Intra versus interspecific interactions of ladybeetles (Coleoptera: Coccinellidae) attacking aphids

Summary The nature and relative strengths of intra versus interspecific interactions among foraging ladybeetle larvae were studied experimentally by measuring short-term growth rates of predators and reductions in population sizes of prey in laboratory microcosms. In these microcosms, ladybeetle larvae foraged singly or as conspecific or heterospecific pairs, for pea aphids on bean plants over a two-day period. Similarly sized third instar larvae ofHippodamia convergens andH. tredecimpunctata, H. convergens andH. sinuata, andH. convergens andCoccinella septempunctata, were tested in experiments designed to ensure that paired larvae experienced moderate competition. Interspecific competition in these experiments did not differ significantly from intraspecific competition, in that an individual's weight gain did not depend on whether its competitor was heterospecific or conspecific. Furthermore, aphid populations were reduced equally by heterospecific and conspecific pairs. These results suggest that there is little or no difference between intra and interspecific interactions among larvae of these ladybeetles when two similarly sized individuals co-occur on a host plant. Thus, the species diversityper se of assemblages of ladybeetle larvae may have little influence over the short term on the reduction of aphid populations by ladybeetle predation.     

Hydroid defenses against predators: the importance of secondary metabolites versus nematocysts

Marine hydroids are commonly thought to be defended by stinging organelles called nematocysts that penetrate predator tissues and inject proteinaceous venoms, but not all hydroids possess these nematocysts. Although an increasing number of bioactive secondary metabolites have been isolated from marine hydroids, ecological roles of these compounds are poorly known. To test the hypothesis that nematocysts and noxious secondary metabolites represent alternative defenses against predation, we examined hydroids from North Carolina, United States for: (1) the palatability of whole polyps before and after nematocysts had been deactivated; (2) the palatability of their chemical extracts; and (3) their nutritional value in terms of organic content, protein content, and levels of refractory structural material (chitin). All hydroids were avoided by a generalist predator, the pinfish Lagodon rhomboides, compared with palatable control foods. Two of these (Halocordyle disticha and Tubularia crocea) became palatable after being treated with potassium chloride to discharge their nematocysts, suggesting that these species rely on nematocysts for defenses against predators. Chemical extracts from nematocyst-defended species had no effect on fish feeding. The four species that remained unpalatable after nematocysts had been discharged (Corydendrium parasiticum, Eudendrium carneum, Hydractinia symbiolongicarpus, Tridentata marginata) possessed chemical extracts that deterred feeding by pinfish. We have isolated and characterized the structures of the deterrent metabolites in two of these species. We found no differences in nutritional content or levels of chitin between nematocyst-defended and chemically defended species, and no evidence that either of these played a role in the rejection of hydroids as prey. Our results suggest that, among hydroids, chemical defenses may be at least as common as nematocyst-based defenses and that the two may represent largely alternative defensive strategies. The four hydroid species with deterrent extracts represent four families and both sub-orders of hydroids, suggesting that chemical defenses in this group may be widespread and have multiple origins. Received: 25 May 1999 / Accepted: 1 February 2000     

Temperature-dependent oviposition model of Scopula subpunctaria (Lepidoptera: Geometridae)

Scopula subpunctaria, is one of important defoliating insect pests of tea plants in China. However, the effect of temperature on its longevity and fecundity had not been reported, and the temperature-dependent oviposition model is lacking. The reproductive characteristics of S. subpunctaria were investigated under eight constant temperatures (13, 16, 19, 22, 25, 28, 31, and 33℃), with a 14L: 10D photoperid and 65–85% RH. The results showed that temperature significantly influence oviposition period, longevity, and fecundity of S. subpunctaria. The mean longevity of female adults increased from 10.8 days (33℃) to 33.9 days (13℃). Male lived shorter than female did. The mean longevity of males was shortest at 33℃ (8.1 days) and longest at 23.8℃ (17.6 days). Total fecundity of S. subpunctaria females ranged from 15.4 eggs at 33℃ to 279.9 eggs at 22℃. The temperature-dependent oviposition model of S. subpunctaria was constructed based on four sub-models (developmental rate model, total fecundity model, oviposition rate model, and survival rate model). The results of this study would be helpful in forecasting and predicting population occurrence of S. subpunctaria in tea plantations.     

Genetic variation for the rate of recruitment of novel insect prey into the diet of a bird

Two lines of quail (Cotumix coturnix japonicus) were selected over three generations for the speed of their response to the red colour form of the two-spot ladybird beede (Adalia bipunctata), an aposematic, semi-palatable prey insect. One line was bred for fast recruitment of die new prey into the diet and die odier for slow recruitment. Differences between the lines suggested additive genetic variation. The birds' response to insect prey with different colour patterns and toxicities (brown beetles, green butterfly pupae, melanic two-spot ladybirds and toxic seven-spot ladybirds) demonstrated that die selection involved a specific response to novel prey. The results are discussed in relation to predator strategy and the evolution of aposematism.     

Can ant‐eating Zodarion spiders (Araneae: Zodariidae) develop on a diet optimal for euryphagous arthropod predators?

Abstract Little attention is paid to the behavioural and physiological adaptations of ant‐eating predators. It is expected that there should be a strong selection for traits related to prey handling, leading to the evolution of morphological, behavioural and nutritional adaptations. Such adaptations may then entail trade‐offs in handling and utilization of alternative prey. To investigate behavioural as well as nutritional adaptations and the occurrence of the corresponding trade‐offs in two ant‐eating spiders of the genus Zodarion [Zodarion atlanticum Pekár & Cardoso and Zodarion germanicum (C. L. Koch)], spiders are reared on two diets: ants (i.e. their preferred prey) and fruit flies (i.e. an alternative prey that is nutritionally optimal for euryphagous spiders). Food consumption is observed and several fitness‐related life‐history parameters are measured. Although spiders readily accept ants, more than one‐third of 35 spiders refuse to consume fruit flies and starve. Furthermore, severe hunger does not induce these individuals to accept fruit flies. Starving spiders die before moulting to the second stadium. Spiders that eat fruit flies increase only little and slowly in weight, and all of these die during the first two stadia. By contrast, spiders on an ant diet increase dramatically in weight, and develop up to the fourth stadium. These data indicate that fruit flies are not suitable for Zodarion, supporting the hypothesis that there are behavioural and nutritional trade‐offs. Taking into account the results of previous studies, it is suggested that nutritional trade‐offs are generally important for stenophagous spiders.     

Determinants of predation on phytophagous insects: the importance of diet breadth

To evaluate the role of predation in the evolution of diet specialization and to determine the effectiveness of various larval defenses, we offered lepidopteran larvae to colonies of the tropical ant Paraponera clavata. We recorded behavioral and physical characteristics of prey items and used log-linear models to analyze their importance as deterrents to predation by P. clavata. The most important determinant of probability of prey rejection by P. clavata was a prey's diet breadth; specialists were rejected by the ants significantly more than generalists. Other less important, but significant, predictors of prey rejection included ontogeny, morphology and chemistry. Late instar caterpillars were rejected more frequently than early instars, hairy caterpillars were rejected more frequently than caterpillars with other morphologies, and one caterpillar species with an unpalatable extract was rejected more frequently than two species with palatable extracts.     

Escaping an evolutionary trap: preference and performance of a native insect on an exotic invasive host

Exotic plants may act as population sinks or evolutionary traps for native herbivores. The native butterfly Pieris oleracea lays eggs on garlic mustard, Alliaria petiolata, but larvae develop very poorly on this exotic invasive plant. We examined oviposition preference of individual females and larval performance of their offspring for individuals from one area where garlic mustard is well established and one where it is absent. These data were used to assess whether garlic mustard is being incorporated into or excluded from the diet. Females from the area without garlic mustard showed a wide range of preference, families had low larval survival on garlic mustard, and larval survivorship showed no correlation with mothers’ preferences. Females from the area with garlic mustard preferred it to the native host, and larval survivorship on garlic mustard was positively correlated with the mother’s preference. Individuals surviving on garlic mustard took longer to pupate and weighed >30% less compared to pupae reared on normal hosts. Our results suggest that where garlic mustard is well established P. oleracea may be adapting to this plant by both improved larval performance and increased adult female oviposition preference for it.     

The potential importance of podocysts to the formation of scyphozoan blooms: a review

Podocysts are cysts with stored reserves of organic compounds produced beneath the pedal discs of polyps of scyphozoans in the orders Rhizostomae (suborder Dactyliophorae) and Semaeostomae. They excyst small polyps that develop into fully active polyps (scyphistomae) capable of further podocyst production and of medusa production by strobilation. They contribute to increasing the number of polyps and also to survival through seasonal periods of reduced food availability or predation. These attributes may help support scyphozoan blooms, but as yet there are few quantitative data. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Intraguild predators and the spatial distribution of a parasitoid

An experimental plot of the aphid Aphis fabae on various host plant species was colonized by natural populations of the aphidiine parasitoid Lysiphlebus fabarum and insect predators, especially coccinellids. Parasitism of A. fabae by L. fabarum was significantly depressed on plants bearing coccinellids. The number of parasitized aphids increased with aphid abundance on three plant species (Papaver dubium, Rumex obtusifolius, Vicia faba), but not on the plant species (Chenopodium album) which bore very high numbers of coccinellids. In complementary laboratory experiments, L. fabarum offered a choice between odours of plants infested with A. fabae and/or coccinellids selected the odour fields from coccinellid treatments at significantly lower frequency than the odour fields of other treatments. It is concluded that avoidance of coccinellids by L. fabarum contributes to the negative association between the abundance of coccinellids and parasitoids in the field. Received: 22 March 1999 / Accepted: 22 March 2000     

Evolution of alternative mating tactics: conditional versus mixed strategies

Intrasexual polymorphisms have evolved in a wide range of organisms.Most of them have been interpreted as the product of conditionalstrategies in which the tactic an individual adopts is determinedby some aspect of state (e.g., age, size, condition). However,there are a few examples that appear to represent an evolutionarilystable mixture of heritable pure strategies that are maintainedby frequency-dependent selection. In the present study, we producea model of a mating system with two morphs: a territorial morphand a sneak morph. By varying the costs and limits associatedwith conditional strategies, mating skew, and the proportionof matings obtained by sneaking males, we examine the conditionsthat favor the evolution of conditional versus pure strategies.Contrary to current thinking, our results show that as longas either costs or limits are greater than zero, conditionalstrategists are never able to entirely replace pure strategists,and equilibrium populations may frequently consist of a mixtureof conditional and pure strategists. Our results suggest thatconditional strategists will be most frequent at intermediatelevels of mating skew. Polymorphisms in which conditional strategistsare rare or absent are most likely to evolve when mating skewis extremely high, the costs and limits of plasticity are veryhigh, or the benefits of being conditional are very low. Thelimited data available suggest that high mating skew is probablythe most important factor.     

Potential hominin plant foods in northern Tanzania: semi-arid savannas versus savanna chimpanzee sites

Savanna chimpanzees are useful as referential models for early hominins, and here potential differences between chimpanzee and early hominin ecology is the focus. Whereas chimpanzees inhabit only a handful of modern African savannas, there is evidence that early hominins occupied relatively more open and arid savannas than those in which chimpanzees live. In order to help expand potential models of early hominin palaeoecology beyond savanna chimpanzee-like scenarios, and to provide a basis for future modeling and testing of actual hominin diets, this study compares the types of plant foods available in modern semi-arid savannas of northern Tanzania to plant foods at savanna chimpanzee sites. The semi-arid savannas are not occupied by modern chimpanzees, but are potentially similar to environments occupied by some early hominins. Compared to savanna chimpanzee habitats, the northern Tanzania semi-arid savanna has a lower density and fewer species of trees that produce fleshy fruits. Additionally, the most abundant potential hominin plant foods are seasonally available Acacia seeds/pods and flowers, grass seeds, and the underground parts of marsh plants, as evidenced by vegetation surveys and by studies of the diets of baboons that forage in similar areas. The information from this study should be useful for framing hypotheses about hominin diets for sites with palaeoenvironmental contexts similar to those of the northern Tanzania semi-arid savannas and for contextualising tests of actual hominin diets (e.g., those based on dental microwear or isotopes).     

Infiltration by alien predators into invertebrate food webs in Hawaii: a molecular approach

Abstract Alien invertebrate predators have been introduced to Hawaii to control pests, particularly in lowland areas where most crops are grown. We developed techniques for assessing the impact of these predators on native food webs in relatively pristine upland areas where, it was hypothesized, few lowland predators might be found. Predator densities were assessed along transects within the Alakaii Swamp on Kaua'i. The most numerous alien biocontrol agents found were Halmus chalybeus (Coccinellidae), a species known to feed on Lepidoptera eggs. Laboratory experiments were conducted using two genera of endemic Lepidoptera, Scotorythra and Eupithecia (Geometridae), that are of considerable conservation value, the former because of its recent speciation across Hawaii, the latter for its unique predatory larvae. Techniques were developed for detecting Lepidoptera DNA within the guts of alien predators using prey-specific PCR primers. General primers amplified fragments of the mitochondrial cytochrome oxidase I gene from beetles and Lepidoptera. The sequences were aligned and used successfully to design target-specific primers for general detection of the remains of Geometridae and for particular species, including Scotorythra rara and Eupithecia monticolans. DNA fragments amplified were short [140-170 base pairs (bp)], optimizing detection periods following prey ingestion. Trials using the introduced biocontrol agent Curinus coeruleus (Coccinellidae) demonstrated detection of Lepidoptera DNA fragments = 151 bp in 85-100% of beetles after 24 h digestion of an early instar larva. This study provides a framework for future use of molecular gut analysis in arthropod conservation ecology and food web research with considerable potential for quantifying threats to endemic species in Hawaii and elsewhere.     

Predation regulation of sedimentary faunal structure: potential effects of a fishery-induced switch in predators in a Newfoundland sub-Arctic fjord

The collapse of the cod fishery in Newfoundland has coincided with marked increases in abundances of snow crab, pandalid shrimp, and other crustaceans that prey on sedimentary infauna. A 3-year sampling program in Bonne Bay, Newfoundland indicates differences in composition and number of these predators in the two main arms of the fjord that coincide with strong differences in benthic community structure. To test whether predation pressure contributes to the observed patterns in sedimentary fauna, exclusion field experiments with full and partial cages were deployed in both arms at 30-m depth and sampled along with ambient sediments at 0-, 4-, and 8-week periods. Predation significantly influenced species composition, abundance and, in some cases, diversity. The most striking changes included increases in the polychaetes Phöloe tecta and Ophelina cylindricaudata in exclusions relative to controls, and concurrent declines in the polychaete Paradoneis lyra and the cumacean Lamphros fuscata. In laboratory experiments, fresh non-disturbed sediment cores from each experimental area were either protected or exposed to snow crab, the most abundant predator in the bay. A snow crab inclusion experiment was also carried out in the field, using cages similar to those used for exclusions. Despite differences in sedimentary faunas in the two arms, both types of experiments detected a predator effect that was very similar to that documented in exclusion experiments. Thus, despite differences in the scales associated with each type of manipulation, our results suggest that crab predation is a significant structuring force in Newfoundland sedimentary communities. Given the historical changes that have occurred in predator composition as a result of cod over-fishing, we hypothesize that broad-scale community changes may be taking place in North Atlantic benthic ecosystems.     

Food web structure of three guilds of natural enemies: predators, parasitoids and pathogens of aphids

1. Most communities of insect herbivores are unlikely to be structured by resource competition, but they may be structured by apparent competition mediated by shared natural enemies. 2. The potential of three guilds of natural enemies (parasitoids, fungal entomopathogens and predators) to influence aphid community structure through indirect interactions is assessed. Based on the biology, we predicted that the scope for apparent competition would be greatest for the predator and least for the parasitoid guilds. 3. Separate fully quantitative food webs were constructed for 3 years for the parasitoid guild, 2 years for the pathogen guild and for a single year for the predator guild. The webs were analysed using standard food web statistics designed for binary data, and using information-theory-based metrics that make use of the full quantitative data. 4. A total of 29 aphid, 24 parasitoid, five entomopathogenic fungi and 13 aphid specialist predator species were recorded in the study. Aphid density varied among years, and two species of aphid were particularly common in different years. Omitting these species, aphid diversity was similar among years. 5. The parasitoid web showed the lowest connectance while standard food web statistics suggested the pathogen and predator webs had similar levels of connectance. However, when a measure based on quantitative data was used the pathogen web was intermediate between the other two guilds. 6. There is evidence that a single aphid species had a particularly large effect on the structure of the pathogen food web. 7. The predator and pathogen webs were not compartmentalized, and the vast majority of parasitoids were connected in a single large compartment. 8. It was concluded that indirect effects are most likely to be mediated by predators, a prediction supported by the available experimental evidence.     

A window of opportunity: Subdominant predators can use suboptimal prey

Introduced species have been linked to declines of native species through mechanisms including intraguild predation and exploitative competition. However, coexistence among species may be promoted by niche partitioning if native species can use resources that the invasive species cannot. Previous research has shown that some strains of the aphid Aphis craccivora are toxic to a competitively dominant invasive lady beetle, Harmonia axyridis. Our objective was to investigate whether these aphids might be an exploitable resource for other, subdominant, lady beetle species. We compared larval development rate, survival, and adult weight of five lady beetle species in no‐choice experiments with two different strains of A. craccivora, one of which is toxic to H. axyridis and one that is nontoxic. Two lady beetle species, Cycloneda munda and Coleomegilla maculata, were able to complete larval development when feeding on the aphid strain that is toxic to H. axyridis, experiencing only slight developmental delays relative to beetles feeding on the other aphid strain. One species, Coccinella septempunctata, also was able to complete larval development, but experienced a slight reduction in adult weight. The other two lady beetle species, Hippodamia convergens and Anatis labiculata, demonstrated generally low survivorship when consuming A. craccivora, regardless of aphid strain. All five species showed increased survival and/or development relative to H. axyridis on the “toxic” aphid strain. Our results suggest that this toxic trait may act as a narrow‐spectrum defense for the aphids, providing protection against only some lady beetle enemies. For other less‐susceptible lady beetles, these aphids have the potential to provide competitive release from the otherwise dominant H. axyridis.     

Chiral escape of bark beetles from predators responding to a bark beetle pheromone

Summary Two species of predatory beetles that locate their prey, Ips pini, by responding to its aggregation pheromone have different chiral preferences to ispdienol than does the herbivore. This suggests that chiral disparity may provide some escape for bark beetles from predation, and that geographic variation in herbivore communication systems may be partially due to predator — imposed selection pressures. These results also suggest ways in which the semiochemical and biological control of North America's most damaging group of forest insects can be improved.     

Importance of protein quality versus quantity in alternative host plants for a leaf-feeding insect

The nutritional value of alternative host plants for leaf-feeding insects such as caterpillars is commonly measured in terms of protein quantity. However, nutritional value might also depend on the quality of the foliar protein [i.e., the composition of essential amino acids (EAAs)]. A lack of comparative work on the EAA compositions of herbivores and their host plants has hampered the testing of this hypothesis. We tested the “protein quality hypothesis” using the tree-feeding caterpillars of Lymantria dispar (gypsy moth) and two taxonomically unrelated host plants, red oak (Quercus rubra) and sugar maple (Acer saccharum). Because L. dispar has higher fitness on oak than on maple, support for the hypothesis would be found if protein were of higher quality from oak than from maple. The whole-body EAA composition of L. dispar larvae was measured to estimate its optimum dietary protein composition, which was compared with the EAA compositions of oak and maple leaves. Contrary to the protein quality hypothesis, the EAA compositions of oak and maple were not significantly different in the spring. The growth-limiting EAAs in both tree species were histidine and methionine. Similar results were observed in the summer, with the exception that the histidine composition of oak was between 10 and 15 % greater than in maple leaves. The two main factors that affected the nutritional value of protein from the tree species were the quantities of EAAs, which were consistently higher in oak, and the efficiency of EAA utilization, which decreased from 80 % in May to <50 % in August. We conclude that the relative nutritional value of red oak and sugar maple for L. dispar is more strongly affected by protein quantity than quality. Surveys of many wild herbaceous species also suggest that leaf-feeding insects would be unlikely to specialize on plants based on protein quality.     

Advantages of a mixed diet: feeding on several foliar age classes increases the performance of a specialist insect herbivore

Two field studies were carried out to determine the influence of Abies balsamea foliage age on the preference and performance of larvae of Neodiprion abietis, a specialist Diprionid sawfly. Preference was determined by examining N. abietis defoliation on all age classes of foliage. Performance was estimated using larval survival, cocoon weights and the percentage of adults that were females. Neodiprion abietis preference for, and performance on, current-year foliage was very low, peaked on 2 or 3-year-old foliage, and declined on older foliage. Thus, sawfly feeding preference was adaptive. However, survival and cocoon weight were highest when sawflies were allowed to feed on all age classes of foliage, demonstrating that an insect specialist may perform better when feeding on several age classes of foliage from a single host plant species. These results indicate that either different larval instars have different nutritional requirements, or that food mixing provides the best diet, permitting the herbivore to obtain needed nutrients while avoiding ingestion of toxic doses of secondary metabolites. In addition, our results suggest that limited availability of varied foliage has more negative consequences for N. abietis females than for males, as the percentage of survivors that were females decreased when juvenile mortality was high. Our results emphasize the importance of considering non-linear changes in foliar quality as leaves age on herbivore preference and performance, and demonstrate how a herbivore can use this variability to maximize its fitness.     

Test for predation effects of single versus multiple species of generalist predators: spiders and their insect prey

The prediction that single spider species (as exemplary generalist predators) limit associated prey populations to the same extent that species assemblages do was tested in a well controlled and replicated old field experiment involving the following treatments: (1) the natural spider assemblage (2) a numerically prominent web building spider, (3) a numerically prominent wandering spider, (4) a biomass prominent web-builder, and (5) a biomass prominent wandering spider. Pest insect numbers were significantly higher in spider removal controls than in any spider treament over the four month period of the study, both in terms of total numbers and per spider effects. The individual spider species, in general, showed reduced prey limitation effects relative to that of the spider assemblage, though the magnitudes of these differences were small when compared to those exhibited between the various treatments and the spider removal control. When insect numbers were partitioned according to taxa, no treatment was found to have limited the predaceous insects nor the phytophagous hemipterans. All treatments, however, showed significant limiting effects on the phytophagous homopterans, coleopterans, and dipterans in the old field system, and other taxa were significantly reduced in at least one treatment in addition to the spider assemblage as a whole.