Predation pressure on an imperfect Batesian micicry complex in the presence of alternative prey

Summary Differential predation pressure and the probability of predation on a Batesian mimicry complex and on alternative prey were estimatedin a field experiment. The mimicry complex was composed of a noxious model (Eleodes obscura (Say)) and a palatable mimic (Stenomorpha marginata (LeConte)). House crickets (Acheta domesticus) (Linn.) were used as alternative prey. The experiment was conducted for 23 nights in August and September to approximate the peak seasonal activity time period during which both models and mimics normally are exposed to predation while foraging and depositing eggs. Each night thirty prey in ratios of 16 models: 7 mimics: 7 crickets were exposed for 2.5 h to a suite of predators consisting of pallid bats (Antrozous pallidus), striped skunks (Mephitis mephitis) and ringtails (Bassariscus astutus) that had free access to the prey. The model-mimic ratio was similar to that found in nature. Predators obtained prey on 11 of the 23 nights and preferred the alternative prey (crickets) in proportions higher than was expected from a predation rate that was equal on all species of prey. Mimics were taken by predators at a rate proportional to their abundance, while models were taken at a rate considerably lower than their relative abundance. This suggests that at least some of the predators could distinguish between models and mimics and were willing to eat the mimics at higher frequencies than they were willing to eat the models. However, although the mimicry is not perfect with respect to the entire predator suite, the mimics still gain an advantage by resembling the models, compared to the predation levels on the alternate prey.     

Prey selectivity inEuseius tularensis [Acari: Phytoseiidae]

Differences in responsiveness to potential prey and pollen were tested in the facultative predator,Euseius tularensis Congdon. Following isolation without food for 16 h, adult female mites were observed with one of the following potential foods: pollen ofMalephora crocea (Jacquin) Schwantes,Scirtothrips citri (Moulton) 2nd instar larvae,Tetranychus pacificus McGregor nymphs and ♂,Panonychus citri (McGregor) nymphs and ♂,E. hibisci (Chant) larvae, andE. tularensis larvae. Responses were compared using the proportion of encounters that resulted in consumption. This proportion was highest when pollen was encountered, followed byS. citri andT. pacificus. Consumption was much lower in response toP. citri and congeneric larvae. WhenS. citri larvae were encountered a 2nd time, 1 to 2 h after the initial capture and consumption, responsiveness increased to a level equal to the response observed with pollen. These results suggest thatE. tularensis is a more effective biological control agent of citrus thrips (S. citri) than of citrus red mite (P. citri).     

The effect of pulse repetition rate on the delay sensitivity of neurons in the auditory cortex of the FM bat,Myotis lucifugus

1. Echo delay is the primary cue used by echolocating bats to determine target range. During target-directed flight, the repetition rate of pulse emission increases systematically as range decreases. Thus, we examined the delay tuning of 120 neurons in the auditory cortex of the bat, Myotis lucifugus, as repetition rate was varied. 2. Delay sensitivity was exhibited in 77% of the neurons over different ranges of pulse repetition rates (PRRs). Delay tuning typically narrowed and eventually disappeared at higher PRRs. 3. Two major types of delay-sensitive neurons were found: i) delay-tuned neurons (59%) had a single fixed best delay, while ii) tracking neurons (22%) changed their best delay with PRR. 4. PRRs from 1-100/s were represented by the population of delay-sensitive neurons, with the majority of neurons delay-sensitive at PRRs of at least 10-20/s. Thus, delay-dependent neurons in Myotis are most active during the search phase of echolocation. 5. Delay-sensitive neurons that also responded to single sounds were common. At PRRs where delay sensitivity was found, the responses to single sounds were reduced and the responses to pulse-echo pairs at particular delays were greater than the single-sound responses. In facilitated neurons (53%), the maximal delay-dependent response was always larger than the best single-sound responses, whereas in enhanced neurons (47%), these responses were comparable. The presence of neurons that respond maximally to single sounds at one PRR and to pulse-echo pairs with particular echo delays at other PRRs suggests that these neurons perform echo-ranging in conjunction with other biosonar functions during target pursuit.     

Changes in diet,prey size and feeding habit in Bagrus bayad,and possible interactions with B. docmac in a Nile canal

Synopsis Bagrids in Bahr Shebeen Nilotic canal depend mainly on fish, insects and shrimp as well as fish embryos for food and their stomachs included runoff materials (e.g. plant foliage, glass, black crystals, coloured gravel). B. bayad maximised its efficiency of catching prey catfish by face to face attack to avoid damage by the prey's pectoral and dorsal spines. In the size classes of 10 to 30 cm standard length, B. bayad and B. docmac show diet overlap and interact with each other especially with respect to tilapias as prey. After this length, B. docmac, aided by its relatively larger mouth, shifted to larger size of tilapias to coexist with B. bayad.     

Intra- and interspecific variation in body size ofProtohermes (Megaloptera: Corydalidae)

The life history of three populations ofProtohermes grandis and two populations ofProtohermes immaculatus (Megaloptera: Corydalidae) was compared. In general, the larvae lived in stream riffles for 2 years and the adults appeared in summer. Adult body size differed between these closely related species and also between the populations ofP. grandis. Dwarfism occurred inP. immaculatus, a species that is endemic to the small, isolated island, Amami Island. The population ofP. grandis on Yaku Island, located between Amami Island and the mainland Kyushu, had an intermediate body size between that ofP. immaculatus and the mainland population ofP. grandis. Despite being an insular population,P. grandis on Tsushima Island had a similar body size to mainlandP. grandis. In these populations with large adults, some larvae lived in the streams for 3 years. The size distribution of benthic animals, which are the prey available toProtohermes larvae, differed between the streams studied. The density of large prey was lowest on Amami Island, intermediate on Yaku Island, and highest on the mainland and Tsushima Island. Different size distributions of available prey may be caused by the differences of benthic fauna; most of Ecdyonuridae and Ephemerellidae (large mayflies) and Perlidae (large stoneflies) were not found on Amami and Yaku Islands. Thus, there is a tendency to dwarfism in the populations ofProtobermes inhabiting streams where the density of large prey is low.     

Searching behaviour and cereal aphid consumption by Bembidion lampros and Pterostichus cupreus, in relation to temperature and prey density

Maximum consumption rates were determined for two carabids, Bembidion lampros Herbst. and Pterostichus cupreus L., feeding on the cereal aphid Rhopalosiphum padi L. at different temperatures in the laboratory. Mean daily consumption increased with increasing temperature for both species, B. lampros consuming a maximum of 16 1–3 instar nymphs and 9 apterous adult aphids at 25°C. P. cupreus was particularly voracious and consumed 125 apterous adult R. padi per day at 20°C. The behaviour of both species was analysed by video filming starved beetles, maintained at different constant temperatures, in arenas sown with spring barley. The behavioural components (1) still; (2) run/walk; (3) search and (4) confrontation were identified and were common to both species. P. cupreus was more active over the temperature range tested; B. lampros was inactive under 10°C. The proportion of time spent searching, number of plants searched, and velocity increased with increasing temperature for both species. When observed in similar arenas seeded with R. padi colonies, individuals of P. cupreus significantly increased their time spent searching in arenas with increasing aphid density. Following discovery of an aphid colony, individuals climbed and searched the host plant and its nearest neighbours. Plants in aphid free arenas were rarely climbed. B. lampros was not observed climbing in either aphid free arenas or in arenas with increasing aphid densities, and did not significantly increase its time spent searching in response to increased prey density. The few B. lampros that found aphids caught them walking on the soil surface. The relative efficiences of these two carabids as predators of R. padi are discussed, and the results are compared with similar studies elsewhere with predators of Sitobion avenae on winter wheat.

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Résumé Les taux maximum de consommation de R. padi L. à différentes températures ont été déterminés au laboratoire chez deux carabes, B. lampros Herbstet P. cupreus L. La consommation moyenne a augmenté avec la température chez les deux espèces, B. lampros consommant un maximum de 15,9 larves des stades 1 à 3 et 9,1 pucerons adultes aptères, à 25°C. P. cupreus a été particulièrement vorace et a consommé 125,3 adultes aptères par jour à 20°C. Le comportement des deux espèces a été observé en filmant en vidéo des carabes à jeun, maintenus à différentes températures constantes, dans des enceintes semées en orge de printemps. Des éléments du comportement, communs aux deux espèces, ont été définis: 1) immabilité, 2) marche et course, 3) recherche, 4) affrontement. P. cupreus a été plus actif à toutes les températures, B. lamprosa été inactif au-dessous de 10°C. La part de temps consacrée à la recherche, le nombre de plantes prospectées, et la vitesse ont augmenté avec la température chez les deux espèces. Dans des enceintes similaires colonisées par R. padi, P. cupreus a significativement augmenté le temps consacré à la recherche dans les enceintes, parallèlement à l'augmentation de la densité des pucerons. Après la découverte d'une colonie de pucerons, P. cupreus escalade et prospecte la plante et ses voisines immédiates; tandis que les plantes des enceintes sans pucerons sont rarement escaladées. B. lampros n'a pas été observé escaladant des plantes d'enceintes avec ou sans pucerons, et il n'a pas accru son temps de prospection en fonction de la densité de pucerons. Les quelques B. lampros qui ont capturé des pucerons l'ont fait lorsque ceux-ci marchaient sur la surface du sol. La discussion a porté sur l'efficacité relative des deux carabes comme prédateurs de R. padi, et les résultats ont été comparés à ceux d'études du même type, menées ailleurs, avec des prédateurs de Sitobion avenae sur blé d'hiver.

     

Density dependent mortality in the springtail species Orchesella cincta due to predation by the carabid beetle Notiophilus biguttatus

Mortality of the collembole species Orchesella cincta (L.), due to predation by the carabid Notiophilus biguttatus F., has been measured in the laboratory at three levels of prey density. Prey densities were chosen such that the predator's consumption at the lowest prey density was a little above the maintenance ration. Between the two lowest prey densities, mortality increased significantly with prey density. We presume that density-dependent search activity by the predator is responsible for the positive density-dependent mortality. A method is proposed to solve the problem of decreasing prey density during the observations.

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Relation positive entre la densité de Orchesella cincta et la mortalité due à la prédation part Nothiophilus biguttarus

Résumé Cet article décrit l'importance de la prédation par le carabe N. biguttatus en fonction de la densité de la proie, O. cincta. Les résultats montrent que, sous l'effet de la prédation, la mortalité de la proie augmente avec sa densité. La quantité minimale de proies qui doit être consommée pour permettre la reproduction du carabe, a été déterminée. La relation entre la fécondité du carabe et l'effectif consommé, montre l'effet stabilisant de la prédation sur les populations de proies.La relation de dépendance de la densité observée avec la mortalité est due à une modification du comportement du carabe en fonction de la densité de sa proie. Nous proposons un modèle permettant de prévoir la régression de la population de proies sous les effets de la prédation.

     

The daily timing of lamprey attacks

Synopsis Laboratory and field data suggest that several species of parasitic lampreys forage primarily at night, but data forPetromyzon marinus are equivocal. Nocturnal foraging may minimize predation risk. This may be particularly important to those species of lampreys that inhabit shallow streams. Nocturnal foraging also may enhance foraging efficiency if potential hosts at night are quiescent and more easily approached. Future experiments should be designed to avoid several potential sources of bias.     

Size of prey ingested by whitefish,Coregonus sp., larvae. Are Coregonus larvae gape-limited predators?

The feeding ecology of whitefish, Coregonus , larvae in oligotrophic Lake Sarnen and eutrophic Lake Hallwil is presented, taking into consideration the size of the prey ingested in relation to the mouth size of larvae and to the availability of zooplankton.
When larvae grow from 10 to 16 mm their mouth width increases from 500 to 1000 μm while their gape height, with the mouth open at 45 and 90°, increases from 200 to 400 μm and from 400 to 700 μm, respectively, whichever lake is considered.
The differences found in the mean prey width between the two lakes arise in part from the composition of diets, and in part from differences in size of the available prey organisms. Whatever the origin of the larvae and the taxon we consider, larvae seem to ingest the most abundant prey size without selecting the biggest ones available even if, from a mechanical point of view, they would be able to ingest them.
Taking into account the generally low density of zooplankton in the uppermost layer of the lake, we conclude that whitefish larvae eat in the manner such as to decrease the cost of prey capture, in terms of energy and time, by choosing the more abundant prey available.