Life tables of the processionary caterpillar Ochrogaster lunifer Herrich-Schäffer (Lepidoptera: Thaumetopoeidae) at local and regional scales

A major ongoing debate in population ecology has surrounded the causative factors underlying the abundance of phytophagous insects and whether or not these factors limit or regulate herbivore populations. However, it is often difficult to identify mortality agents in census data, and their distribution and relative importance across large spatial scales are rarely understood. Here, we present life tables for egg batches and larval cohorts of the processionary caterpillar Ochrogaster lunifer Herrich-Schäffer, using intensive local sampling combined with extensive regional monitoring to ascertain the relative importance of different mortality factors at different localities. Extinction of entire cohorts (representing the entire reproductive output of one female) at natural localities was high, with 82% of the initial 492 cohorts going extinct. Mortality was highest in the egg and early instar stages due to predation from dermestid beetles, and while different mortality factors (e.g. hatching failure, egg parasitism and failure to establish on the host) were present at many localities, dermestid predation, either directly observed or inferred from indirect evidence, was the dominant mortality factor at 89% of localities surveyed. Predation was significantly higher in plantations than in natural habitats. The second most important mortality factor was resource depletion, with 14 cohorts defoliating their hosts. Egg and larval parasitism were not major mortality agents. A combination of predation and resource depletion consistently accounted for the majority of mortality across localities, suggesting that both factors are important in limiting population abundance. This evidence shows that O. lunifer is not regulated by natural enemies alone, but that resource patches ( Acacia trees) ultimately, and frequently, act together to limit population growth.     

The effects of food and space on the occurrence of cannibalism and predation among larvae of Anopheles gambiae s.l.

Competitive interactions among the aquatic stages of the malaria mosquito Anopheles gambiae s.l. (Diptera: Culicidae) may affect the resulting adult densities and, hence, the risk of malaria. We investigated the impact of the presence of a fourth‐instar larva (An. gambiae Giles s.s. or An. arabiensis Patton), the quantity of food, and the available space on the survival and development of freshly hatched larvae of An. gambiae s.s. and An. arabiensis. To analyse the results, two proportional hazard models were constructed. The first estimated the effects of all covariates on mortality rate and the second estimated the effects of the covariates on development rate into the third larval instar (L3). A time‐dependent covariate for density, which changed during the experiment as a result of death or development to L3, was included in both models. In the presence of a fourth‐instar larva (L4), survival of the experimental larvae was significantly reduced, but no difference was detected between the presence of L4 An. gambiae and L4 An. arabiensis. The observation that the majority of dead larvae were not recovered in trays with an L4 present suggested that cannibalism and predation occurred readily. Limitation in space significantly increased mortality of larvae, whereas a limitation of food reduced larval development rate, but did not cause mortality per se. From this, we concluded that both cannibalism and predation were enhanced as a result of more frequent interactions within smaller environments, but did not occur for reasons of food shortage. This study shows that inter‐ and intraspecific interactions among larvae of the An. gambiae complex strongly affect survival and development, and that the quantity of food and the available space are important determinants of the outcome of these interactions. Implications of the results are discussed with respect to the population dynamics of both malaria vectors in the field.     

Sublethal parameters in morphologically deformed Chironomus larvae: clues to understanding their bioindicator value

1. Parameters of condition between normal and deformed fourth instar larvae of Chironomus gr. thummi (Kieffer 1911) were compared in four populations: one reference (PE) and two metal-polluted sites (NP and SCH) in the River Dommel, and one site polluted by domestic sewage and copper in the River Ijse (NEI).
2. The site PE ranked lowest for metal body burdens, deformities, mortalities and emergence duration, while SCH and NEI ranked highest.
3. Deformed and non-deformed larvae most often did not differ in length and weight ( in situ end-points for growth); when differences occurred, deformed larvae tended to be smaller.
4. The energy content and dry weights in one population (NEI) were lower in normal larvae than in the weakly deformed ones. The percentage of ash-free dry weight was lower in deformed larvae of the polluted Dommel sites, compared to the normal ones.
5. The in vitro emergence rate (end-point for development of fourth instars) for the reference population PE, both in its own sediment and in artificial cellulose substrate, was better than for the other sites. In two populations (NEI, SCH) the development of deformed larvae in their own sediment was slower, with higher mortality, than for the normal larvae. In one population (NP), normal and deformed larvae survived and developed equally well. The emergence rates of the respective populations were similarly ranked when the larvae were raised in an artificial cellulose substrate.
6. Elution peaks of alleged metal-binding proteins were lower in deformed larvae from SCH and NEI, but higher in deformed larvae from NP, than in normal larvae.
7. A different development rate and mortality of deformed larvae in non-adapted populations and the possibility of metal adaptation, as in site NP, may modulate the final outcome of deformity frequencies, thus having an impact on the biomarker value of deformities in benthic midge larvae.     

Vulnerability of larvae of two species of aphidophagous ladybirds, Adalia bipunctata Linnaeus and Harmonia axyridis Pallas, to cannibalism and intraguild predation

Vulnerability of larvae of two species of aphidophagous ladybirds, Adalia bipunctata Linnaeus and Harmonia axyridis Pallas, to cannibalism and intraguild predation was assessed in the laboratory. In the first experiment, a first instar of one of the two above species was kept with a fourth instar of the other species in a Petri dish. The number of times each first instar larva was encountered by the fourth instar larva and the fate of the first instar was determined over a period of 10 min. The fourth instar larvae captured and killed all the first instar larvae of their own species at the first encounter. However, when presented with fourth instar larvae of the other species the first instar larvae of A. bipunctata and H. axyridis were encountered 6.4 ± 1.3 ( n  = 10) and 19.4 ± 2.1 ( n  = 10), respectively. In this experiment no first instar larvae of H. axyridis , whereas all those of A. bipunctata , were killed.     

Quantifying the Impact of Woodpecker Predation on Population Dynamics of the Emerald Ash Borer (Agrilus planipennis)

The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle that has killed millions of ash trees (Fraxinus spp.) since it was accidentally introduced to North America in the 1990s. Understanding how predators such as woodpeckers (Picidae) affect the population dynamics of EAB should enable us to more effectively manage the spread of this beetle, and toward this end we combined two experimental approaches to elucidate the relative importance of woodpecker predation on EAB populations. First, we examined wild populations of EAB in ash trees in New York, with each tree having a section screened to exclude woodpeckers. Second, we established experimental cohorts of EAB in ash trees in Maryland, and the cohorts on half of these trees were caged to exclude woodpeckers. The following spring these trees were debarked and the fates of the EAB larvae were determined. We found that trees from which woodpeckers were excluded consistently had significantly lower levels of predation, and that woodpecker predation comprised a greater source of mortality at sites with a more established wild infestation of EAB. Additionally, there was a considerable difference between New York and Maryland in the effect that woodpecker predation had on EAB population growth, suggesting that predation alone may not be a substantial factor in controlling EAB. In our experimental cohorts we also observed that trees from which woodpeckers were excluded had a significantly higher level of parasitism. The lower level of parasitism on EAB larvae found when exposed to woodpeckers has implications for EAB biological control, suggesting that it might be prudent to exclude woodpeckers from trees when attempting to establish parasitoid populations. Future studies may include utilizing EAB larval cohorts with a range of densities to explore the functional response of woodpeckers.     

Life history traits of Aedes caspius (Diptera: Culicidae): a laboratory study of larval stages

The larval survival and development times of Aedes caspius (Diptera: Culicidae) were examined in the laboratory. These life history traits were estimated using life tables constructed for two populations, one of which had been subjected to a long-term larvicide control program. Traits were evaluated for eight different population densities. The effects of population, larval stage and larval density were investigated using a general linear model. Density was positively correlated with larval survival but did not affect development time. The fourth instar and pupae had the lowest larval survival rates. First and fourth instar larvae had the longest development times. These traits were not significantly different between the two populations. The effect of larvicide control on these traits is discussed.     

Seasonal variation in natural mortality factors of Tuta absoluta (Lepidoptera: Gelechiidae) in open‐field tomato cultivation

The seasonal variation in natural mortality of phytophagous insects is determined by the relative importance of biotic and abiotic factors in agroecosystems. Knowledge regarding these factors throughout the year represents a key concern for IPM programmes. Seasonal population fluctuations of tomato pinworm, Tuta absoluta, led to an investigation of its natural mortality factors during the rainy season when the population level is low and during the dry season when population peaks occur. The aim of this study was to verify the seasonal variation in T. absoluta mortality factors in tomato crops. Immature stages of T. absoluta were obtained from laboratory‐rearing in the laboratory. These were taken to the field and monitored over two years. The mortality causes for each stage of insect development from egg to adult were assessed daily. Multiple biotic and abiotic mortality factors affected the immature T. absoluta stages such as rainfall, physiological disturbances, diseases, parasitoids and predators. The key T. absoluta mortality factor during summer–spring was predation. In addition, larvae predation correlated positively with temperature, wind velocity, photoperiod and rainfall. Nevertheless, during winter–fall, the key mortality factor was parasitism. Therefore, the critical stage for mortality was 3rd‐ and 4th‐instar larvae, being more vulnerable to natural control factors. Finally, the results showed the importance of vertical and horizontal action on natural mortality factors.     

Sex pheromone of the larch caterpillar moth, Dendrolimus superans, from northeastern China

A study was set up to determine the sources and rates of mortality of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) on field‐grown cassava in Uganda. Using a cohort‐based approach, daily direct observations were used to construct partial life tables for 12 generations of egg and nymph populations which were studied over a 1‐year period. Mortality was categorized as dislodgement, predation, parasitism (for nymphs only), unknown death, and inviability (for eggs only). The highest mean rate of marginal mortality across all stages was attributed to parasitism, with dislodgement and predation following, respectively. Across all factors, the highest mean rate of marginal mortality was observed in the fourth instar followed by the eggs, first‐, second‐, and third‐instars, respectively. Key factor analysis revealed that dislodgement was the major mortality factor contributing to generational mortality in eggs while for nymphs, parasitism in the fourth instar was the main driving force behind the observed generational mortality. Highest irreplaceable mortality in both the egg and nymph stages was attributed to dislodgement followed by parasitism and predation, and least was due to unknown death. Across stages, highest irreplaceable mortality rates were observed in the eggs and the fourth‐instar nymphs. The other stages had relatively low rates of irreplaceable mortality. Rain‐protection experiments revealed no significant differences in marginal mortality rates when compared to the open field situation.     

Effects of prey density, prey instar, and patch size on the development of the predatory mosquito, Toxorhynchites towadensis

Effects of prey density, prey instar, and patch size on the development of the predatory mosquito larva, Toxorhynchites towadensis, were investigated in the laboratory. Survivors of T. towadensis showed different developmental patterns in relation to prey age structure. All predatory larvae in containers with only second instar prey developed into the third instar. However, in several containers with fourth instar prey, mortality of predators was observed. During the third instar, no predatory larva died, but both prey density and prey instar significantly affected the survival of predators during their fourth instar. Large prey size promoted large predator adults, and predatory larvae which grew up in small surface containers responded by developing to large sizes than those in large containers. Larval developmental time of the predators differed in each treatment. During first and second instars, faster predator development was observed in containers with small surface areas and containing young prey individuals. However, when development was enhanced by the presence of old prey individuals, no surface effect was observed. The fastest predator development was observed with prey of mixed instars and high density. This study suggests that a small surface container containing prey of mixed instars and high density is suitable for development of predators.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

A Sensitive Bioassay for the Detection of Cry1A Toxin Expression in Transgenic Cotton

In the process of development of insect resistant transgenic plants and also to evaluate the consistency in expression of the toxin under greenhouse and field conditions, immunological and bioassays are commonly used. The assay being described in this report, is based on the high levels of sensitivity of a cotton leaf feeding insect, the semilooper, Anomis flava (Fabricius) to Cry toxins (Cry1Aa, Cry1Ab and Cry1Ac). The assay is sensitive, quick and reproducible. Cry1Ac was the most toxic followed by Cry1Ab and Cry1Aa. LC 50 s of the three toxins on first instar larvae ranged from 0.79-6.08 ng cm -2 of leaf. LC 50 s of Cry1Ac for the fourth instar larvae ranged from 12.91-21.14 ng cm -2 while LC 50 s for Cry1Aa and Cry1Ab were in the range 53.0-138 ng cm -2 . The fiducial limits (at 95% probability) of the probit assay data indicated that there was no difference in response between the three different populations to each of the three toxins. The data from all assays were pooled for each of the three toxins separately and subjected to regression analysis to obtain a cumulative log dose response for first and fourth instar larvae. These can be used as standard curves to quantify toxin expression in plants based on mortality response of either first or fourth instar A. flava larvae. Apart from being used to detect expression in putative Bt cotton transgenic plants, the assay can also be used to follow the activity of Cry toxins in transgenic cotton plants in the field during the growing season.     

Snail predation by larvae of Sepedon scapularis Adams (Diptera: Sciomyzidae), a potential biocontrol agent of snail intermediate hosts of schistosomiasis in South Africa

Malacophagous larvae of the fly Sepedon scapularis Adams were shown experimentally to be effective predators of three species of aquatic pulmonate snails tested as prey: Bulinus africanus (Krauss) an important intermediate host of Schistosoma haematobium (Bilharz), Bulinus tropicus (Krauss) and the invasive species Physa acuta Draparnaud. Survival of S. scapularis larvae from instar to instar was negatively affected by the size of prey snails, since larvae tended to be asphyxiated by the mucous secretions of the snails, or by the larval hydrofuge hairs becoming entangled in snail faeces. In experiments to test the choice of S.scapularis larvae for different species and sizes of snails, B.africanus was significantly disfavoured compared with the other two snail species. Small snails (< 3 mm) were more frequently killed by all three larval instars of S.scapularis. First instars killed few, if any, large snails (> 7 mm), whereas second and third instars preyed effectively on all sizes of snails. Third instars killed significantly more snails than younger instars. For larvae offered only one species of snail as prey, the mean total number of snails killed per larva during its entire development was 49 B.africanus, 45 B.tropicus or 34 P.acuta. It is concluded that S.scapularis is potentially useful as a biological control agent for use against indigenous Bulinus and exotic Physa snails.     

Larval interactions in aphidophagous predators: effectiveness of wax cover as defence shield of Scymnus larvae against predation from syrphids

Small-sized predators in the aphidophagous guild of Aphis gossypii Glover colonies on hibiscus trees in Japan exploit aphids at low prey abundance. Scymnus (Pullus) posticalis Sicard beetles were the first predatory species to attack aphids in the spring, and their larvae co-occurred with larvae of Eupeodes freguens (Matsumura) syrphids in aphid-infested leaves of hibiscus for 3 weeks in absence of large-sized coccinellid predators. Larval interaction between Scymnus and syrphid predators was examined in relation to effectiveness of wax cover of Scymnus against predation from syrphids. Waxless first instar larvae were not protected but wax-covered larvae of second, third and fourth instars were protected from predation by syrphid larvae. The protection was lower in the second instar which has a thin wax cover and significantly higher in the third and fourth instars having a thick wax cover. In addition, larvae from which the wax was removed were significantly more vulnerable to predation. Vulnerability of Scymnus larvae to predation from syrphids was directly related to the thickness of wax cover. Results suggest that the wax cover of Scymnus larvae act as an effective defence mechanism against predation from syrphid larvae.     

Disarmed by domestication? Induced responses to browsing in wild and cultivated olive

Populations of plants and animals are almost always made up of individuals of different sizes. In populations where cannibalism is common, this size variation can influence rates of mortality and growth and affect population regulation. Size variation can be caused by a variety of mechanisms. One of these is due to size-specific responses to the threat of predation by potentially cannibalistic conspecifics. We investigated the role of anti-predator behavior in size structure development within single-aged cohorts of Arizona tiger salamander larvae. In a laboratory experiment, we show that size variation increases over time within groups of salamanders, even if they are fed in isolation. We also show that increasing the size of neighbors decreases the feeding rate of small salamander larvae. However, increasing density of neighbors did not have a significant effect on feeding rate. These results are consistent with the hypothesis that size variation among tiger salamander larvae is, in part, a result of size-specific responses to predation risk. We discuss the potential for feedback between size structure development, predation risk, and rates of cannibalism. Received: 5 October 1999 / Accepted: 8 February 2000     

Effect of cannibalism and predation on the larval performance of two ladybird beetles

The relative impact of cannibalism and predation on the development and survival of fourth instar larvae was assessed in two species of aphidophagous ladybird beetles, Coccinella septempunctata and Harmonia axyridis. The effect of eating aphids, conspecific larvae or heterospecific larvae on larval performance differed in the two species: aphids were the best food for C. septempunctata and survival of C. septempunctata larvae was significantly lower when offered heterospecific larvae rather than conspecific larvae or aphids as food, indicating that H. axyridis larvae were not suitable food for C. septempunctata. However, as the different foods did not affect the larval performance of H. axyridis, this species appears to be more polyphagous. Both intraguild predation by the aggressive larvae of H. axyridis and the polyphagous food habit of this species may account for its dominance in ladybird assemblages and its displacing other ladybird beetles in several places in the world.     

Larval growth and development in the caddisfly Cheumatopsyche brevilineata under natural thermal regimes

Cheumatopsyche brevilineata (Iwata) is a filter‐feeding caddisfly without distinct or distinguishable cohorts. In a semi‐natural channel, we reared fourth and fifth instar larvae of C. brevilineata in individual cages with hourly recording of water temperature. We calculated the individual growth rate from the wet‐weight gain of each larva, and the development rate from the ratio of larvae that progressed to the next instar or pupal stage during each rearing experiment. We analyzed the linear regressions of growth (increase in size) and development (physiological and morphological progression toward maturity) rates against the statistical parameters of water temperatures during each rearing period, i.e. mean and given percentiles of water temperatures. We presumed that the most appropriate parameter of water temperature to explain larval growth and development would show a peak value of the determination coefficients (r²) in the linear regressions. There were highly significant regressions in the growth rates for fourth and fifth instar larvae and in development rates for fourth instar larvae against every statistical parameter of water temperature, but not in the development rates for fifth instar larvae. For the growth of fourth and fifth instar larvae, we could not specify the most appropriate parameters of water temperatures, because we observed no clear peaks in the determination coefficients. For the development of fourth instar larvae, this parameter could be the 65th percentile value, where the development zero temperature and effective degree‐days were 11.1°C and 56 degree‐days, respectively.     

A laboratory investigation of mosquito larval predation by Toxorhynchites moctezuma on Aedes aegypti

1. Functional responses of predatory Toxorhynchites moctezuma (Dyar & Knab) larvae feeding on Aedes aegypti (L.) larvae (Diptera: Culicidae) were found to be type II of Holling (1959) and Rogers (1972). 2. Estimates of searching rate were generally higher for later instar predators. The search rate of second instar predators declined as prey instar increased, but fourth instar Tx. moctezuma had the highest search rate for second instar Ae. aegypti. 3. Prey handling times were higher for early instar predators and late instar prey. 4. When presented with mixtures of two instars of Ae. aegypti, second instar Tx. moctezuma showed frequency independent selectivity for the early instars, whereas fourth instar predators showed frequency independent selectivity for the late instars of Ae. aegypti. There was no evidence of frequency dependent predation. Preferences appeared to be transitive. 5. Extended random predator equations, using parameters derived from the functional responses, did not adequately describe the outcome of predation in the prey mixture experiment, even when the possibility of optimal switching behaviour was accounted for.     

Greenhouse Evaluation of Dose– and Time–Mortality Relationships of Two Nucleopolyhedroviruses for the Control of Beet Armyworm, Spodoptera exigua, on Chrysanthemum

Dose– and time–mortality relationships of baculoviruses in pest insects are important for the determination of effective spraying regimes. A series of experiments with Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Spodoptera exigua MNPV (SeMNPV) against synchronized populations of S. exigua larvae in greenhouse chrysanthemum was conducted. Dose– and time–mortality relationships of different virus concentrations and S. exigua target stages were determined and the area foliage consumption was measured. Crop injury was greatly reduced when S. exigua were controlled as second or third instar larvae, whereas virus applications against fourth instar larvae could not prevent considerable crop injury, even at high concentrations. SeMNPV was approximately 10 times as infectious as AcMNPV when applied on greenhouse chrysanthemum. The relative virulence of AcMNPV and SeMNPV corresponded reasonably well with previously published laboratory bioassay data. SeMNPV killed second and fourth instar S. exigua larvae approximately 12 h faster than did AcMNPV in chrysanthemum, but no difference in speed of action was found for third instar larvae. The relative speed of action of AcMNPV and SeMNPV determined in chrysanthemum and in laboratory bioassays did not correspond for third instar S. exigua larvae; laboratory bioassay data can therefore not simply be extrapolated to the crop level.     

Effects of food nutritive quality and Bacillus thuringiensis onfeeding behaviour, food utilization and larval growth of spruce budworm Choristoneura fumiferana (Clem.) when exposed as fourth- and sixth-instar larvae

Abstract 1 Feeding behaviours, and lethal and sublethal (growth, development and food utilization) effects of Foray 48B, a commercial formulation of Bacillus thuringiensis (kurstaki), were investigated on fourth‐ and sixth‐instar spruce budworm larvae according to food nutritive quality. Nitrogen and soluble sugar content of artificial diets were modified to obtain three different qualities of food, simulating variations in nutritive quality of host tree. 2 Larval development times were longer for Bt‐treated larvae and pupal weights were reduced for sixth‐instar larvae only. Bt‐induced mortality levels were influenced by food quality. Ingested dose of Bt and feeding inhibition times were strongly affected by the Bt treatment, but food quality affected only fourth‐instar larvae. Except for food digestibility, nutritional indices were negatively affected by the Bt treatment and by the reduction in food quality. 3 Contrary to early treated larvae (fourth instar), larvae treated at the beginning of the sixth instar were not able to compensate for Bt injury and were consequently more affected by the Bt‐treatment both in terms of lethal and sublethal effects. 4 Bt efficacy was not directly related to the ingested dose. 5 Increase in larval vulnerability to Bt was more likely a consequence of a general stress induced by a less suitable food than a direct interaction between Bt and food nitrogen or sugar compounds. 6 The application of Bt on late‐instar larvae could be a successful operational strategy at low population levels when field sprays target the insect instead of foliage protection.     

Effects of temperature on the development of overwintering immature stages of the near‐threatened butterfly Leptalina unicolor (Bremer & Grey) (Lepidoptera: Hesperiidae)

Overwintering larvae of multivoltine and univoltine populations of Leptalina unicolor were reared under various constant and fluctuating temperatures superimposed on a photoperiod of either 12 h of light and 12 h of darkness (LD 12:12) or LD 15:9. Diapause of the larvae terminated in midwinter (by early February). All the larvae of both populations pupated after two molts without feeding and the head capsule width of the final instar larvae was smaller than that of the penultimate instar ones. The photoperiod did not significantly affect larval development, but long‐day conditions (LD 15:9) hastened pupal development. The thermoperiod had a significant effect on the development of the multivoltine population. When multivoltine population larvae were kept under a low fluctuating temperature regime (cryophase/thermophase = 14/20°C), the period until adult eclosion was shorter than that under a constant temperature of 17°C. On the contrary, when larvae were kept under a high fluctuating temperature regime (24/30°C), the period until adult eclosion was longer than that under a constant temperature of 27°C. However, the univoltine population did not show such a reaction to the fluctuating temperature. The durations of final instar larva and pupa of the multivoltine population were shorter than those of the univoltine population. The developmental zeros of penultimate and final instar larvae and pupae of the univoltine population were lower than those of the multivoltine population. The head capsule width of penultimate instar larvae and the forewing length of adults of the univoltine population were larger than those of the multivoltine population for both sexes.