The relationship between conspecific superparasitism and the outcome of in vitro contests staged between different larval instars of the solitary endoparasitoid Venturia canescens

When endoparasitoid wasps oviposit into hosts which have already been parasitized (= superparasitism), this results in potentially lethal interlarval competition. For solitary species, the decision to lay additional eggs should therefore be based on the probability of superparasite survival in any superparasitized host. In this study, in vitro contests staged between three larval instars of Venturia canescens Grav. (Hymenoptera: Ichneumonidae) reveal that the age difference between competitors affects the outcome of interlarval competition. Three parameters were used to assess parasitoid performance: the number of fights initiated, the number of bites inflicted, and the duration of each bite. When fighting takes place between two first instars, then both competitors were found to be evenly matched. However, at greater age differences, first-instar competitors appeared to win more fights than their larger second instar rivals. The advantage shown by younger competitors is most pronounced in contests staged between first and third instar larvae. These findings are consistent with the increasingly high levels of conspecific superparasitism shown by V. canescens in the first 5 days after initial host attack, suggesting that this parasitoid can deploy her use of deliberate superparasitism in an adaptive way. Received: 13 December 1995/Accepted after revision: 5 March 1996     

Effects of a Biological Control Introduction on Three Nontarget Native Species of Saturniid Moths

Abstract: Damage to nontarget (native) invertebrates from biological control introductions is rarely documented. We examined the nontarget effects of a generalist parasitoid fly , Compsilura concinnata ( Diptera: Tachinidae), that has been introduced repeatedly to North America from 1906 to 1986 as a biological control agent against 13 pest species. We tested the effect of previously established populations of this fly on two native, nontarget species of moths ( Lepidoptera: Saturniidae) , Hyalophora cecropia and Callosamia promethea , in Massachusetts forests. We estimated survivorship curves for newly hatched H. cecropia larvae (n = 500), placed five per tree in the field and found no survival beyond the fifth instar. We simultaneously deployed cohorts (n = 100) of each of the first three instars to measure the effect of parasitoids during each stage of development. C. concinnata was responsible for 81% of H. cecropia mortality in the first three instars. We deployed semigregarious C. promethea in aggregations of 1–100 larvae in the field and recorded high rates of parasitism by C. concinnata among C. promethea larvae exposed for 6 days (69.8%) and 8 days (65.6%). We discovered a wild population of a third species of silk moth, the state-listed (threatened) saturniid Hemileuca maia maia, and found that C. concinnata was responsible for 36% (n = 50) mortality in the third instar. Our results suggest that reported declines of silk moth populations in New England may be caused by the importation and introduction of C. concinnata .     

Life cycle and population dynamics of Pycnogonum litorale (Pycnogonida) in a natural habitat

A natural population of Pycnogonum litorale Ström was examined every 4 weeks over a period of 15 months and thereafter at yearly intervals for 15 years. Adult pycnogonids – mating couples and males carrying egg batches – and freshly hatched protonymphon larvae within these egg batches were found throughout the year. The second, third, and fourth instar larvae were only found from April to July, during the vegetation period of their hydroid host Clava multicornis. After metamorphosis to the fifth instar (first juvenile instar) the pycnogonids have a significantly larger proboscis than during the larval period, and they feed on the sea anemone Metridium senile. First juvenile instars were found on M. senile from May to August. Older and larger juvenile stages were found over longer time spans throughout the year, and the maximum number of successive instars shifted slowly from June to December. Freshly moulted adults occurred throughout the year. Males, which on the average are smaller, usually reach the adult stage during late autumn of the first year and females, at the end of the following spring. We conclude that in nature the development from egg to adult stage is completed within one year. Continuous reproduction and asynchronous embryonic development provide offspring throughout most of the year. The annual cycle is synchronized by the vegetation period of C. multicornis, the only host of these pycnogonid larvae in the investigated habitat, and by the arrest of growth during low winter temperatures. The low level of locomotory activity of P. litorale probably requires an environment in which both host species coexist. The abundance of C. multicornis, M. senile, and juvenile pycnogonids decreased from 1990 to 1996, maybe due to hydrographic conditions.     

Influence of food concentration,temperature and salinity on the larval development ofBalanus amphitrite

Influence of food concentration (0.5, 1 and 2 x 10⁵ cell ml^–1 ofSkeletonema costatum), temperature (20 and 30°C) and salinity (15, 25 and 35) on the larval development ofBalanus amphitrite (Cirripedia: Thoracica) was examined. The mortality rate at 20°C was lower than at 30°C in general. Increase in food concentration from 0.5 to 1 x 105 cells ml^–1 improved the survival rate, but this was not evident when food concentration was increased to 2 x 105 cells ml^–1. The results indicate that food availability and temperature jointly determine the energy allocation for metamorphic progress. It was observed that the influence of the tested variables varied with instar. At 20 °C the mean duration of the second instar exceeded 3 d and was much longer than other instar durations. The fourth, fifth and sixth instars and the total naupliar period showed that the effect of different salinities at given food concentrations was negligible at 20°C, while at 30°C there was a marked decrease in duration with increasing salinity.     

Larval behavior of predacious sister-species: orientation,molting site,and survival in Chrysopa

Field and laboratory studies compared two features of larval behavior in a pair of predacious sisterspecies of green lacewings: one (Chrysopa slossonae) a specialist on a single species of colonial aphids (the woolly alder aphid) that occur on branches and trunks of alder trees, the other (C. quadripunctata) a general aphid feeder whose primary prey is dispersed on foliage of diverse types of trees. First, a few hours after hatching, larvae of the two species develop significantly different phototactic responses; the differences correspond well with the spatial distributions of their prey. Most C. slossonae exhibited negative phototaxis, a response that helps move hatchlings inward on alder trees toward the woolly alder aphid colonies, whereas most C. quadripunctata hatchlings showed positive orientation to light, a response that tends to keep them in tree canopies with their prey. Second, in greenhouse experiments, a significantly greater proportion of C. slossonae larvae (second instars) molted within woolly alder aphid colonies and remained with the aphids than did C. quadripunctata larvae. These differences indicate that the specialist larvae have evolved a high degree of behavioral fidelity to their prey. However, larvae (second instars) of the two species that were released near ant-tended woolly alder aphid colonies in the field had similar recovery (= survival) rates. Consequently, natural selection may not act on behavioral traits that influence larval fidelity to prey during the late second and early third instars.     

Intraguild predation by <Emphasis Type="Italic">Harmonia axyridis</Emphasis> on coccinellids revealed by exogenous alkaloid sequestration

Summary.  Under laboratory conditions, the multicolored Asian lady beetle, Harmonia axyridis is well known as an intraguild predator of other ladybirds. However the real impact of this exotic species on native species was poorly investigated in the field. Because many ladybird species produce alkaloids as defensive compounds, we propose here a new method of intraguild predation monitoring in coccinellids based on alkaloid quantification by GC-MS. In laboratory experiments, adaline was unambiguously detected in fourth instar larvae of H. axyridis having ingested one egg or one first instar larva of Adalia bipunctata. Although prey alkaloids in the predator decreased with time, traces were still detected in pupae, exuviae and imagines of H. axyridis having ingested one prey when they were fourth instar larvae. Analysis of H. axyridis larvae collected in two potato fields shows for the first time in Europe the presence of exogenous alkaloids in 9 out of 28 individuals tested. This new method of intraguild predation detection could be used more widely to follow the interactions between predators and potential chemically defended insect preys.     

Comparative efficacy of certain plant extracts alone and combination with profenofos against Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae)

Ethyl alcohol, acetone, and petroleum ether extracts of three plant species belonging to three different botanical families [Strychnos nux-vomica (Loganiaceae), Euphorbia lathyrus (Euphorbiaceae), and Datura stramonlum (Solanaceae)], a chemical insecticide; profenofos and their combinations were tested against second and fourth instars of Spodoptera littoralis under lab conditions. Results revealed that the ethanol extract of S. nux-vomica was the most effective among all plant extracts, where the corrected mortality% were 92, 81, 58, and 27% to 2nd instar and 89, 74, 34, and 11% to 4th instar at concentrations 0.5, 0.25, 0.125, and 0.0625%, respectively. Calculated LC₅₀'s were 0.11, 0.22, and 0.34% to 2nd instar and 0.17, 0.37, and 0.52% to 4th instar for ethanol, petroleum ether, and acetone extracts, respectively. Acetone extracts of all plants were of lower effect. The chemical insecticide profenofos displayed higher efficacy than plant extracts (LC₅₀ = 0.002 and 0.003% for S. littoralis 2nd and 4th larval instars, respectively). The co-toxicity factor reached 76 and 60 when mixing S. nux-vomica + profenofos and D. stramonlum + profenofos at ratio 1:1 against S. littoralis 2nd instar larvae, thus indicating a potentiating effect. While treatment of the 4th instar larvae by the same mixtures resulted in a co-toxicity factor below 20 at all mixing ratios indicating, only, an additive effect against this instar.     

The scent of alarm: ontogenetic and genetic variation in the osmeterial gland chemistry of Papilio glaucus (Papilionidae) caterpillars

Within a species, ontogenetic and genetic variation in defensive chemistry can provide the basis for natural selection from different predator types. The osmeterial chemistry of fifth (last) instar Papilio glaucus caterpillars is known to differ qualitatively from the composition of early instar caterpillars. However, the osmeterial chemistry of early instar caterpillars has not been thoroughly characterized and may change as the caterpillars undergo their first three molts. We have used GC/MS to identify a suite of about 50 different terpene compounds in the osmeterial secretions of P. glaucus caterpillars, and found the relative amounts of these compounds changed significantly with each molt. These quantitative changes preceded the more dramatic qualitative switch to the production of 2-methylbutyric and isobutyric acids after the molt to the fifth instar. We also examined the effects of diet and genetic background on the relative quantities of 15 terpenes present in the secretions of third instar caterpillars. Parentage was found to affect the percentages of many more of the individual components than did diet, although both exerted an effect. The ontogenetic and genetic variations in the composition of the osmeterial secretions appear to have an effect on would-be predators. In the laboratory, terpene secretion was found to discourage attack by ants, whereas the switch from terpene to acid production rendered the caterpillars less palatable to a larger predator, the green anole. In the field, the presence of functional osmeteria did not seem to dramatically increase survival in a field study, and only a small, non-significant advantage was seen. Similarly, field data was suggestive that parentage might affect the likelihood of survival in a natural setting, but the stage of the caterpillar and the field site significantly affected survivorship. Further studies with greater replicates will be needed to determine whether and to what extent chemical differences in osmeterial components as well as behavior contribute to differences in outcomes in the field.     

Cuscuta hyalina Roth., an insect development inhibitor against common house mosquito Culex quinquefasciatus Say

A preliminary screening of crude acetone extract of Cuscuta hyalina Roth. was conducted against the laboratory reared preadult stages of common house mosquito Culex quinquefasciatus (Say) (Diptera: Culicidae). 24 h LC50 of third and fourth instar larvae and pupae were 303 ppm, 306.44 ppm and 97.66 ppm respectively. The extract completely restrained the adult mosquito emergence at sublethal doses (50 ppm for third instar and 75 ppm for fourth instar), by disrupting the normal morphogenesis. The extract was found to be an effective oviposition deterrent at 80 ppm. The plant was also found to be effective in suppressing the adult emergence when directly sprinkled as dry powder on the water surface.     

Development of the escape response in larval walleye pollock (Theragra chalcogramma)

The development of the escape response of walleye pollock (Theragra chalcogramma) larvae from attacks by macrozooplanktonic and small-fish predators was quantified in laboratory experiments. Behavior was recorded using video cameras with silhouette illumination from infrared-emitting diodes and by visual observation. Laboratory-reared larvae of 1, 3, 8, 10, 12, 18, 22, 27, 42 days post-hatching, ranging in size from 4 mm to 10 mm total length, were used in the experiments. Even the youngest larvae were observed to exhibit a fast startle response. The percentage of successful larval escapes from the different predators increased as the larvae developed. Euphausiids (Thysanoessa raschii) and amphipods (Calliopiella pratti) often touched larvae but the larvae were usually able to escape and no successful captures of larvae over 22 days old were observed. Although successful escape from initial attacks by three-spine sticklebacks (Gasterosteus aculeatus) increased ontogenetically, sticklebacks were able to consume most larvae, even of the oldest age group, by repeated attacks. Day-old larvae had the lowest percent of escapes after encounters with jellyfish (Sarsia sp.), but the percentage of escapes increased dramatically for 3-day-old larvae. Escape speeds after an attack also increased with age, and tended to be higher after stickleback attacks and lower after jellyfish attacks. This study revealed that the escape response of larval pollock to attack by predators improves rapidly with development during the early larval stage.     

Size regulation in larvae of the crustacean Balanus eburneus (Cirripedia: Thoracica)

We tested the hypothesis that larval size in the acorn barnacle Balanus eburneus Gould (Cirripedia Thoracica) varies in relation to food availability. In March–November 1980, and March–July 1981, larvae were obtained from adult Balanus eburneus collected in the Newport River, North Carolina, USA. Carapace length and width of larvae reared at three different food concentrations were measured. Mean naupliar instar size was independent of food concentration. Mean size of the cypris instar increased with increasing food level. Greater cypris size could be attributed to increased food reserves in the preceding naupliar stage, and was coinciden with inmarked increase in metamorphic success. Variation in instar size remained constant or declined during naupliar development, but increased sharply at the molt to the cyprid. Naupliar size regulation involved: (1) conservation of a molt increment specific for each naupliar-naupliar molt, (2) an inverse relationship between premolt size and the molt increment during the first five naupliar instars, and (3) an increase in the precision of the molt increment at the molt to the sixth naupliar instar. Experimental evidence implies that size regulation in Balanus eburneus limits variation about a fixed final naupliar size (e.g. volume). Measurement of naupliar size, accumulated energy reserves, survival and development time, and cypris metamorphic success indicated that naupliar cuticular growth is the most conservative feature of larval development. The data suggest that maximum naupliar size is limited by escalating metabolic costs during development, while minimum naupliar size is limited by size-related feeding effectiveness.     

Health-dependent vulnerability to predation affects escape responses of unguarded chinstrap penguin chicks

Predators may select more often to attack the more vulnerable prey or those with an inferior health status. Thus, prey should be able to assess their own vulnerability to predation and modify their antipredatory behavior accordingly. When approached by predator skuas, unguarded penguin chicks flee short distances, and usually aggregate in dense packs, but there is a clear interindividual variability in their responses under similar conditions. We hypothesized that this variability in escape responses might be related to the perceived vulnerability to predation of each individual chick. We simulated predator attacks to chinstrap penguin chicks and analyzed the sources of variation in their escape response, such as the presence of adults or the density of other chicks, and the sex, age, body condition, and health status of responding chicks. Chicks allowed shorter approach distances when they had a better health condition (i.e., a greater T-cell-mediated immunity, CMI), when they were younger, and when the density of adults around was higher. Sex and density of other chicks were not important. Similarly, chicks fled from the experimenter to longer distances when they had a lower CMI and when the density of adults was lower. Therefore, escape characteristics of chicks depended on the presence of adults that can deter predators and on the health-dependent vulnerability of chicks.     

Effects of hydrolysable tannins on a herbivorous insect: fate of individual tannins in insect digestive tract

Summary. We investigated the effects of four chemically characterised galloylglucoses (GGs, a subgroup of hydrolysable tannins) and their hydrolysis product, gallic acid (GA), on consumption and performance of larvae of the autumnal moth Epirrita autumnata. Larvae were fed with birch (Betula pubescens) leaves that had been painted individually with each of the compounds at two levels, 5 and 20 mg/g. In addition, we investigated the fates of the leaf-painted GGs and GA in the E. autumnata digestive tract by comparing phenolics in leaves consumed and in faeces. In general, GGs reduced leaf consumption by E. autumnata during the second and fourth instars, although there was high compound- and instar-specific variation. However, GGs did not affect the leaf consumption rates by the most voracious fifth instar larvae. This resulted in approximately the same loss of total biomass by the experimental tree, regardless of the nature and level of GGs enriched to its foliage. The characteristic fate of hydrolysable tannins, i.e. hydrolysis, was evidenced in the larval digestive tract for three of the four leaf-painted GGs. In addition to hydrolysis, the almost total absence of GGs in larval faeces was presumably related to the oxidation of GGs. The dose-dependent excretion percentage of ingested GA showed that it's faecal content should not be used, although it commonly is, to calculate the level of GG hydrolysis. Moreover, by comparing the non-uniform appearance of faecal tetragalloylglucoses, whether ingested as such or hydrolysed from pentagalloylglucose, we concluded that a major part of oxidation of GGs occurs before their hydrolysis in the digestive tract of E. autumnata. Criticism against the common use of tannic acid, a heterogeneous mixture of GA and GGs, in ecological studies is presented. Received 15 May 2002; accepted 16 July 2002     

阿特拉津对家蚕生殖发育的影响

为探讨环境激素(EH)对鳞翅目昆虫的性激素作用,用添加阿特拉津(AT)的人工饲料饲养家蚕,调查AT对家蚕生殖发育的影响.结果显示:1)染毒72、96、120h的半数致死浓度LC50分别为0.348、0.326、0.234mmol·kg-1(即75.1、70.3、50.5mg·kg-1),属中等毒性.2)AT染毒可导致雄性家蚕5龄幼虫生殖腺指数显著升高,成虫(蛾)生殖腺指数显著降低.从整个世代看,0.05mmol·kg-1的AT对家蚕精巢的生长即表现出不良影响.AT染毒对雄蚕5龄幼虫、蛹前期精细胞的形成有促进作用,而对蛹后期和成虫(蛾)的精子形成有限制作用,可能AT染毒阻碍了精细胞向精子的发育.3)雌性家蚕幼虫期AT染毒后,5龄幼虫和蛹的卵细胞数量显著少于对照组(p<0.05).从不同发育时期的影响程度看,AT对卵细胞发育的抑制作用具有显著的时间效应(p<0.05).实验浓度下,AT对雌蚕造卵数和产卵数影响不显著,然而随着AT浓度的增加,不受精卵越来越多,AT染毒对雌蚕受精率具有显著抑制作用.AT通过抑制家蚕生殖腺的生长和生殖细胞的发育表现出雌激素效应.     

Wait before running for your life: defensive tactics of spiny mice (<Emphasis Type="Italic">Acomys cahirinus</Emphasis>) in evading barn owl (<Emphasis Type="Italic">Tyto alba</Emphasis>) attack

Raptor–prey encounters were studied to evaluate the strategies and success rate of both predator attack and prey defense. We compared the success of barn owls in catching stationary simulated prey (food item) with that of moving prey (food item that was pulled in various directions). We also tracked real encounters between barn owls and spiny mice in a captive environment. It was found that owls had higher success in attacking stationary prey and that they seemed to attack the prey as soon as it became motionless. When attacked, only a few spiny mice remained immobile (freeze response) whereas most fled and usually avoided capture by the owls. It was also found that spiny mice displayed a preference to escape in those directions in which owls had demonstrated a lower success in catching the simulated prey. Escape initiation dichotomized to a short or long (but rarely intermediate) distance between the spiny mouse and the owl with more successful avoidance at short-distance (last-moment) escapes. The best predictor of escape success was the velocity of the spiny mouse, and the second best predictor was its flight initiation distance (FID). We present an update for Ydenberg and Dill’s model for optimal FID in close encounters, suggesting that fleeing at the last moment is advantageous. However, a last-moment attempt to escape is also more risky with a split second differing between life and death, and is therefore appropriate mainly for agile prey under close-distance attack.     

Experimental evidence for effective and altruistic colony defence against natural predators by soldiers of the gall-forming aphid Pemphigus spyrothecae (Hemiptera : Pemphigidae)

Summary The thick-legged first instar soldiers of the gall-forming aphid Pemphigus spyrothecae Pass. are able to protect the aphids in the gall from being eaten by a range of insect predators. In artificial galls, the soldier aphids were able to kill first instar ladybirds Adalia bipunctata (L.) (Coleoptera: Coccinellidae), early third instar hoverfly larvae Eupeodes (Metasyrphus) corollae (Fab.) (Diptera: Syrphidae), and first-third instar Anthocoris nemoralis (Fab.) (Hemiptera: Cimicidae). Almost all the aphids that attacked the predators were themselves killed. The soldiers were also able to kill predators introduced into natural galls. Experiments were devised in which individuals of Anthocoris minki, which is the most important insect predator of the gall generations of Pemphigus at the study site, were free to enter and leave the gall: the soldiers were effective both in preventing the predator's access to the gall and in killing those predators that did manage to get in. In galls with experimentally manipulated numbers of soldiers and non-soldiers, it was clearly shown that it is the soldiers alone that kill the predators (Anthocoris minki and 1st instar Adalia bipunctata) (see Table 3). Even though many soldiers may die during these encounters, the selective advantage of killing the predators is high, since observations show that individual A. minki can pass through more than one instar inside a gall and kill all the aphids therein. The aphids were not observed to attack conspecific aphids from other galls or the cohabiting aphid Chaitophorus leucomelas Koch. The primary role of the soldier caste is therefore the defence of the aphid colony against predators.     

Gene flow and local adaptation in a sunfish-salamander system

There is increasing evidence that populations may not be well adapted to their local environments, and as a result, recent interest has focused on understanding factors that constrain adaptive evolution. This study presents data suggesting gene flow may constrain the ability of larvae of the streamside salamander Ambystoma barbouri to avoid predation by fish via escape behavior and life history tactics. Streamside salamander larvae face conflicting selection pressures in different streams. Some streams are ephemeral, where larvae should be active to feed, grow, and reach metamorphosis before stream drying. Other streams contain predatory fish, where larvae should be generally inactive to avoid predation. Previous work has shown that streamside salamander larvae exhibit ineffective antipredator behavior by having inappropriately high activity levels with fish, resulting in high predation in laboratory and field experiments. This study investigated the possibility that gene flow from larvae in ephemeral habitats may reduce the escape performance of larvae from populations with fish and alter their life history characteristics to increase their susceptibility to fish predation. I assayed escape behavior (speed, acceleration, and duration of escape) and life history characteristics (hatching date, size, stage) associated with predator avoidance among laboratory-reared larvae from four populations. As predicted, two populations (one with fish and the other fishless and ephemeral) connected by gene flow were not significantly different in almost all assays. In contrast, larvae from an isolated population with fish had significantly stronger escape behaviors and delayed hatching than both an isolated population that lacked a history of fish co-occurrence and the population with fish but gene flow from a fishless population. These results support theory suggesting that gene flow can constrain adaptive evolution. Received: 22 February 1999 / Received in revised form: 4 April 1999 / Accepted: 26 April 1999     

Autecology of the tailed jay butterfly Graphium agamemnon (Lepidoptera : Rhopalocera : Papilionidae)

The Tailed Jay Graphium agamemnon is one of the attractive papilionid butterflies that enliven the environment of Visakhapatnam. It occurs throughout the year. It lays eggs singly on young leaves of the mast tree Polyalthia longifolia var. pendula (Annonaceae). The eggs take 3-4 days to hatch. The larvae go through 5 instars over a period of 15-16 days. The pupal period is 13-14 days. The total period from egg to adult emergence spans over 33-36 days. Based on this short life cycle, and larval and pupal development success studied every month, this butterfly can be multivoltine with a minimum of 7-8 broods in a year. Both CI and GR decreased with the age of larva, their average figures being 3.78 and 0.43 respectively. AD values are high (average 92%) and decreased through successive instars. Both ECD and ECI followed a similar pattern with an increase from instar I up to II, then a decrease up to IV and again an increase in instar V and the highest value is with fifth instar. Adults frequently visited flowers (12-35 flowers in a min) spending 1.0 to 3.2 seconds on a flower. The nectar concentration ranged between 16 and 58%. Peak foraging activity mostly fell between 0900-1000 h. The proboscis received pollen in most of the floral species visited, thus satisfying one of the characteristics of butterfly pollination. Being a fast and strong flier it is treated as "high energy" pollinator promoting cross-pollination.     

The costs and benefits of cooperation between the Australian lycaenid butterfly,Jalmenus evagoras,and its attendant ants

Summary The larvae and pupae of the Australian lycaenid butterfly, Jalmenus evagoras associate mutualistically with ants in the genus Iridomyrmex. Four ant exclusion experiments in three field sites demonstrated that predation and parasitism of J. evagoras are so intense that individuals deprived of their attendant ants are unlikely to survive. Larvae and pupae of J. evagoras aggregate, and the mean number of attendant ants per individual increases with larval age and decreases with group size. Field observations showed that young larvae could gain more attendant ants per individual by joining the average size group of about 4 larvae than by foraging alone. Aggregation behaviour is influenced by ant attendance: young larvae and pupating fifth instars aggregated significantly more often on plants with ants than on plants where ants had been excluded. In return for tending and protecting the larvae, ants were rewarded by food secretions that can amount to as much as 409 mg dry biomass from a single host plant containing 62 larvae and pupae of J. evagoras over a 24 h period. Larval development in the laboratory lasted approximately a month, and larvae that were tended by ants developed almost 5 days faster than larvae that were not tended. However, tended individuals, particularly females, pupated at a significantly lower weight than their untended counterparts, and the adults that eclosed from these pupae were also lighter and smaller. On average, pupae that were tended by ants lost 25% more weight than untended pupae, and in contrast with larvae, they took longer to eclose than pupae that were not tended. These experimental results are discussed in terms of costs and benefits of association for both partners, and of aggregation for the lycaenids.     

Predicting functional response and size selectivity of juvenile Notonecta maculata foraging on Daphnia magna

A mechanistic model was developed to assess the impact of predation of juvenile Notonecta maculata on size structured Daphnia magna populations and to provide a framework for quantifying the backswimmers uptake of food. Results of experiments and model predictions clearly demonstrate selective predation of backswimmers when fed with a choice of daphnid size classes, with patterns of selectivity differing across N. maculata instars. The model describes the foraging process empirically on the base of a general predation cycle including four conditional events instead of using classic functional response curves. For model parameterisation components of predation, namely probability of encounter, attack and success as well as time spent on handling prey was directly observed by means of video tracking experiments. Since attack rate, capture success and handling time appeared to be a function of prey size differing between Notonecta instars, the model takes into account ontogenic changes in both predator and prey characteristics. Independent data of functional response and size selectivity experiments were used for model validation and proved the model outcome to be consistent with observations.