevil attack decreased plant survival by up to 43%, reduced plant height by 54%, increased the number of shoots by up to four–fold and delayed seed ripening, but had no significant negative effect on seed production. Plants infested in spring allocated less biomass to aboveground plant parts, and remained smaller than plants attacked in autumn, indicating that the latter were able to partly compensate for weevil attack. Increasing weevil density rarely had an effect on A. petiolata performance, and did not increase F₁ recruitment, suggesting strong intraspecific competition. At field sites, C. scrobicollis attack is spread over a long time period, which probably alleviates intraspecific competition. In summary, attack by the root-crown feeding weevil, C. scrobicollis, can substantially reduce growth and survival of A. petiolata. If introduced as a biological control agent into North America, C. scrobicollis is likely to decrease the fitness and competitive superiority of A. petiolata.     

Implications for conservation biocontrol of spatio‐temporal relationships between carabid beetles and coleopterous pests in winter oilseed rape

1 The spatio‐temporal distributions of predatory carabid beetles (Coleoptera: Carabidae) and their potential prey, the larvae of three coleopterous pests, Meligethes aeneus (Fabricius) and Ceutorhynchus spp. [Ceutorhynchus pallidactylus (Marsham), the cabbage stem weevil, and Ceutorhynchus assimilis (Paykull), the cabbage seed weevil], were studied within a crop of winter oilseed rape. The distributions of Collembola were recorded as potential alternative prey. Insect distributions were analysed and compared using Spatial Analysis by Distance Indices. 2 Mature larvae of the pests dropped from the crop canopy to the soil for pupation in temporal succession from May to early July. Their distributions within the crop were irregular and differed with species. 3 Adults of seven species or genera of carabid were abundant and active within the crop during May and June: Nebria brevicollis (Fabricius), Anchomenus dorsalis (Pontoppidan), Loricera pilicornis (Fabricius), Amara similata (Gyllenhal), Asaphidion spp., Pterostichus madidus (Fabricius) and Pterostichus melanarius (Illiger). 4 During May, N. brevicollis was spatially associated with peak numbers of M. aeneus larvae and with Collembola. Anchomenus dorsalis was spatially associated with Ceutorhynchus spp. larvae during two peaks in the abundance of the latter in early and late June. Nebria brevicollis and A. dorsalis coincided in both time and space with larvae of the three coleopterous pests when they were most vulnerable to predation by epigeal predators and are therefore good candidates for conservation biocontrol. 5 The importance of carabid beeetles in the natural enemy complex in winter oilseed rape and their potential for biocontrol of spring and summer pests are discussed in relation to husbandry practices for the crop and its adjacent areas which could be manipulated to promote carabid survival for integrated pest management.     

Between-clutch interactions affect a benefit of group feeding for pipevine swallowtail larvae

Abstract.  1. In California, early instar larvae of the pipevine swallowtail ( Battus philenor ) develop at an accelerated rate when feeding in large groups compared with small groups due to a plant-mediated response to feeding group size. Larvae benefit from accelerated growth because the time larvae remain in early stages, where mortality is highest, is reduced. Occasionally, multiple clutches are laid on the same plant stem. Clutch size modification by females ovipositing on plant with previously laid clutches and the effect of kinship and group size on larval behaviour was examined. The direct and indirect interactions between clutches were investigated to determine if group size and time between clutch establishment affects the performance of early instar larvae.
2. Larger groups consume the young foliage more quickly and develop at an accelerated rate compared with smaller groups. Older foliage available to later clutches is an inferior food resource compared with younger foliage.
3. There was no evidence that females adjust clutch size in response to the presence of conspecific clutches.
4. Second groups of larvae readily joined previously established feeding groups. There were no observed behavioural differences between sibling and mixed-family groups.
5. The effect of a second group on the growth of the initial group was dependent on the size of both groups and the time interval between the arrival of the two groups.
6. Accelerated growth associated with larger feeding aggregations was absent when these groups were introduced to plants with previously established groups.
7. It is beneficial for ovipositing females to avoid plants with previously laid clutches because direct and indirect interactions with established clutches compromises larval performance.     

Seed growth suppression constrains the growth of seed parasites: premature acorn abscission reduces Curculio elephas larval size

Abstract.  1. The abscission of seeds infested by insects is common in many plants and has been proposed as a defensive mechanism, although its negative consequences for insects have rarely been assessed.
2. We assessed the consequences of seed abscission studying the interactions between the holm oak Quercus ilex and the chestnut weevil Curculio elephas , its main pre-dispersal seed predator. Female weevils oviposit into the acorns and the larvae must complete their development inside a single acorn feeding on the cotyledons. The growth of the infested acorns is suppressed because they are prematurely abscised.
3. Female weevils oviposit along the acorn growing period; hence, the size of the infested acorns increased with date. The growth of the larvae inside the smaller, early infested, acorns was constrained because food (i.e. cotyledons) was frequently depleted. Larval size increased with the date along with the size of the infested acorns, but it declined slightly in the latest dates as a result of the higher conspecific competition provoked by the larger number of larvae per acorn.
4. The present results demonstrate that premature seed abscission by Q. ilex had negative consequences for C. elephas , as a smaller acorn size reduced food availability and constrained larval size, a key insect life history trait. At the same time, it is suggested that the growth suppression of infested acorns may condition the oviposition phenology of these insects according to their body size. These results need to be considered in further research in the context of the evolutionary significance of premature seed abscission as a defensive mechanism.     

Resistance of kale varieties to attack by Mamestra brassicae

1 The objectives of this work were to study the resistance of six kale ( Brassica oleracea acephala group) varieties to cabbage moth Mamestra brassicae (L.) expressed as antibiosis and to determine the effect of plant age on larval survival and development.
2 The influence of plant age on resistance was determined using leaves from seedlings and from mature plants. Survival and development of M. brassicae larvae and feeding rates were determined in laboratory bioassays.
3 Leaves from seedlings were more suitable than those of mature plants for establishing differences in resistance. There were significant differences between kale varieties in larval survival, growth rate, leaf feeding, and time to pupation but not pupal weight. The varieties MBG-BRS0031, MBG-BRS0351, and MBG-BRS0287 reduced survival of M. brassicae larvae. Larvae that fed on MBG-BRS0060 were the heaviest and took the longest time to pupation. MBG-BRS0031 was consumed significantly less by larvae than were all the other varieties examined. Leaves from mature plants of MBG-BRS0142 and MBG-BRS0170 were defoliated significantly less than those of other varieties.
4 In conclusion, the variety MBG-BRS0031 may be a promising source of resistance to M. brassicae . Leaf antibiotic resistance was shown to play a role in defense against M. brassicae attack but it is not the only possible mechanism of resistance.     

Determining the host specificity of the biological control agent Trichomalus perfectus (Hymenoptera: Pteromalidae): the importance of ecological host range

We determined the host range of the parasitoid Trichomalus perfectus (Walker), a candidate for classical biological control of cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), an important pest of canola in Canada. Studies were conducted in Europe and in North America. In laboratory experiments, the levels of parasitism (acceptance) of Ceutorhynchus turbatus Schultze, C. cardariae Korotyaev, C. omissus Fall and C. querceti (Gyllenhal) by T. perfectus were not significantly different than of the target host C. obstrictus. Although C. typhae (Herbst), C. pallidactylus (Marsham), C. americanus Buchanan, C. neglectus Blatchely and Ceutorhynchus sp. nr. nodipennis were parasitised by T. perfectus, the levels of parasitism were significantly lower on these species than on C. obstrictus. Ceutorhynchus peyerimhoffi Hustache, C. erysimi (Fabricius), C. alliariae H. Brisout, C. roberti Gyllenhal, Mogulones borraginis (Fabricius), Mononychus vulpeculus (Fabricius) and the leaf-mining fly Scaptomyza flava (Fallén) were not attacked. Ecological host range surveys in Europe corroborated the prediction that T. perfectus would attack C. cardariae at similar rates to C. obstrictus. In North America, the recent discovery of T. perfectus in a C. omissus population suggests that laboratory findings predicting that C. omissus is a preferred host may be the case in the field. We found that T. perfectus attacks larvae of some Ceutorhynchus spp. feeding on Brassicaceae and does not attack species outside of that host range. Thus, the parasitoid can be defined as narrowly oligophagous. These results demonstrate the value of ecological host range studies in the area of origin to validate hypotheses generated through laboratory host range experiments.     

The consequences of larval aggregation in the butterfly Chlosyne lacinia

1. Females of Chlosyne lacinia (Geyer) (Lepidoptera: Nymphalidae, Melitaenae), the bordered patch butterfly, clump eggs in a few large clusters on their host plant, Helianthus annuus. Resulting larvae form sibling aggregations to at least the third instar.
2. The effect of group size on survival and development of C. lacinia larvae was tested experimentally in the field. Larvae developed faster and survived better in larger groups.
3. The effects of various predator guilds (ground-dwelling arthropods, aerial arthropods and avian predators) on survival of larvae was then tested while controlling group size. Ground-dwelling arthropods, mainly fire ants Solenopsis xyloni , reduced larval survival greatly but other solitary invertebrate and avian predators did not alter survival. Group defences and aposematism of C. lacinia larvae are probably ineffective against predatory ants that attack en masse and recruit other colony members.
4. In laboratory experiments, two possible mechanisms underlying faster development of larvae in larger groups were tested: (i) overcoming the physical toughness of host plant leaves, and (ii) social stimulus to feed. Results support the physical toughness hypothesis but not the social stimulus hypothesis.
5. Feeding in large groups by C. lacinia larvae confers multiple advantages, including protection from solitary predators and increased feeding efficiency because grouped, early-instar larvae can initiate feeding wounds on tough sunflower leaves. These advantages of larval gregariousness, coupled with reduced desiccation at the egg stage, apparently outweigh disadvantages of aggregation, such as interference and exploitative competition among larvae.     

Biological studies on <Emphasis Type="Italic">Ceutorhynchus portulacae</Emphasis>, a potential natural enemy of the purslane weed <Emphasis Type="Italic">Portulaca oleracea</Emphasis>

Ceutorhynchus portulacae (Coleoptera Curculionidae) is identified as a potential natural enemy of Portulaca oleracea, which is a serious crop weed in many countries of the world. Detailed biological studies on its life cycle, fecundity, longevity and mortality factors revealed that the weevil possess positive attributes of successful biological control agents like short life cycle, prolonged longevity, high fecundity, small size, high rate of increase, phenology of attack, multivoltinism and versatile feeding behaviour. Based on the present study, it could be concluded that C. portulacae could contribute considerably to reduce purslane weed in crop fields.     

Superparasitism in Cotesia glomerata: response of hosts and consequences for parasitoids

Abstract.  1. Superparasitism occurs in Cotesia glomerata (Hymenoptera: Braconidae), a gregarious endoparasitoid of Pieris spp. (Lepidoptera: Pieridae). The response of P. brassicae larvae to superparasitism and the consequences for the parasitoid were examined in order to elucidate the ecological significance of this behaviour.
2. Field surveys of a Swiss population revealed that C. glomerata brood sizes from P. brassicae larvae ranged from three to 158, and both the female ratio and the body weight of emergent wasps correlated negatively with brood size. In the laboratory, single oviposition on P. brassicae larvae did not produce any brood size larger than 62, but brood size increased with superparasitism.
3. Laboratory experiments demonstrated that both naive and experienced female wasps were willing to attack hosts that had been newly parasitised by themselves or conspecifics. Superparasitism reduced survivorship but increased food consumption and weight growth in P. brassicae larvae. Superparasitism lengthened parasitoid development and prolonged the feeding period of host larvae.
4. Despite a trade-off between maximising brood size and optimising the fitness of individual offspring, two or three ovipositions on P. brassicae larvae resulted in a greater dry female mass than did a single oviposition on the host. Thus, superparasitism might be of adaptive significance under certain circumstances, especially when host density is low and unparasitised hosts are rare in a habitat.     

Effects of temperature and delayed initial feeding on the survival and growth of Japanese flounder larvae

The effects of the timing of initial feeding (0, 1, 2, 3 and 4 days after yolk exhaustion) and temperature (15, 18 and 21° C) on the point‐of‐no‐return (PNR), survival and growth of laboratory‐reared Japanese flounder Paralichthys olivaceus larvae were studied under controlled conditions. The larvae reached PNR on 7·7, 5·2 and 4·2 days‐post‐hatching (dph) at 15, 18 and 21° C, respectively. At each temperature, larval growth did not differ significantly among the delayed initial feedings 1 day before PNR but decreased significantly in larvae first fed after that. In the treatments where initial feeding was equally delayed, larvae grew significantly faster at 18 and 21° C than at 15° C. The larvae survived apparently better at 15 and 18° C than at 21° C when initial feeding was equally delayed. At each temperature, survival of the larvae first fed before PNR did not differ noticeably, while delayed initial feeding after that apparently reduced their survival. These results indicated that there existed a negatively temperature‐dependent PNR in the Japanese flounder larvae. Survival and growth of the larvae strongly depended on temperature as well as the timing of initial feeding. High temperature accelerated the yolk exhaustion and growth of the larvae and thus reduced their starvation tolerance and survival. To avoid potential starvation mortality and obtain good growth, the Japanese flounder larvae must establish successful initial feeding within 2 days after yolk exhaustion at 15° C and within 1 day at both 18 and 21° C.     

Thermal ecology of western tent caterpillars Malacosoma californicum pluviale and infection by nucleopolyhedrovirus

Abstract 1. Western tent caterpillars hatch in the early spring when temperatures are cool and variable. They compensate for sub-optimal air temperatures by basking in the sun.
2. Tent caterpillars have cyclic population dynamics and infection by nucleopolyhedrovirus (NPV) often occurs in populations at high density.
3. To determine whether climatic variation might influence viral infection, the environmental determinants of larval body temperature and the effects of temperature on growth and development rates and larval susceptibility to NPV were examined.
4. In the field, larval body temperature was determined by ambient temperature, irradiance, and larval stage. The relationship between larval body temperature and ambient temperature was curvilinear, a property consistent with, but not necessarily limited to, behaviourally thermoregulating organisms.
5. Larvae were reared at seven temperatures between 18 and 36 °C. Larval growth and development increased linearly with temperature to 30 °C, increased at a lower rate to 33 °C, then decreased to 36 °C. Pupal weights were highest for larvae reared between 27 and 30 °C.
6. The pathogenicity (LD₅₀) of NPV was not influenced by temperature, but the time to death of infected larvae declined asymptotically as temperature increased.
7. Taking into account larval growth, the theoretical yield of the virus increased significantly between 18 and 21 °C then decreased slightly as temperatures increased to 36 °C.
8. Control and infected larvae showed no difference in temperature preference on a thermal gradient. The modes of temperature preference were similar to those for optimal growth and asymptotic body temperatures measured in the field on sunny days.
9. Warmer temperatures attained by basking may increase the number of infection cycles in sunny springs but do not protect larvae from viral infection.     

Tracking larval insect movement within soil using high resolution X-ray microtomography

Abstract.  1. In contrast to above-ground insects, comparatively little is known about the behaviour of subterranean insects, due largely to the difficulty of studying them in situ .
2. The movement of newly hatched (neonate) clover root weevil ( Sitona lepidus L. Coleoptera: Curculinidae) larvae was studied non-invasively using recently developed high resolution X-ray microtomography.
3. The movement and final position of S. lepidus larvae in the soil was reliably established using X-ray microtomography, when compared with larval positions that were determined by destructively sectioning the soil column.
4. Newly hatched S. lepidus larvae were seen to attack the root rhizobial nodules of their host plant, white clover ( Trifolium repens L.). Sitona lepidus larvae travelled between 9 and 27 mm in 9 h at a mean speed of 1.8 mm h⁻¹.
5. Sitona lepidus larvae did not move through the soil in a linear manner, but changed trajectory in both the lateral and vertical planes.     

Survival at low temperature of larvae of the pine processionary moth Thaumetopoea pityocampa from an area of range expansion

1 Larvae of Thaumetopoea pityocampa (Lepidoptera: Notodontidae) develop throughout the winter, although their feeding activity and survival can be impaired by adverse climatic factors. The present study investigated the survival at low temperature of larvae originating from a population with range expansion in an alpine valley in Northern Italy.
2 The supercooling point of individually analysed larvae averaged at −7 °C. This value insufficiently described the cold hardiness of the larvae; 39% of the tested larvae were alive when returned to room temperature immediately after freezing. When larval colonies inside their nest were exposed to −17 °C for 1 h after gradual temperature decrease, survival was 70.4%.
3 Rearing of larvae in the laboratory at different day/night temperatures indicated an effect of cumulative chill injury on larvae. A logistic regression explained the relationship between negative thermal sum (h°C below 0 °C) received in the laboratory experiment and larval survival. A similar relationship was demonstrated between negative thermal sum and survival of larval colonies in the field.
4 In the laboratory experiment, some tested larvae were able to survive for up to 8 weeks without feeding depending on rearing temperature. As expected, feeding occurred only when larvae were reared at temperatures of 9 °C day/0 °C night.
5 We classify the larvae of T. pityocampa as being moderate freezing tolerant. The winter behaviour allows this species to track climate warming by a rapid expansion into those areas that become compatible with the insect's development.     

A comparative study of larval cannibalism in three species of ladybird

Abstract. 1. Cannibalism was studied under laboratory conditions in three species of Coccinellidae (Coleoptera): Cycloneda sanguinea , Olla v-nigrum , and Harmonia axyridis . Larval cannibalism varied among species, C. sanguinea  > H. axyridis  ≥  O. v-nigrum .
2. Larvae of all species cannibalised more in response to reduced food availability (14 h starvation daily) than in response to reduced food quality (dry Ephestia eggs).
3. Larvae of H. axyridis cannibalised siblings at lower rates than non-siblings, but larvae of C. sanguinea and O. v-nigrum did not. Rates of cannibalism increased significantly with increasing size disparity among larvae of all three species.
4. Cannibalism in C. sanguinea and H. axyridis , but not in O. v-nigrum , increased with larval density, suggesting that not all attacks on conspecifics are driven by hunger.
5. Costs, not benefits, were the predominant effects of exclusively and partially cannibalistic diets. A diet of conspecific eggs yielded survival equivalent to the Ephestia egg diet for H. axyridis and O. v-nigrum , but developmental time was extended and adults were smaller. Larvae of C. sanguinea had superior survival and faster development on conspecific eggs than on the Ephestia egg diet. Only H. axyridis larvae survived as well on diets comprised exclusively or partially of conspecific larvae as on Ephestia eggs, although they developed more slowly.     

Using food for different purposes: female responses to prey in the predator Coccinella septempunctata L. (Coleoptera: Coccinellidae)

Abstract.  1. Although predatory insects often feed on diverse prey, their reproductive activity may be linked most strongly to a more restricted range of prey. The propensity of adult females of the ladybird beetle, Coccinella septempunctata L., to attack two natural prey species, pea aphids [ Acyrthosiphon pisum (Harris)] and alfalfa weevil larvae [ Hypera postica (Gyllenhal)], was compared, and the degree to which ladybird egg production depends on consumption of aphids vs. weevils was assessed.
2. Coccinella septempunctata females more readily attacked aphids than weevil larvae. This was true regardless of whether females had fed previously on aphids or on weevil larvae.
3. When females were provided with few to many aphids daily, or few aphids plus an excess number of weevil larvae, their rates of egg production depended primarily on the number of aphids consumed.
4. Addition of weevil larvae to diets of limited numbers of aphids increased egg production, but only modestly. Thus, consumption of weevil larvae may have served mostly for self-maintenance, thereby enabling females to use for egg production more of the nutrients and energy obtained from limited consumption of aphids.
5. The females' linkage of egg production primarily to aphid rather than weevil consumption may be adaptive, as their offspring are much less able as larvae to survive and mature on a diet of weevils rather than aphids.     

Feeding by short-snouted weevils on seedlings of seven tree species planted beneath Norway spruce at differing stand densities

1 Short-snouted weevils, including Strophosoma spp. and related species, may damage tree seedlings. We investigated the damage caused by these weevils feeding on seedlings of seven tree species planted in clear-cuts and under shelterwoods with three densities (control, dense, and sparse).
2 There were no pronounced differences in damage caused by short-snouted weevils in the various shelterwood densities. Most feeding occurred in the clear-cuts during the first year, whereas more seedlings were damaged in the shelterwoods of all densities in the second year. In the third year, virtually no feeding occurred in either the clear-cuts or shelterwoods. We cannot explain this pattern of damage, but a fallow period of 2 years seems to prevent short-snouted weevil damage.
3 We determined the general feeding preferences for short-snouted weevils to be cherry, lime > beech, oak, spruce > maple, ash. However, although Strophosoma mellanogrammum [Correction added after online publication 8 December 2008: Strophosoma melanogrammum corrected to Strophosoma mellanogrammum ] was observed feeding on seedlings, no full short-snouted weevil inventory was conducted, so caution must be exercised when drawing conclusions from this study regarding weevil damage patterns and feeding preferences.
4 Short-snouted weevils can sometimes occur in large numbers and may destroy entire reforestation projects, but in our study their influence on seedling growth and survival was minor.     

Glucosinolate profile and oviposition behavior in relation to the susceptibilities of Brassicaceae to the cabbage seedpod weevil

Understanding how host‐plant characteristics affect behavioral and physiological responses of insect herbivores is of considerable importance in the development of resistant crop germplasm. Feeding, oviposition preference, larval development, and oviposition behavior of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (= Ceutorhynchus assimilis Payk.) (Coleoptera: Curculionidae), were investigated on eight Brassicaceae species that differed in their glucosinolate profiles. The least preferred host plants for feeding and oviposition were the Sinapis alba L. lines while the Brassica carinata L. line was most preferred. Larval development occurred most rapidly on Brassica rapa L. and slowest on S. alba. Larval weight was highest on B. napus L. and lowest on S. alba. Total glucosinolate levels did not influence C. obstrictus larval growth or development; however high levels of specific glucosinolates such as p‐hydroxybenzyl and 3‐butenyl glucosinolate were associated with increased developmental time or reduced weight. The time required for oviposition behavioral events was measured on different host‐plant species: B. rapa, B. napus, B. napus×S. alba, B. tournefortii Gouan., B. juncea (L.) Czern, B. carinata, B. nigra (L.) Koch., and S. alba. The early steps in the sequence were completed faster on more susceptible host plants (B. carinata, B. napus, and B. rapa) than on relatively resistant ones (B. tournefortii and B. juncea). Females explored pods of B. nigra and S. alba, but oviposition occurred only rarely on these species. There was no significant difference in the location on the pod on which oviposition occurred among the different plant species. Mean eggs laid per female weevil were highest on the B. napus×S. alba hybrid and lowest on B. nigra and S. alba.     

Limiting the success of stem borers (Prodoxus quinquepunctellus) in yuccas: indirect effects of ants, aphids, and fruit position

Abstract.  1. Although the moth–yucca mutualism is often studied as a pairwise interaction, yucca plants are also the sole host for a variety of other visitors. One of these additional visitors is a stem-boring moth, Prodoxus quinquepunctellus.
2. In this study, it is shown how the reproductive success of Prodoxus indirectly depends on the interactions between yuccas and their pollinators ( Tegeticula , Prodoxidae) as well as the indirect effects of ants and aphids.
3. Aggressive wood ants foraging on yuccas will attack adult Prodoxus moths while attempting to oviposit. This reduces the number of eggs laid in yucca stalks, leading to fewer larvae feeding in the stalks.
4. Once in the stalk, the survival of Prodoxus eggs/larvae depends upon the rate at which the flowering stalks dry out during fruit maturation. Portions of the stalk above the highest fruit dry out quickly and survivorship approaches zero in these dry sections, while larvae in green sections of the flowering stalk have significantly higher survival rates. The presence of aphids feeding on the stalk slows down the rate of stalk drying and could lead to increased survival of Prodoxus larvae.
5. Overall, ants have strong indirect effects on P. quinquepunctellus by controlling how many eggs are laid in the stalk and by influencing the distribution of aphids. However, it is primarily the presence and position of the fruit that can affect larval survivorship, and fruit position is a function of pollinator visits and resource limitation. These complex interactions illustrate the importance of studying the yucca–moth mutualism in a community context.     

Herbivory and abiotic factors affect population dynamics of Arabidopsis thaliana in a sand dune area

Population dynamics of the annual plant Arabidopsis thaliana (L.) Heynh. were studied in a natural habitat of this species on the coastal dunes of the Netherlands. The main objective was to elucidate factors controlling population dynamics and the relative importance of factors affecting final population density. Permanent plots were established and plants were mapped to obtain data on survival and reproductive performance of each individual, with special attention to herbivore damage. In experimental plots we studied how watering, addition of nutrients, artificial disturbance, and natural herbivores affected survival and growth. Mortality was low during autumn and early winter and high at the time of stem elongation, between February and April. A key factor analysis showed a high correlation between mortality from February to April and total mortality. The specialist weevils Ceutorhyncus atomus and C. contractus (Curculionidae) were identified as the major insect herbivores on A. thaliana, reducing seed production by more than 40 %. These herbivores acted in a plant size-dependent manner, attacking a greater fraction of the fruits on large plants. While mortality rates were not affected by density, fecundity decreased with density, although the effect was small. Adding water reduced mortality in rosette and flowering plant stages. Soil disturbance did not increase seed germination, but did have a significant positive effect on survival of rosette and flowering plants. Seed production of A. thaliana populations varied greatly between years, leading to population fluctuations, with a small role for density-dependent fecundity and plant size-dependent herbivory.