The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs overa period of several days. Differences in the escape responseof fish larvae during the hatching period have not hithertobeen studied. In this study, the escape response of walleyepollock (Theragra chalcogramma) larvae over the hatching periodwas examined. Escape speed, response to multiple touches witha fine probe, response to water currents generated by a predatorand predation by euphausiids (Thysanoessa inermis) and amphipods(Pleusirus secorrus) were measured in the laboratory. Otolithmeasurements of field-collected larvae support a broad hatchingperiod for walleye pollock eggs in the sea similar to that observedin the laboratory. The escape response of walleye pollock larvaewas affected by rank in the order of hatching, thus with respectto predation, hatching order may affect the survival of larvaein the sea. Early hatching larvae were smaller, less sensitiveto tactile stimulation, had a slower, weaker escape responseand higher laboratory rates of predation mortality than thosethat hatched later.     

Laboratory studies of predation by the ctenophore Mnemiopsis leidyi on the early stages in the life history of the bay anchovy, Anchoa mitchilli

The lobate ctenophore, Mnemiopsis leidyi, consumed eggs andlarvae of the bay anchovy, Anchoa mitchilli, in laboratory experiments.This ctenophore exhibited a type I functional response to increasesin egg densities without reaching saturation at high prey densities.Clearance rate increased with increasing experimental containervolume. There was a 3-fold increase in the volume of water clearedby 2.0–2.5-cm ctenophores and a >5-fold increase for4.5–5.0-cm ctenophores in 15–1 versus 100–200–1containers. Clearance rate was dependent on tenophore lengthbut was probably underestimated for the larger animals due tocontainer effects. The presence of various densities of alternateprey, Acartia hudsonica or Anemia sp. nauplii, in addition toAnchoa mitchilli eggs did not affect the clearance rates onthe eggs alone. Comparison of clearance rates of 2.0–2.5-cmctenophores on various ages of starved and fed bay anchovy larvaeindicated that predation may be higher on yolk-sac larvae thanon eggs but decreases as the larvae grow. After 3 days posthatch starved anchovy larvae become more vulnerable to predationthan fed larvae. The ctenophore, M.leidyi has the potentialto inflict substantial predation pressure on early stages inthe life history of bay anchovy. ¹ Present address: University of Maryland, Center for Environmentaland Estuarine Studies, Chesapeake Biological Laboratory, POBox 38, Solomons, MD 20688-0038, USA     

Predation by a carnivorous marine copepod,Euchaeta norvegica Boeck,on eggs and larvae of the North Atlantic cod Gadus morhua L.

The predatory behavior of a carnivorous marine copepod, Euchaeta norvegica Boeck, feeding on eggs and larvae of the North Atlantic cod Gadus morhua L. was examined. In the laboratory, adult females of Euchaeta norvegica did not feed on eggs. Predation rates on yolk-sac larvae and starved post-yolk-sac larvae did not vary significantly with age up to 14 days old because of little change in size or activity of the larvae. This differs from E. elongata Esterly, a temperate congener, which selectively feeds on middle yolk-sac-stage larvae of the Pacific hake Merluccius productus Ayres. The subarctic congener Euchaeta norvegica appeared to detect tailbeats of the cod larvae. The functional response was measured for E. norvegica feeding on 2–4-day-old yolk-sac larvae. Maximum ingestion was achieved at 5 larvae · 1⁻¹ with a rate of 6.3 ± 1.2 larvae·copepod⁻¹·day⁻¹ or 10.5% of its body weight. Estimates of short-term feeding rates, determined from gut-evacuation curves, indicate that E. norvegica, when preying on cod larvae only, must feed for at least 4 h to achieve this maximum ingestion rate. Presence of copepods as alternative prey for E. norvegica depresses its predation rate on cod, although the ingestion of cod greatly supplements the ration consumed. Copepods fed cod larvae form black melanin-pigmented fecal pellets in which larval cod otoliths have been found. Approximately 0.5 larva was required to form one fecal pellet. The last three developmental stages of the predatory copepod were able to ingest larvae and form dark-pigmented fecal pellets. The feeding of this carnivorous marine copepod may contribute to the mortality noted in the larval stages of cod because E. norvegica is numerous in the center of the cod-spawning area of Skrova in the Lofoten Islands, northern Norway.     

Planktivory by larval Odontesthes bonariensis (Atherinidae: Pisces) and its effects on zooplankton community structure

This paper describes the effect of predation by larvae of theatherinid Odontesthes bonariensis upon the zooplankton communityduring a 3-month experiment Three-day-old larvae were stockedin 45 m² concrete tanks at rates of 100 and 200 fish m^–2Gut content analyses showed that the larvae consumed relativelysmall prey all along the experiment Morphological (mouth width)and physiological (gastric inefficiency) constraints seems tohave precluded the capture of the largest prey. The prey community,however, showed all the symptoms generally ascribed to size-selectivepredation upon the largest individuals: decrease in maximumzooplankton size and mean cladoceran size, decrease in cladoceranand copepod biomass, extinction of the largest zooplankton species(Daphnia stmilis), increase in rotifer biomass, etc It is concludedthat size-selective predation is not a necessary condition forthe commonly observed decrease in zooplankton size after theincrease in density of a vertebrate predator. The ecologicalimplications of this result are discussed. Present address: Department of Biology, Lehigh University, WilliamsHall 31, Bethlehem, PA 18015-3189, USA     

Predation impact of Cyclops vicinus on the rotifer community in Lake Constance in spring

The predation impact of Cyclops vicinus on rotifers was studiedunder near-natural conditions in small enclosures to evaluatewhether copepod predation is responsible for the decline ofrotifers in Lake Constance in spring. Cyclops vicinus fed selectivelyon Synchaeta spp.; Keratella and Polyarthra spp. were not selectedfor. Predation rates increased with prey density up to a maximumof 37 Synchueta day^–1 at a density of 1.6 x 10⁶ Synchaetam^–2, i.e. at -1200 Synchaeta l^–1. Calculation ofcropping rates suggests that Cyclops alone can control the abundanceof Synchaeta in spring, i.e. that mainly Cyclops is responsiblefor the decline of Synchaeta species in Lake Constance in May.     

Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied,focusing on fish larvae and their copepod prey. The major objectiveswere to describe distributional overlap and evaluate the predatoryimpact by larval fish. The study was carried out across DoggerBank in the North Sea, August-September 1991. Sampling transectscrossed tidal fronts off the Bank and plankton at all trophiclevels showed peak abundance within frontal zones. Also Verticallythere was a significant overlap in distributional patterns ofthe plankton. Seven species of fish larvae were abundant, ofthese sprat (Sprattus sprattus) dominated. The abundance ofone group of fish larvae peaked in the shallow water close tothe Bank, whereas other species, including sprat, were foundin deeper water. Prey preference and predation pressure of fishlarvae were assessed using information on prey sizes and growthrates of larvae and the copepod prey. We estimated larval removalof preferred prey sizes to 3–4% day^–1, counterbalancedby a 3–7% day^–1' replenishment from copepod productionand growth. Additional predation pressure on copepods by aninvertebrate predator was estimated to 1–3%day^–1.In conclusion, the dynamics of fish larvae and other zooplankterswere closely linked. At peak abundances of fish larvae (>35mg dry weight m^–2), the accumulated predation on specificsize ranges of copepods, exerted by larvae and other predators,could exceed the ability of copepod replenishment and intra-/interspecificcompetition among predators might take place.     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA     

Foraging behavior of the predaceous cladoceran, Leptodora kindti, and escape responses of their prey

Using silhouette video photography we have made the first quantitativeobservations of foraging behavior in Leptodora kindti, a predaceouscladoceran (Haplopoda). Leptodora swims with a mean velocityof 13.4?4.0 mm s^–1 and initiates an attack only upon directcontact with potential prey. The attack sequence is as follows:Leptodora swims randomly through the water column with all fivepairs of thorac appendages spread to form a ‘feeding basket’and, seemingly by chance, encounters prey. Shortly after preymake contact with any part of Leptodora's body (usually ventral),the abdomen is rapidly pulled forward, clamping itself underthe feeding basket so that the telson closes it at the posteriorend. The duration of this movement is always the same and weconclude that it is an indiscriminate reflex. If the prey isencountered anywhere but a short distance directly in frontand slightly below the Leptodora, it is not captured. The speedof copepod escape responses effectively allows them to avoidcontact with the predator. Daphnia's escape response, particularlythat of juveniles, is slower and leaves them far more susceptibleto Leptodora predation.     

Predation by ciliates on a metalimnetic Cryptomonas population: feeding rates, impact and effects of vertical migration

Deep algal maxima are frequently overlayed by dense populationsof ciliates, rotifers and crustaceans. This has been interpretedas evidence of heavy predation on the algae, although the impactof this predation has never been determined experimentally.We determined the vertical and seasonal distribution of thealga Cryptomonas phaseolus and its most relevant predators,the ciliates Coleps sp. and Prorodon sp., forming metalimneticmaxima in Lake Cisó. On several dates, in situ feedingrates of ciliates were determined by three independent methods:(i) epifluorescence counts of ingested algal cells togetherwith estimates of the food turnover time of the ciliates; (ii)in situ incubations with radioactively labeled algae: (iii)HPLC determination of alloxanthine content in the predator sizefraction. Feeding rates varied between 0.07 and 0.64 Cryptomonasciliate^–1 h^–1. We then calculated integrated predationon the algae. using the functional response of the ciliatesand the vertical distribution of each population. We found thateven though the ciliates were always food saturated, their predationimpact on Cryptomonas was not very large: as an average, 5–25%of the biomass of Cryptomonas was removed daily by the ciliates.Finally, we studied the effects of the diel vertical movementsof these populations on predation impact. By migrating intothe sulfide-rich hypolimnion during the night, Cryptomonas couldreduce its predation losses by 38%. Thus, the algae were protectedfrom predation during several hours of each diel cycle and maintaineda very large biomass throughout stratification, although thisresulted in a very slow growth. Slow growth, coupled to largebiomass, seems to be a general feature of metalimnetic accumulationsof organisms.     

Impact of Cheiracanthium pelasgicum (Araneae: Miturgidae) and Chrysoperla carnea (Neuroptera: Chrysopidae) intraguild predation on the potential control of cotton pest Helicoverpa armigera (Lepidoptera: Noctuidae)

Antagonist interactions such as intraguild predation (IGP) or cannibalism among predatory arthropods can reduce the impact of these invertebrates on pest limitation in agroecosystems. Here, the effects of IGP between two major natural enemies of cotton pests, the cursorial spider Cheiracanthium pelasgicum (C.L. Koch) and the common green lacewing Chrysoperla carnea (Stephens), were studied under laboratory conditions. First, a feeding preference test was carried out to determine the degree of C. pelasgicum preference for lacewing larvae, using second-instar Helicoverpa armigera larvae as alternative prey. In a second bioassay, the effects of predator interactions on potential predation of H. armigera larvae were analysed using three treatment combinations (plus a control with no predator): (1) spider alone, (2) lacewing larvae alone, (3) spider + lacewing larvae. Potential predation by C. pelasgicum on lacewing eggs was also studied. C. pelasgicum showed no significant preference for either of the two species, indicating that this spider may impact negatively on the green lacewing population. Findings revealed no additive effects and an antagonist interaction between C. pelasgicum and green lacewing larvae, which adversely affected H. armigera suppression; both predators displayed lower predation rates when kept together than either predator alone. However, presence of lacewing larvae and subsequent unidirectional IGP did not affect the predation capacity of C. pelasgicum. Finally, predation rates of C. pelasgicum on lacewing eggs were very low (mean 2.35 ± 0.71 eggs, 24 h after offering) indicating that the impact of C. pelasgicum on lacewing populations may be limited.     

The role of pelagic water mites in the control of cladoceran population in a temperate lake of the southern Andes

In a small temperate lake of the southern Andes, Bosmina longirostrisand Ceriodaphnia dubia coexist with the predaceous water miteLimnesia patagonica. Sampling of natural populations and laboratoryexperiments were carried out. The field population of Limnesiadid not show a numerical response to the density or biomassof its prey. Laboratory experiments showed that the water miterejected C.dubia adults and juveniles as prey, but consumedBosmina. The maximum predation rate was 40 prey predator^–1day^–1 and a linear relationship between predation rateand prey density was obtained (R² = 66%). The contribution ofmortality due to predation and the predation risk are too lowto provoke a prey suppression. By means of computer modelling,the densities of predator necessary to explain half of the totalprey mortality were calculated. These densities were one ortwo orders of magnitude higher than those in the field. It isconcluded that Limnesia could really be a suppressor, but thepotential depends greatly on its density.     

Behavioral interactions between a cyclopoid copepod predator and its prey

Behavioral observations on the predatory interactions betweenMesocyclops edax and several different types and sizes of preyrevealed that prey size alone was less important than otherspecific morphological and behavioral characteristics of theprey in deterring successful predation by the copepod. The behavioralresponses of Bosmina and Asplanchna to an attacking copepodwere passive and consisted of a simple retraction of vulnerableswimming appendages which made the prey more difficult to grasp.Daphnia and Diaphanosoma on the other hand exhibited very activeswimming escape responses. Tropocyclops usually avoided M. edaxby fleeing before the larger predator could detect them. Thehard carapaces of Daphnia, Bosmina and Keratella were effectiveat reducing ingestion following capture by M. edax. The resultsof these behavioral observations were supported by enclosureexperiments in which the predator was offered a choice betweentwo prey simultaneously. Cyclopoid copepods are capable of successfullyattacking, capturing and ingesting prey organisms several timestheir own body length. Although size alone may influence thepreference of cyclopoid copepods on large and small individualsof the same or similar prey species, it is not a dependabledeterminant of the preference of cyclopoids on multispecificprey assemblages. ¹Present address: Department of Biology, Williams Hall No. 31,Lehigh University, Bethlehem, PA 18015, USA     

Evergreen foliage allows early hatching in a pine processionary moth and escape from predation

1. Seasonal variation in leaf quality and climate conditions often imposes constraints on the temporal occurrence of tree‐feeding insect larvae, but the seasonal effects of predation have received limited attention. In temperate climate zones, both the abundance and activity of predators can be expected to vary over time. 2. The study reported herein examined the impact of temporal variation in predator activity levels on the life history of an herbivorous insect feeding on a constant food source: previous‐year needles of Scots pine (Pinus sylvestris L.). In field experiments, the survival and growth rates of colonies of Thaumetopoea pinivora Treitschke larvae that had been manipulated to hatch at three different dates were compared. Eggs of T. pinivora usually hatch by mid‐April in southern Sweden, which is earlier than most other herbivorous insects that overwinter as eggs in this region. 3. Predator exclusion experiments indicated that larvae which hatched later than April experienced a higher level of predator activity, mainly by ants. The final larval size and the timing of pupation were not affected by hatching date. First instar larvae were more extensively preyed on than second instars. 4. The life history of herbivore species can be affected by seasonal variation in predation pressures. This study suggests that early hatching in a lepidopteran species can allow a temporal escape from predation during the vulnerable early life stages.     

Clutch detachment in a copepod after capture by a predator

When the egg-carrying copepod, Eudiaptomus gracilis, is capturedby larvae of predatory Chaoborus species (C.obscuripes or C.flavicans)in laboratory experiments, the external egg clutch detachesin 16-29% of caw, thereby allowing the eggs to escape from ingestion.Clutch detachment may be due to active removal by the femalebefore she is eaten or the result of accidental loss by thepredator during handling. Detached clutches were significantlylarger than thase that were ingested along with the female.Clutch detachment is advantageous to the female since ingestedeggs are lost, whereas detached eggs hatch normally and maypotentially propagate the genotype. Chaoborus predation maytherefore influence the evolution of clutch size and the degreeof iteroparity in Eudiaptomus.     

Implications of the feeding current structure of Euchaeta rimana, a carnivorous pelagic copepod, on the spatial orientation of their prey

Many marine planktonic organisms create water currents to entrainand capture food items. Rheotactic prey entrained within thesefeeding currents often exibit escape reactions. If the directionof escape is away from the feeding current, the prey may successfullydeter predation. If the escape is towards the center of thefeeding current, the prey will be re-entrained towards its predatorand remain at risk of predation. The direction of escape isdependent on (i) the ability of the prey to escape in a directiondifferent than its pre-escape orientation and (ii) the orientationcaused by the interaction of the prey's body with the movingfluid. In this study, the change in orientation of Acartia hudsonicanauplii as a result of entrainment within the feeding currentof Euchaeta rimana, a planktonic predatory copepod, was examined,When escaping in still water, A.hudsonica nauplii were ableto vary their pre-escape direction by only 10. This allowsonly a limited ability to escape in a direction different thantheir pre-escape orientation. Analyses of the feeding currentof E.rimana show the flow speed to be most rapid in the centralregion with an exponential decrease in speed distally. In contrast,flow vorticity is minimal in the center of the feeding currentand maximal at 1.75 mm along the antennae. As a result, thedegree of rotation of the prey towards the center of the feedingcurrent shows a strong dependency on the prey's location withinthe feeding current. The feeding current of E.rimana rotatedthe prey 14 when near the center of the flow field and up to160 when located more distal in the feeding current Since theprey's escape abilities cannot compensate for the rotation dueto the flow, this mechanism will maintain the escaping preywithin the feeding current of their predator. Therefore, thefeeding current facilitates predatory copepods in capturingprey by (i) increasing the amount of water which passes overtheir sensors and through their feeding appendages and (ii)controlling the spatial orientation of their prey prior to escape.     

A study of feeding in predacious ciliates using prey ciliates labeled with fluorescent microspheres

Feeding in predacious estuarine ciliates was investigated ina series of laboratory experiments using a new method of preylabeling which facilitates microscopic indentification of ingestedprey items. Ingestion rates of Mesodinium pulex, Euplotes vannusand E.woodruffi were estimated using the appearance, insidethe predator, of bacteriovorous ciliates (Metanophrys sp., Cyclidiumsp.and Pleuronema sp ) labeled with fluorescent microspheres. Preyremain motile and have presumably unaltered surface characteristics.Ingestion rates of log-growth phase predators increased withprey density. Mesodinium pulex ingested 0 15–0.32 cellsh^–1 over a prey concentration of 60–2300 ml^–1.Maximum ingestion rates of E. woodruffi and E. vannus were 4.5and 3.4 cells h^–1 respectively, estimated at prey abundancesof 75 and 172 cells ml^–1 respectively. Comparisons offeeding rates on prey of different sizes, and the effects ofstarvation, indicated that ingestion is likely limited by differentfactors in ‘raptorial’ (M pulex) and ‘filterfeeding’ (Euplotes spp.) predators.     

菜蛾盘绒茧蜂多分DNA病毒的特性及其对小菜蛾幼虫的生理效应

对菜蛾盘绒茧蜂Cotesia plutellae多分DNA病毒的特性及其对寄主小菜蛾Plutella xylostella幼虫的生理效应进行了研究。结果表明：菜蛾盘绒茧蜂雌蜂输卵管萼中含有大量的多分DNA病毒（polydnavirus, PDV）;一个PDV内含多个核衣壳,最多可达16个;核衣壳长40～168 nm,直径39～40 nm;PDV仅在输卵管萼细胞内复制;雌蜂产卵时,随蜂卵将PDV注入寄主血腔,并扩散到寄主的许多组织中;PDV可能先通过脱膜再侵染寄主组织。雌蜂经Co⁶⁰辐射处理后再寄生（即假寄生）小菜蛾2龄、3龄和4龄初期的幼虫,被寄生后的寄主幼虫几乎全部不能化蛹,但末龄（即4龄）幼虫期显著延长,并在寄生后期,幼虫胸部有褐色的短翅芽出现;即将化蛹的4龄末小菜蛾幼虫被假寄生后,即使每头寄主被过寄生9次,依然能正常化蛹,但不能羽化。假寄生与正常寄生后寄主的脂肪体数量和形态结构有明显的不同,推测在正常寄生的情况下蜂卵孵化时释放的畸形细胞及随后的幼蜂可能对脂肪体的结构产生了作用。     

槲栎种子雨进程中昆虫的捕食特征

为了了解昆虫捕食与槲栎Quercus aliena种子大小和产量的关系以及在栎林更新中的作用,于2008和2009年秋季,分别在暖温带伏牛山系天池山国家森林公园内研究了两个年度槲栎的种子雨过程及昆虫捕食特征。结果表明：（1）种子雨过程从8月中下旬到9月底约经历40 d,存在高峰期且时段较明显,高峰期下落种子量分别占全部种子雨量的78.13%（2008年）和75.91%（2009年）;（2）槲栎的种子雨强度年间存在较大差异,2008年明显小于2009年（两年分别为31.75±16.65粒/m²和51.92±29.26粒/m²）,但2009年的橡子明显比2008年小（两年分别为1.94±0.61 cm³和2.46±0.57 cm³）;（3）种子雨构成比例在两个年份间存在差异,完好种子的量分别为59.05%（2008年）和36.12%（2009年）,虫蛀率在大量结实的2009年显著提高;（4）虫蛀橡子（2.29±0.42 cm³）显著大于完整橡子（1.59±0.32 cm³）,且虫蛀种子中所含虫卵数与种子大小显著正相关,昆虫有选择大种子产卵寄生的偏好。本研究的结果说明,昆虫对槲栎大种子有寄生选择偏好;大部分槲栎种子遭遇象甲虫蛀而降低生命活力,这可能是影响槲栎林更新的主要因素之一。     

Distribution and abundance of European hake Merluccius merluccius (L.), eggs and larvae in the North East Atlantic waters in 1995 and 1998 in relation to hydrographic conditions

Plankton samples from surveys in 1995 and 1998 were analysedin order to study the spatial distribution of hake (Merlucciusmerluccius) eggs and larvae. The cruises covered an extensivearea from the south of the Bay of Biscay to the north-west ofIreland. This spatial distribution has been interpreted in relationto the direction and intensity of the dominant winds. An importantdecrease of eggs (69%) and larvae (27%) was observed from 1995to 1998. Hake showed continuous spawning from February to Julyin the area. The centroids of hake egg distribution indicateda northward displacement of the peak of spawning as the seasonprogresses. Spawning of hake was located close to the 200 mdepth isobath in the southern area (Bay of Biscay) and it spreadover the shelf in the area of the Celtic Sea. Eggs were mostabundant at 20 m depth temperatures between 12 and 12.5°C.The centroids of hake larvae distribution also showed a northwarddisplacement although not as strong as that of hake eggs. Thespatial distribution of hake larvae by size ranges showed importantdifferences between years: in 1995 it suggested inshore transportof larvae, from the spawning areas (close to the 200 m depthisobath) to nursery areas (closer to the coast) while in 1998it suggested offshore transport. This transport does not correspondwith the general wind regimes during those years. Thereforeit is concluded that the transport of the hake larvae to nurseryareas must be controlled by different hydrographic mechanismssuch as geostrophic currents in the Bay of Biscay and tidalcurrents in the northern areas.     

The relationship between predator density, community composition, and field predation of Colorado potato beetle eggs

Conservation biological control programs seek to increase natural enemy densities through the adoption of more benign farming practices, under the assumption that higher predator densities will lead to more effective pest suppression. However, predator–predator interference may lead to diminishing returns in improved pest control as predator densities increase. We examined the relationship between predator density and predation rates on Colorado potato beetle eggs in production potato fields. These potato fields naturally spanned a 10-fold range in predator density, due to differences in management practices. Periodically through the growing season we simultaneously measured predator densities and subjected sentinel eggs masses to predation, allowing us to correlate predator density and egg predation for each field on each sample date. Egg predation rates were significantly positively correlated with total predator densities, a correlation that was not improved when predator densities were scaled to reflect differences in feeding rates on potato beetle eggs of the constituent predator taxa. There was no correlation between per-capita egg predation rates and predator density, and so no evidence that predator interference increased with increasing predator density. We divided predators into six dominant taxa—dwarf spiders, crab spiders, minute pirate bugs, big-eyed bugs, damsel bugs, and Lygus bugs (together constituting 93% of all predators collected), and a seventh group, “other predators” that included all other, less common, taxa—and examined correlations between all predator combinations and egg predation rates. The highest correlation was between combined densities of the six most common predator taxa, excluding only the “other predators” grouping. This suggests that predators may be largely equivalent in their impact on Colorado potato beetle eggs, and that field scouts might be able to ignore uncommon predator taxa when sampling for natural enemies.