Effect of entomopathogenic fungus, Beauveria bassiana on larvae of three species of mosquitoes

Beauveria bassiana, an entomopathogenic fungus, was evaluated for its potential against second and third instar larvae of Culex quinquefasciatus, Anopheles stephensi and Aedes aegypti. Conidiospores of this fungus were effective in causing infection leading to mortality of different larval instars. Larvae of Cx. quinquefasciatus were more susceptible to infection than An. stephensi and the second instar larvae of these two species were more susceptible than third instar larvae. Larvae of Ae. aegypti were resistant to infection by B. bassiana.     

Shifts in developmental diet breadth of Lymantria xylina (Lepidoptera: Lymantriidae)

The moth Lymantria xylina Swinhoe (1903) (Lepidoptera: Lymantriidae) is a major defoliator of hardwood and fruit trees in Taiwan. Although the plants identified as host plants of L. xylina usually refer to plants used as food or as shelter, most of the host plant identifications have not considered the role of these hosts on larval development. This study investigated various instars feeding on different plants to assess the developmental diet breath of L. xylina. Forty-seven plant species, belonging to 25 families were used in feeding trials. Various bioassays, including first instar survival and long-term feeding trials, indicated the most suitable host plants for the different developmental stages. Results of the first instar survival trial indicated that first instars could survive only on 13 of the tested plant species. In addition, first instars could only successfully grow to pupa on seven of these 13 test plants species. To assess the developmental diet breath shifts of this moth, 38 plant species (excluding those nine plant species that the first instars did not feed upon) were fed to third and fifth instars in long-term feeding trials. Survival to pupa was noted on 12 and 13 test plant species for the third and fifth instars, respectively. In short, we found that the larvae performed differently when fed on various host plants and that the host plant range increased with the larval stage. Therefore, it is necessary to adjust the host plant range of this moth and to consider host plant breadth together with the developmental stages of caterpillars.     

Classification of immature mosquito species according to characteristics of the larval habitat in the subtropical province of Chaco, Argentina

To classify mosquito species based on common features of their habitats, samples were obtained fortnightly between June 2001-October 2003 in the subtropical province of Chaco, Argentina. Data on the type of larval habitat, nature of the habitat (artificial or natural), size, depth, location related to sunlight, distance to the neighbouring houses, type of substrate, organic material, vegetation and algae type and their presence were collected. Data on the permanence, temperature, pH, turbidity, colour, odour and movement of the larval habitat's water were also collected. From the cluster analysis, three groups of species associated by their degree of habitat similarity were obtained and are listed below. Group 1 consisted of Aedes aegypti. Group 2 consisted of Culex imitator, Culex davisi, Wyeomyia muehlensi and Toxorhynchites haemorrhoidalis separatus. Within group 3, two subgroups are distinguished: A (Psorophora ferox, Psorophora cyanescens, Psorophora varinervis, Psorophora confinnis, Psorophora cingulata, Ochlerotatus hastatus-oligopistus, Ochlerotatus serratus, Ochlerotatus scapularis, Culex intrincatus, Culex quinquefasciatus, Culex pilosus, Ochlerotatus albifasciatus, Culex bidens) and B (Culex maxi, Culex eduardoi, Culex chidesteri, Uranotaenia lowii, Uranotaenia pulcherrima, Anopheles neomaculipalpus, Anopheles triannulatus, Anopheles albitarsis, Uranotaenia apicalis, Mansonia humeralis and Aedeomyia squamipennis). Principal component analysis indicates that the size of the larval habitats and the presence of aquatic vegetation are the main characteristics that explain the variation among different species. In contrast, water permanence is second in importance. Water temperature, pH and the type of larval habitat are less important in explaining the clustering of species.     

Achieving high sexual size dimorphism in insects: females add instars

Abstract.  1. In arthropods, the evolution of sexual size dimorphism (SSD) may be constrained by a physiological limit on growth within each particular larval instar. A high SSD could, however, be attained if the larvae of the larger sex pass through a higher number of larval instars.
2. Based on a survey of published case studies, the present review shows that sex-related difference in the number of instars is a widespread phenomenon among insects. In the great majority of species with a sexually dimorphic instar number, females develop through a higher number of instars than males.
3. Female-biased sexual dimorphism in final sizes in species with sexually dimorphic instar number was found to considerably exceed a previously estimated median value of SSD for insects in general. This suggests a causal connection between high female-biased SSD, and additional instars in females. Adding an extra instar to larval development allows an insect to increase its adult size at the expense of prolonged larval development.
4. As in the case of additional instars, SSD is fully formed late in ontogeny, larval growth schedules and imaginal sizes can be optimised independently. No conflict between selective pressures operating in juvenile and adult stages is therefore expected.
5. In most species considered, the number of instars also varied within the sexes. Phenotypic plasticity in instar number may thus be a precondition for a sexual difference in instar number to evolve.     

Dosage-mortality and time-mortality studies of a granulosis virus in a laboratory strain of the codling moth,Laspeyresia pomonella

Different doses of a granulosis virus were administered to first- and fifth-instar larvae of the codling moth Laspeyresia pomonella. Virus was very pathogenic for both larval instars. The LD₅₀ values for first- and fifth-instar larvae were 5 and 49 capsules/larva, respectively. However, fifth-instar larvae were much more variable in their response to virus than first-instar larvae. Using probit methods it was calculated that 1 capsule could cause death in about 25% of both larval instars but 1578 capsules were required to cause 70% mortality of fifth-instar larvae as compared to 12 capsules for first-instar larvae. This is the first report of a decided difference in variability of response to virus by two larval instars of the same species. A bimodal response by both larval instars was observed in time-mortality studies. Apparently, about 20% of the larvae were very resistant to virus infections.     

Larval parasitoids and pathogens of the fall armyworm in honduran maize

Parasitism by natural enemies of all larval instars of the fall armyworm (FAW),Spodoptera frugiperda, was quantified in 5 Honduran maize fields during the 1984 growing season. Overall, 42.0% of the host larvae were killed by the natural enemy complex. The braconid parasitoidChelonus insularis was the most common natural enemy accounting for 36.8% of the complex and causing 15.5% mortality of FAW larvae. Other important natural enemies included the nematodeHexamermis sp., the tachinidLespesia sp. and the imperfect entomopathogenic fungusNomuraea rileyi. Distinct ranges of larval instars were attacked by several natural enemy species.Chelonus insularis was an important FAW natural enemy at all larval densities observed. Florida Agricultural Experiment Station Journal Series No. 8650 and MIPH-EAP No. 146.     

Intraspecific variability in number of larval instars in insects

The number of larval instars varies widely across insect species. Although instar number is frequently considered to be invariable within species, intraspecific variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies, we show that intraspecific variability in the number of larval instars is widespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, food quality and quantity, humidity, rearing density, physical condition, inheritance, and sex are the most common factors influencing the number of instars. We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according to which instar number increases in adverse conditions when larvae fail to reach a species-specific threshold size for metamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more specific cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional generations in multivoltine species, or increasing the probability of surviving in long-lasting adverse conditions.     

Larval growth and biosynthesis of reserves in mosquitoes

Developmental instars of four species of mosquitoes have been analyzed for growth and synthesis of biomass with respect to their caloric content of protein, lipids, and carbohydrates for each instar of Aedes aegypti and Culex pipiens of the subfamily Culicinae, and Anopheles albimanus, and An. gambiae of the subfamily Anophelinae. The diameter of the thorax grows during the intermolt, reflecting continuous increase in biomass because it correlates significantly with the larval synthesis of total protein, lipids, and carbohydrates. For Ae. aegypti the fourth instar was sexed to disclose the sex-specific synthetic potential. In Ae. aegypti the protein increased in linear proportion with larval body size, whereas lipid synthesis followed a significant, exponential regression, which was clearly steeper in male larvae and most pronounced in the last instar. When normalized for size, the size-specific protein and lipid contents showed minimal levels of 0.25 and 0.1, respectively, regardless of standard or crowded rearing conditions. The rate of lipid synthesis in Ae. aegypti was determined by incubating fourth instar larvae with (14)C-acetate and estimating the lipids. The rate was highest in the early larvae and decreased towards the end shortly before pupation; in male larvae incorporation was twice the rate of female larvae. Cx. pipiens reached the largest body sizes of all species tested, with protein and lipids increasing linearly with size. Their minimal levels of size-specific caloric contents were around 0.35 for protein and 0.25 for lipids. Anopheles also showed a linear relationship between larval size and caloric protein and lipid contents. Their minimal threshold levels in size-specific contents were 0.35 for protein and 0.2 for total lipids, similar to Culex, but slightly higher than in Aedes. Starvation of Ae. aegypti larvae and subsequent feeding partially improved their lipid contents, but never to the levels of non-starving, optimal controls. Conversely, well-fed final instars exposed to complete starvation showed a tremendous reduction of the protein and lipids contents in the surviving imagines, accompanied by 73% mortality. These results demonstrate the biosynthetic plasticity and the significance of the phagoperiod in Ae. aegypti during the final fourth instar for growth. The characteristic differences between these two subfamilies in their larval physiology are discussed in relation to ecological factors as encountered in the field under natural conditions, and in relation to our earlier findings on the reproductive physiology.     

Dynamics of immature stages of Anopheles arabiensis and other mosquito species (Diptera: Culicidae) in relation to rice cropping in a rice agro-ecosystem in Kenya.

We determined changes in species composition and densities of immature stages of Anopheles arabiensis mosquitoes in relation to rice growth cycle in order to generate data for developing larval control strategies in rice ecosystems. Experimental rice paddies (6.3m x 3.15m) exposed to natural colonization of mosquitoes were sampled weekly for two rice growing cycles between February 2004 and March 2005. Overall, 21,325 Anopheles larvae were collected, of which 91.9% were 1st and 2nd instars and 8.1% were 3rd and 4th instars. An. arabiensis was the predominant species (84.1%) with other species, An. pharoensis (13.5%), An. funestus (2.1%), An. coustani (0.3%), and An. maculipalpis (0.1%) accounting for only a small proportion of the anophelines collected. Culex quinquefasciatus (65.7%) was the predominant species among the non-anopheline species. Others species collected included: C. annulioris (9.9%), C. poicilipes (7.3%), C. tigripes (7.2%), C. duttoni (0.6%), Aedes aegypti (5.3%), Ae. cumminsii (3.5%), and Ae. vittatus (0.7%). The densities of the major anopheline species were closely related to rice stage and condition of the rice field. An. arabiensis, the predominant species, was most abundant over a three-week period after transplanting. Low densities of larvae were collected during the late vegetative, reproductive, and ripening phases of rice. An increase in larval density ten days post-transplanting was found to correlate with the application of fertilizer (sulphate of ammonia). Culicine and aedine species densities were significantly higher during the post-harvesting period. Our results suggest that the transplanting stage is favorable for the growth of immature stages of An. arabiensis and provides a narrow window for targeted larval intervention in rice.     

Cohort studies on mosquito (Diptera: Culicidae) larvae living in the water-filled floral bracts of Heliconia aurea (Zingiberales: Musaceae)

Abstract. 1. An experimental and observational study of the three species of mosquito larvae living in the water-filled floral bracts of Heliconia aurea at Rancho Grande, Venezuela, was designed to determine if each mosquito species forms instar cohorts and excludes conspecifics of younger instars. This study was motivated by a statement of Maguire (1971) who suggested that mosquito larvae in Puerto Rican Heliconia inflorescences form instar cohorts and exclude younger mosquitoes from the floral bracts.
2. Field observations of naturally occurring populations of mosquito larvae from fifty-three H.aurea inflorescences give no indication that cohorts are formed for Wyeomyia felicia and Culex bihaicolus . However, there is strong evidence that cohorts of Trichoprosopon digitatum occur. These cohorts are not structured by larval competition but by female preference of the youngest bracts as oviposition sites and by large T.digitatum larvae feeding on smaller con-specifics.
3. Field experiments show that first instar W.felicia and C.bihaicolus survive as well over a 6 day period in the presence of third instar conspecifics as they do when all mosquito larvae in an experimental bract are first instars.     

Inter- and intra-specific differences in the number of larval instars in British populations of 24 species of stoneflies (Plecoptera)

1. Ontogenetic changes during the life cycle of aquatic insects are important not only in life‐history studies but also in evaluating food‐web structure. They require information on the growth and number of larval instars but such information is lacking for many species, including Plecoptera. Therefore, the chief objectives of the present study were to determine inter‐ and intra‐specific differences in the number of larval instars in British populations of 24 species of stoneflies, to test Dyar’s hypothesis that growth followed a geometric progression, and to synthesise this information with previously published values for four British species. 2. Larvae were reared at constant temperatures in the laboratory from eggs from 63 populations (one to six populations per species). First instars from each population were divided into three batches and each batch was reared at one of three constant temperatures. For each species, the rearing temperature and source population had no significant effect on the mean size of each larval instar. 3. The relationship between the geometric mean length of each instar and instar number was well described by an exponential equation (P < 0.001, r² > 0.9 for all species), thus supporting Dyar’s hypothesis. Only one species, Brachyptera risi, had the same number of instars for males and females (12–13). For the other 15 herbivorous species and the four smaller carnivorous ones, the number of instars was higher for females than males (range 11–16 for males, 12–17 for females). The larger size of the females was due to their additional instars, not a sex difference in growth rates. In contrast, there was a clear growth separation of the sexes after the 9th or 10th instar for the four largest carnivores. The number of larval instars was highest for these four species (range 16–19 for males, 18–23 for females), and females were much larger than males. 4. A multiple regression equation with data from the present and previous studies (n = 27) showed that variability in the mean length of the first instar and the maximum number of larval instars for each species accounted for 88% and 91% of the variability in the mean length of the final instar for males and females, respectively. 5. Values for Plecoptera in other countries were in general agreement with those in the present study, especially in the same families. Two old, but widely quoted, high values are doubtful. The present study and four previous ones provide a sound basis for ontogenetic studies on 28 species of Plecoptera and their role in aquatic ecosystems.     

Morphometric comparison of Simulium perflavum larvae (Diptera: Simuliidae) in relation to season and gender in Central Amazônia, Brazil

Number of larval instars, age structure and environmental effects on these parameters represent basic information in the study of insect population biology. When species have economic importance, this information is essential in order to choose the best period to apply different control methods and to determine the stages of the life cycle of the insect that are most susceptible to each treatment. The family Simuliidae has many species of medical/veterinary importance in the world, and some studies in the temperate region have suggested that the number of larval instars and the larval size can vary according to the season, gender and some environmental factors, such as temperature and diet. This study, with the zoophilic species Simulium perflavum Roubaud, is the first in the Neotropics observing some of these factors and will serve as a template for other species of medical importance in the region. S. perflavum larvae were collected in five streams in Central Amaz?nia (Manaus and Presidente Figueiredo counties, State of Amazonas), in Sept./Oct. 1996 (dry season) and Feb./Mar. 1997 (rainy season). These larvae were measured (lateral length of head capsule and width of cephalic apodema) to determine the number of larval instars (n=3985), to compare the larval size between seasons and genders (last and penultimate larval instars, n=200). Seven larval instars were determined for this species using frequency distributions, t-tests and Crosbys growth rule. Significant differences were not detected (t-test, p>0.05) in larval size between seasons and genders. Our results differ from some found in temperate regions suggesting that in the Neotropical region the larval size in different seasons and different genders remains constant, although some environmental parameters, such as diet, change depending on the season.     

Effects of temperature on Anoplophora glabripennis (Coleoptera: Cerambycidae) larvae and pupae

Developmental thresholds, degree-days for development, larval weights, and head capsule widths for each larval instar and the pupal stage of Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae) were studied at eight constant temperatures (5, 10, 15, 20, 25, 30, 35, and 40°C) for two source populations (Ravenswood, Chicago, IL [IL], and Bayside, Queens, NY [NY]). The estimated lower threshold temperature for development of instars 1-5 and the pupal stage was near 10°C and was near 12°C for the higher instars. Developmental rate was less temperature sensitive for instars 5-9 compared with instars 1-4. Development for all but the first instar was inhibited at constant temperatures >30°C, and all instars failed to develop at 40°C. Although the two source populations had similar responses to temperature, IL larvae were heavier than those from NY. Temperature and its influence on larval weight had profound impacts on whether a larva proceeded to pupation. Based on the temperature effects detailed here, larval development and pupation should be possible in most of the continental United States where suitable hosts are available. These data can be used to develop a degree-day model to estimate beetle phenology; however, at least 2°C should be added to air temperatures to adjust for the mediation of temperature by the wood. These data provide a basis for predicting the potential geographical range of this species and for developing phenological models to predict the timing of immature stages, both of which are important for management programs.     

Selective and frequency dependent predation of aquatic mosquito predator Diplonychus indicus Venkatesan & Rao (Hemiptera: Belostomatidae) on immature stages of three mosquito species

Frequency dependent mosquito larval size (II and IV instars) and species selection by the water bug Diplonychus indicus against three mosquito species Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi was studied in the laboratory. The different frequencies used for each species selection were 20:30:50, 30:50:20, 50:20:30, 25:35:40, 35:40:25 and 40:25:35 of fourth instars of the respective three prey species. All nymphal water bugs (I–V instars) selected IV instar mosquito larvae and the mean proportion of late (larger) larvae eaten by the predator instars was significantly higher than the mean proportion of early (smaller) larvae eaten (F= 2.28; P < 0.001). In all six ratios used to determine the frequency dependent mosquito species selection, all the stages of the water bug selected Ae. aegypti over the other two species (F= 452.43; P < 0.001). The mean number of mosquito larvae eaten increased as its density increased based on various ratios of larvae offered. The study indicated that the predatory efficiency of D. indicus was high when Ae. aegypti was offered as prey, suggesting the utility of this mosquito predator in the control of dengue vectors.     

Developmental polymorphism in a Newfoundland population of the hemlock looper, Lambdina fiscellaria (Lepidoptera: Geometridae)

The hemlock looper [Lambdina fiscellaria (Guenée)], a widespread and highly polyphagous Geometridae, is considered one of the most economically important defoliators of North American coniferous forests. Variations in the number of larval instars between geographic populations of this species have been previously reported in the literature. However, whether such developmental polymorphism occurs within a given population is unknown. In this study, we report the presence of both four and five larval instar individuals within a population of hemlock looper in Newfoundland when reared on balsam fir. For both sexes, the majority of individuals reared on balsam fir shoots went through four larval instars, but more than one third of the females (35.3%) went through five larval instars. Females with four larval instars developed faster and had smaller pupal weight than females with five larval instars. However, a growth-related index (weight gain per unit of time) was similar for the two ecotypes (four or five larval instars). No significant difference was observed between the two ecotypes in terms of reproductive capacity (fecundity and egg size). We also found significant differences in life history traits between males and females. Results indicate that developmental polymorphism, in this case, the variation in the number of larval instars, might provide some adaptive attributes that allowed exploitation of a broader ecological niche.     

Factors influencing the abundance of Japanese encephalitis vectors in ricefields in India – I. Abiotic

Mosquitoes of the Culex vishnui subgroup (Diptera: Culicidae) are the most important vectors of Japanese encephalitis (JE) and ricefields are their most productive breeding sites in south India, where predominant species of this subgroup are Culex tritaeniorhynchus Giles, Cx. pseudovishnui Colless and Cx. vishnui Theobald sensu stricto. The relationship of 13 abiotic variables with the abundance of Cx. vishnui subgroup immatures was investigated in transplanted rice fields for 3 years (1991-94) covering three different crop seasons. The results from the multiple regression model suggested paddy height (- ve), water temperature (+ ve), dissolved oxygen (- ve), ammonia nitrogen (- ve) and nitrate nitrogen (+ ve) to be the best predictor variables associated with the immature abundance, nearly always consistent in their effects within and between seasons. Application of synthetic nitrogenous fertilizers to the rice fields was followed by a rise in concentration of ammonia nitrogen and a subsequent increase in nitrate nitrogen level in the rice field water, during which an increase in the density of larval instars was observed.     

Caste Determination in Bombus terrestris: Differences in Development and Rates of JH Biosynthesis between Queen and Worker Larvae

To study the possible role of juvenile hormone in caste determination in Bombus terrestris, we measured development and rates of juvenile hormone biosynthesis in vitro in larvae destined to develop into either workers or queens. Larvae of both castes developed through four instars and had the same growth rates. However, the duration of the instars was longer for queen larvae, and their head width at the third and fourth instars was significantly larger. After validating the well-known radiochemical assay of JH for bumble bee larvae, we show that worker larvae corpora allata exhibited a constant and low rate of JH biosynthesis, never more than 5 pmol JH/h/pair. Queen larvae, in contrast, had two peaks of JH biosynthesis: a small one during the first instar, which has previously been correlated with caste determination; and a large peak, previously undetected, above 40 pmol JH/h/pair, during the second and third instars. We suggest that caste determination in this species is mediated by JH and that the duration of larval instars is a key factor. The possibility that the queen influences caste determination via an effect on instar duration is also discussed. Copyright 1997 Elsevier Science Ltd. All rights reserved     

Laboratory evaluation of Alsystin, a chitin synthesis inhibitor, against Culex pipiens pipiens L. (Dip., Culicidae): effects on development and cuticle secretion

Alsystin, a benzoylphenylurea derivative, was evaluated on Culex pipiens pipiens L. Treatment was made on newly third- and fourth-instar larvae for 24 h. Mortality occurred at various developmental stages following treatment. However, death as larvae was relatively important compared with the mortality recorded for the other later developmental stages. Treatment resulted in a dose-dependent reduction in adult emergence. Moreover, treatment caused morphological aberrations depending upon the importance of larval–adult transformation and significantly increased the duration of both third and fourth larval instars. A histological study conducted on fourth instar larval integument, revealed that Alsystin delayed the ecdysis and significantly reduced the thickness of both larval and pupal cuticles secreted compared with controls. Moreover, the apolysis that occurred at day 4 in controls was not affected by treatment.     

骆驼斯氏副柔线虫病传播媒介西方角蝇和截脉角蝇的幼虫龄期划分

【目的】本研究旨在找出区分西方角蝇Haematobia irritans和截脉角蝇H.titillans幼虫龄期划分标准,为准确鉴定两种角蝇各龄幼虫,研究斯氏副柔线虫在角蝇体内的发育过程,以及制定防控骆驼斯氏副柔线虫病的有效措施等奠定基础。【方法】采用实验室人工孵育两种角蝇幼虫的方法,分别测量不同发育阶段幼虫的虫体长、咽骨体长和咽骨体宽3项指标,利用SPSS Statistics 19.0统计软件对数据进行处理,结合Crosby生长法则和线性回归的方法进行分析,比较两种角蝇幼虫之间差异,以确定两种角蝇幼虫最佳龄期划分标准。【结果】结果表明,两种角蝇的幼虫均分为3龄,咽骨体是两种角蝇幼虫龄期划分的特征性结构,两种角蝇各龄幼虫相同指标的测量值随龄期的增长呈现出相同的增长规律。咽骨体长是划分两种角蝇幼虫龄期的最佳测量指标,咽骨体宽可作为分龄的辅助指标;两种角蝇相邻龄期幼虫的体长变化范围存在相互重叠,不能准确划分角蝇幼虫龄期。【结论】研究表明通过西方角蝇和截脉角蝇幼虫咽骨体的形态特征可简便、快速和准确地鉴定两种角蝇幼虫的龄期。