Mechanisms of intra-and interspecific interference between larval stoneflies

Summary Behavior of focal individuals of two potentially competing sympatric stonefly species, Megarcys signata and Kogotus modestus (Perlodidae), was videotaped in flow-through plexiglass arenas placed in the East River, Gunnison County, Colorado. Focal individuals were observed alone and in pairs with conspecifics and allospecifics at four prey (Baetis bicaudatus, Baetidae, Ephemeroptera) densities to determine whether competitors and prey resource levels affected prey capture rates. Presence of conspecific or allospecific competitors reduced stonefly prey capture rates, especially for Kogotus, the smaller of the two species, due to a significant decline in predator-prey encounter rates with competitors present. This competitive effect was not observed at the lowest and highest prey densities due to very low or very high predator-prey encounter rates, respectively. Thus, interference affected feeding rates only at intermediate prey densities. Competitors had no effect on the probability of attacks per prey encounter or capture success per attack. Within each stonefly species the effects of intra-and interspecific interference on feeding rates were similar, even though behavioral responses by both stoneflies to interspecific encounters were more frequent than to encounters with conspecifics. Kogotus showed the highest levels of response to encounters with other stoneflies, maintaining those high levels of response to Megarcys over all prey densities. Further, male Kogotus, which are the smaller sex, responded more frequently to competitive interactions than did females. These data are consistent with the hypothesis that interspecific interference was asymmetrical with Megarcys, the larger species, being the superior competitor.     

Fitness and community consequences of avoiding multiple predators

We investigated the fitness and community consequences of behavioural interactions with multiple predators in a four-trophic-level system. We conducted an experiment in oval flow-through artificial-stream tanks to examine the single and interactive sublethal effects of brook trout and stoneflies on the size at emergence of Baetis bicaudatus (Ephemeroptera: Baetidae), and the cascading trophic effects on algal biomass, the food resource of the mayflies. No predation was allowed in the experiment, so that all effects were mediated through predator modifications of prey behaviour. We reared trout stream Baetis larvae from just before egg development until emergence in tanks with four treatments: (1) water from a holding tank with two brook trout (trout odour), (2) no trout odour + eight stoneflies with glued mouthparts, (3) trout odour + stoneflies and (4) no trout odour or stoneflies. We ended the experiment after 3 weeks when ten male and ten female subimagos had emerged from each tank, measured the size of ten male and ten female mature nymphs (with black wing pads), and collected algal samples from rocks at six locations in each tank. To determine the mechanism responsible for sublethal and cascading effects on lower trophic levels we made day and night observations of mayfly behaviour for the first 6 days by counting mayflies drifting in the water column and visible on natural substrata in the artificial streams. Trout odour and stoneflies similarly reduced the size of male and female Baetis emerging from artificial streams, with non-additive effects of both predators. While smaller females are less fecund, a fitness cost of small male size has not been determined. The mechanism causing sublethal effects on Baetis differed between predators. While trout stream Baetis retained their nocturnal periodicity in all treatments, stoneflies increased drift dispersal of mayflies at night, and trout suppressed night-time feeding and drift of mayflies. Stoneflies had less effect on Baetis behaviour when fish odour was present. Thus, we attribute the non-additivity of effects of fish and stoneflies on mayfly growth to an interaction modification whereby trout odour reduced the impact of stoneflies on Baetis behaviour. Since stonefly activity was also reduced in the presence of fish odour, this modification may be attributed to the effect of fish odour on stonefly behaviour. Only stoneflies delayed Baetis emergence, suggesting that stoneflies had a greater sublethal effect on Baetis fitness than did trout. Delayed emergence may reduce Baetis fitness by increasing risks of predation and parasitism on larvae, and increasing competition for mates or oviposition sites among adults. Finally, algal biomass was higher in tanks with both predators than in the other three treatments. These data implicate a behavioural trophic cascade because predators were not allowed to consume prey. Therefore, differences in algal biomass were attributed to predator-induced changes in mayfly behaviour. Our study demonstrates the importance of considering multiple predators when measuring direct sublethal effects of predators on prey fitness and indirect effects on lower trophic levels. Identification of an interaction modification illustrates the value of obtaining detailed information on behavioural mechanisms as an aid to understanding the complex interactions occurring among components of ecological communities. Received: 20 March 1997 / Accepted: 29 September 1997     

Intraspecific competition in larvae of the ant-lion Morter sp. and interspecific interactions with Macroleon quinquemaculatus

Abstract.

• 1 Intraspecific competition was investigated in the pit-digging larvae of the ant-lion Morter sp., which normally occurs at low density in a food-rich habitat.
• 2 Morter larvae interact by displays, challenges, and sand tossing. Challenges/displays affected pit spacing while sand tossing affected pit size. Pit ownership determined the outcome of contests regardless of the relative sizes or feeding condition of the contestants.
• 3 Competition had no effect on the pit size of winners but losers showed a density dependent reduction in pit size. Interference competition caused a density dependent reduction in larval growth rates.
• 4 Macroleon quinquemaculatus (Hagen) always won contests with Morter, even when smaller. Cannibalism occurred in 60% of these contests.
• 5 Little intraspecific competition occurs in Morter in the field. Morter is more willing to incur energy costs than Macroleon.

     

Egg size affects larval performance in a coleopteran parasitoid

Abstract 1. Optimal progeny size models assume that the more eggs a female produces, the lower the amount of resource allocated per egg. As egg size generally correlates with the fitness of the emerging immature, this trade‐off can be expressed as a choice between the production of numerous low quality or fewer high quality progeny. 2. The first‐instar larvae of the coleopteran parasitoid Aleochara bilineata have to search for and parasitise dipteran pupae. The present study found a positive correlation between egg size and larval weight, but not between egg size and development time. Larger first‐instar larvae survived longer, were more active, and found and parasitised their host more rapidly. 3. Female A. bilineata may invest smaller larvae in conditions of high host density and low intraspecific competition, but investing fewer, larger larvae would bring them more fitness when hosts are scarce and competition high.     

Population regulation of a tropical damselfly in the larval stage by food limitation,cannibalism, intraguild predation and habitat drying

The relative importance of intraspecific, interspecific, and seasonal causes of larval mortality were investigated for aquatic larvae of the giant damselfly Megaloprepus coerulatus in Panama. These larvae live in water-filled holes in fallen and living trees, where they and three other common odonate species are the top predators. By mid wet season, M. coerulatus larvae were found in nearly half of all tree holes that harbored odonates. Although M. coerulatus were typically, but not always, eliminated from holes inhabited by larger hetero-specifics, M. coerulatus were more likely to encounter conspecifics than other odonate species. Hole with less than 11 of water rarely contained more than a single larva. In large holes where M. coerulatus was the only odonate species present, multiple larvae coexisted at a density of one larva per 1–21 of water. There the absence of 2–4 of the 5 larval size classes, despite a continuous input of eggs, suggested that cannibalism was a common cause of mortality. The size of the final instar, which determined adult body size, was correlated positively with tree hole volume for male, but not female, larvae. Experiments showed that when two larvae were placed together in 0.4–1 holes with abundant tadpole prey, the larger larva killed the smaller one. Often the larva that was killed was not eaten. Small larvae were more tolerant of each other than were pairs of medium or large larvae. Before killing occurred, the presence of larger larvae reduced the growth of smaller individuals, relative to controls. Obligate killing was density-dependent. In 3.0–1 holes with ad libitum prey, conspecific killing occurred until the larval density stabilized at one larva per 1–1.5 I, similar to the density found in large holes under field conditions, For M. coerulatus, cannibalism functions to reduce the number of potential competitors for food in addition to providing nutrition. When interactions between paired larvae in small holes were experimentally prevented, competition for food reduced the growth of one or both larvae relative to controls. Holes that were watered during the dry season supported larval densities similar to those in the wet season. Thus, dry season mortality could not be attributed to a decrease in available prey. Rather, M. coerulatus larvae could not survive more than 1 month of complete drying. Because the dry season typically lasts more than 6 weeks, habitat drying is a secondary source of mortality, affecting second- or third-generation larvae that fail to emerge before tree holes dry out completely.     

Do damselfly larvae recognize and differentially respond to distinct categories of macroinvertebrates?

Zygopteran larvae normally encounter other aquatic macroinvertebrates that are predators, competitors, and prey and should therefore demonstrate varied responses when faced with different categories of opponent. In a laboratory experiment individual final-instarIschnura posita (Hagen) larvae were observed in interactions with six categories of invertebrate opponents. The opponent categories were a nonconspecific damselfly and a small crayfish, which represented threatening opponents because they were larger than theI. posita subject larva, and a mayfly and a stonefly, which constituted nonthreatening opponents because they were smaller than the subject larva. The levels of threat posed by conspecific larvae of final and penultimate instar were inferred by comparison to the other opponent categories. Multivariate analysis showedI. posita's response differed between the two larger opponents, but responses were statistically indistinguishable between the two smaller opponents. Larvae retreated, moved around the stalk, and struck their opponents with their lamellae more often in the presence of a crayfish than the nonconspecific zygopteran. In contrast, they assumed an S-bend posture frequently with the zygopteran. Responses toward final-instar conspecifics differed from responses toward the larger opponents. Effectively, larvae wagged their abdomens only in the presence of final-instar conspecifics and retreated and moved around the stalk less frequently in these trials. Responses toward the smaller conspecifics differed from the responses to the small opponents. Larvae struck penultimate-instar conspecifics with their lamellae more frequently than the other small opponents. Our results suggest that larval zygopteran behaviors (such as S-bend and SCS) that have previously been described as intraspecific displays are of a more general nature and used toward a variety of opponents, whereas wag is unique to intraspecific interactions inI. posita.     

Interference competition in ant-lion (Macroleon quinquemaculatus) larvae

Abstract.

• 1 The effects of density, feeding regime, and body size on interference competition in the pit-digging larvae of the ant-lion Macroleon quinquemaculatus (Hagen) were investigated in laboratory experiments.
• 2 Competition had little effect on the pit size of winners but losers constructed much smaller pits than isolated larvae. Losers were less likely to dig or maintain pits and more likely to move than winners.
• 3 Competition was much stronger between well-fed larvae than between hungry ones, and well-fed competitors showed reduced growth rates. Well-fed larvae orientated themselves so that they could throw sand into their neighbour's pit whereas hungry larvae faced away from each other. Differences in hunger level reversed the competitive advantage of larger larvae only when individuals were of similar size.
• 4 Cannibalism was density-dependent and most frequent in hungry, similar-sized, larvae; the smaller larva was usually the victim.
• 5 Displays/challenges between larvae affected the distance between pits. Body size was the main determinant of contest outcome though pit ownership and hunger level also had an effect.

     

Intraspecific competition of Glyptotendipes paripes (Diptera: Chironomidae) larvae under laboratory conditions

Glyptotendipes paripes larvae were reared in wells of tissue culture plates, in groups of 2, 4, 8, 16, and 32 (representing densities of about 1,300, 2,600, 5,200, 10,400, and 20,800 larvae per m², respectively). Larval groups were supplied with one of two concentrations (low or high) of food and larvae were individually observed to evaluate the effects of density on mortality, growth, development, behavior, and adult body size. Increased larval densities resulted in higher mortality, as well as slower larval growth and development. The distribution of developmental time became flatter at higher density, with a wider range of values, or even became bimodal. This was a consequence of the most rapidly developing individuals at higher densities emerging as adults sooner than the fastest developing individuals at lower densities, although overall mean developmental time was longer at higher densities. At higher densities, growth and development of smaller larvae were slowed, based on the relative difference in body length between competitors. When larger competitors emerged as adults or died, the growth of smaller larvae may have accelerated, resulting in increased variability of developmental times. The effect of larval density on adult body size was complex, with the largest body size found at the lowest density and a second peak of adult size at high-middle densities, with smaller adult body sizes found at low-middle, and high densities. Similarly, as with developmental time, the range of body size increased with increasing density. Examined food concentrations had no effect on larval mortality, but significantly affected developmental time, growth rate, and adult body size. At higher densities, larvae spent more time gathering food and were engaged in aggressive or antagonistic behaviors.     

Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle. Received: 1 July 1997 / Accepted: 12 November 1997     

Local Competition Between Foraging Relatives: Growth and Survival of Bruchid Beetle Larvae

Kin selection theory states that when resources are limited and all else is equal, individuals will direct competition away from kin. However, when competition between relatives is completely local, as is the case in granivorous insects whose larval stages spend their lives within a single seed, this can reduce or even negate the kin-selected benefits. Instead, an increase in competition may have the same detrimental effects on individuals that forage with kin as those that forage with non-kin. In a factorial experiment we assessed the effects of relatedness and competition over food on the survival and on fitness-related traits of the bruchid beetle Callosobruchus maculatus. Relatedness of competitors did not affect the survival of larvae. Larval survival substantially decreased with increasing larval density, and we found evidence that beetles maturing at a larger size were more adversely affected by competition, resulting in lower survival rates. Furthermore, females showed a reduction in their growth rate with increasing larval density, emerging smaller after the same development time. Males increased their growth rate, emerging earlier but at a similar size when food was more limited. Our results add to the growing number of studies that fail to show a relationship between relatedness and a reduction in competition between relatives in closed systems, and emphasize the importance of the scale at which competition between relatives occurs.     

Triclads (Turbellaria: Tricladida) as predators of lake-dwelling stonefly and mayfly nymphs

It has been demonstrated by using a serological technique that Polycelis nigra (Müller) and P. tennis (Tjima) feed on mayfly and stonefly nymphs in unproductive lakes. Such feeding is less in productive lakes where these insects form a smaller proportion of the invertebrate fauna of the stony littoral zone. These nymphs form a significantly larger part of the diet of P. nigra than of P. tenuis; both triclad species feed more on stoneflies than mayflies.     

Differentiated egg size of the cannibalistic salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis>

Larvae of the salamander, Hynobius retardatus, are carnivorous, and even though there are two morphs, a typical morph and a broad-headed or “cannibal” morph, both are cannibalistic. They also sometimes eat other large prey, for example larvae of the frog, Rana pirica. In natural habitats, use of both conspecific and R. pirica larvae as food may contribute more strongly to high survival and substantially to fitness when larval densities are higher, because early-stage H. retardatus larvae sometimes experience scarcity of their typical prey. In cannibalistic oviparous amphibians, larger individuals that developed from larger eggs can more efficiently catch and consume larger prey and thus their survival may be better than that of smaller individuals developed from smaller eggs. Populations might therefore diverge in respect of egg size in response to variation in the density of conspecific and R. pirica larvae in natural ponds, with eggs being larger when larval density is higher. I examined how variance in hatchling size correlated with the incidence of cannibalism, and whether increasing larval density in natural ponds correlated with increasing egg size. Variance in initial larval body size facilitated cannibalism, and egg size increased as larval density in the ponds increased. In ponds with high larval density, where cannibalism and large prey consumption is a critical factor in offspring fitness, the production of fewer clutches with larger eggs, and thus of fewer and larger offspring, results in greater maternal fitness. Variation among the mean egg size in populations is likely to represent a shift in optimum egg size across larval density gradients.     

Competition between Bufo larvae in a eutrophic pond

Competition between larvae of two anuran species (Bufo bufo and B. calamita) was investigated under field conditions likely to disfavour cell-mediated interference mechanisms. The experiment used triplicated cage treatments in an unshaded farm pond, a poor habitat for the unicellular pathogen Anurofeca richardsi implicated in interference competition between these anurans in sand dune ponds. The farm pond experienced lower maximum temperatures than a nearby dune pond but sustained larger numbers of eukaryotic algae and therefore had higher primary productivity. Survival and growth of B. calamita larvae were inversely related to density in all treatments but interspecific effects were much more severe than intraspecific ones. There was no evidence of A. richardsi in any treatment and competition between the Bufo larvae was therefore intense in the absence of Anurofeca-mediated interference effects. Anuran larvae reduced the standing crop and altered the community composition of algae in the treatment cages but larval growth rates were not simply related to food availability. Algal cell numbers in larval guts, a measure of food acquisition, were however inversely related to tadpole density in both species. Feeding niche overlap was high but decreased as larval density increased. Resource competition was implicated as the most probable major mechanism. Received: 1 September 1999 / Accepted: 20 January 2000     

Effects of nutrition and density in Culex pipiens

Mosquito larvae face numerous biotic and abiotic challenges that affect their development and survivorship, as well as adult fitness. We conducted two experiments under semi‐natural conditions to evaluate the effects of intraspecific competition, nutrient limitation and sub‐lethal doses of malathion on individual life history traits in adult Culex pipiens (Diptera: Culicidae). In the first experiment, larvae of Cx. pipiens were reared at different intraspecific densities and exposed to sub‐lethal doses of malathion. In the second experiment, different intraspecific densities of Cx. pipiens larvae were reared under conditions of low or high larval nutrients, and subsequent adults were fed on either water or 10% sucrose solution. Malathion treatment had relatively minor effects compared with density, which had significant negative effects on development rate, survivorship to adulthood, body size (wing length) and longevity. As larval density increased, a sex ratio distortion in survivorship to adulthood emerged, in which a bias towards males was apparent. Nutrient‐rich larval environments alleviated, in part, the effects of increasing density and extended the lifespan of mosquitoes fed on water and 10% sucrose. Density‐dependent alterations in adult longevity attributable to the larval environment are complex and show contrasting results depending on interactions with other environmental factors. This study suggests that larval resource availability and competition influence Cx. pipiens population growth correlates and have lasting effects on traits that relate to a mosquito's ability to vector pathogens.     

Influence of the larval environment on performance and adult body size of the wood-boring beetle Phoracantha semipunctata

Large body size confers a reproductive advantage to adults of the wood‐boring beetle Phoracantha semipunctata (F.) (Cerambycidae: Cerambycinae: Phoracanthini). Larvae of this species feed subcortically in stressed and dying eucalypt trees and logs. We evaluated the influence of the larval environment on larval performance and adult body size by manipulating the post‐felling age of host logs (from freshly cut to 2‐weeks‐old) and the density of colonizing neonates (low density with minimal competition for resources vs. high density with intense competition). Adult beetles emerged in greater numbers from logs that had been subjected to the aging treatment which reduced bark moisture content and favored colonization by neonates. Survival was greatest in larger logs having lower densities of neonates, but was greatly diminished in all treatments by mortality during pupation. Development time varied from 2 months to more than a year and was shortest in smaller logs having high densities of larvae. The size of adult beetles emerging from a log was not influenced by larval density, but was positively correlated with the age of logs when the neonates colonized, and log size. These findings suggest that the optimal developmental conditions for P. semipunctata larvae, in terms of larval performance and adult body size, are available in large, aged host logs having low densities of larvae. Manipulation of the larval environment in this study resulted in a considerable variation in adult body size, but large individuals were relatively more common in the wild population that was the source of neonates for the experiment. Potential body size may have been constrained by our use of only one host species and a narrow range of log dimensions.     

The interaction effect between intraspecific competition and seed quality on the life‐history traits of the seed‐feeding beetle Acanthoscelides macrophthalmus

It has been shown that intraspecific competition and resource quality may affect life‐history traits of insects, such as body size, fecundity, and survival. However, intraspecific competition and resource quality may interact with each other. The study of such interacting effects is crucial for understanding the influence of these ecological variables on the selection of specific life‐history traits. Here, we investigated whether the interaction between intraspecific larval competition and variation in resource quality affects adult emergence and survival, egg size, fecundity, body size, and sexual size dimorphism (SSD) of the seed‐feeding beetle Acanthoscelides macrophthalmus (Schaeffer) (Coleoptera: Chrysomelidae: Bruchinae) when infesting Leucaena leucocephala (Lam.) De Wit (Fabaceae), its host plant. In the laboratory, beetles were reared on seeds that differed in quality (e.g., different hardness, seed size, water content), in the presence or absence of larval competition. Body size and SSD did not differ between treatments (with and without competition), nor were they affected by varying resource quality. Females subjected to competition during the larval stage and females emerging from seeds of higher quality, displayed the highest fecundity. The proportion of emergent adults was higher in the absence of competition. In addition, larger eggs were laid on the low‐quality resource in the absence of competition, showing a trade‐off between egg size and egg number. Adult survival differed among treatments and resource qualities, suggesting a higher investment in adult survival for individuals emerging from seeds of low quality in the presence of competition. Whether changes in specific traits could be selected for in detriment of others will depend on the strength of intraspecific competition, the variation in resource quality, and the plasticity in the life‐history traits investigated. This needs further clarification.     

Effect of carnivory on larvae and adults of a detritivorous caddisfly, Lepidostoma complicatum: a laboratory experiment

Lepidostomatid caddisfly larvae are typical detritivores, but they occasionally eat small dead animals. A laboratory feeding experiment was conducted with Lepidostoma complicatum (Kobayashi) larvae using two different feeding treatments: leaves or leaves and dead chironomids. L. complicatum larvae showed significantly higher growth rates and adult emergence weight and a significantly earlier emergence for the leaves and dead chironomids treatment than for the leaves alone treatment. However, the adult emergence rate was not different between the two feeding treatments. Thus, it is apparent that ingestion of dead chironomids by detritivorous L. complicatum larvae positively influences larval growth rates, adult emergence weight, and larval development time.     

Larval consumption rates,interspecific predation,and local guild composition of egg-overwintering Agabus (Coleoptera,Dytiscidae) species in vernal ponds

Summary In vernal ponds in the boreal region, egg-over-wintering Agabus species form a guild that feeds mainly on larvae and pupae of aedine mosquitoes. The regular co-existence of very similar Agabus species indicates local communities not structured by interspecific competition. However, the lower number of species in local guilds than in the regional species pool poses a problem of limited membership. We suggest that the species of this guild display habitat differences mainly with respect to water temperature, pond size and prey density. In this view, habitat selection reflects body size and thermal growth response of the species, mainly in connection with larval development. We present field data from two northern Swedish vernal ponds. Based on these data, feeding experiments were performed to test the hypothesis outlined above. At a high prey density, larvae of all instars of the larger species A. erichsoni Gemm. & Har. had a significantly higher consumption rate than those of the smaller species A. opacus Aubé. At a low prey density the differences were smaller, and only the third instar larvae differed significantly. At 2° C, larvae of A. opacus had a significantly higher consumption rate than those of A. congener (Thunberg). At 15° C, no significant difference was observed. In studies of within-guild interspecific predation, always the larger larvae consumed the smaller ones. Field data show that egg hatching is spread out in time, and show interspecific differences. Consequently, the effects of unexpected droughts differ with species.     

Plant-mediated interactions between the rice water weevil and fall armyworm in rice

Greenhouse studies were conducted to investigate plant-mediated interactions between an above-ground and a below-ground herbivore when sharing a common host plant, rice (Oryza sativa L). Two common pests of rice were used: the rice water weevil (RWW), Lissorhoptrus oryzophilus Kuschel, as the root herbivore, and the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) as the foliage-feeding herbivore. Rice water weevil larval performance was assessed by measuring larval density and average weight in response to different levels of defoliation by FAW larvae. The reciprocal experiment was done to evaluate FAW performance (growth rate) in response to RWW feeding. Severe defoliation by FAW decreased RWW densities by 32% and reduced larval weights by 48% compared to larvae on roots of non-defoliated plants. Effects in the converse experiments were not as strong. FAW growth rates were reduced 9–37% when feeding on rice leaves from plants damaged by RWW compared to larvae feed leaves from the no damage treatment. These reciprocal negative effects show that RWW and FAW are potential competitors when sharing a rice plant. Because RWW and FAW did not interact directly, competition was plant-mediated.     

Relations of intraspecific variations in fecundity, clutch size, and oviposition frequency to the body size in three species of stoneflies, Sweltsa sp., Isoperla aizuana, and Stavsolus japonicus

Most stoneflies oviposit several times during their adult stage. In this study, the relations among oviposition frequency, the number of eggs per egg mass, and body size were examined in the chloroperlid stonefly Sweltsa sp. and two perlodid stoneflies, Isoperla aizuana and Stavsolus japonicus. It was found that larger individuals tended to oviposit more frequently than smaller ones, but the relation was significant only in Isoperla aizuana. In Sweltsa sp. and Isoperla aizuana, the number of eggs per egg mass was higher for the less-frequently ovipositing individuals than for those ovipositing more frequently, and the number of eggs per egg mass decreased with successive oviposition events. Stavsolus japonicus showed the same tendency but it was not statistically significant.