Middle Instars Research Articles

Oviposition and development of the gregarious parasitoid Homalotylus eytelweinii (Hymenoptera: Encyrtidae) in relation to host stage of predaceous ladybirds

ABSTRACT Though predaceous ladybirds (Coleoptera: Coccinellidae) are known to be attacked by a wide array of parasitoids, the biology of these parasitoids remains largely unknown. This study was focused on the gregarious parasitoid Homalotylus eyteweinii that attacks multiple taxa of predaceous ladybirds, to understand how females allocate brood and their offspring develop in relation to host stage at parasitism with the seven-spot ladybird Coccinella septempunctata as the host. The parasitoid spent the same length of time on ovipositor insertion in response to different host instars. All instars of the host were equally susceptible to parasitism. Parasitism success from oviposition to adult emergence was greater for the 2nd instar than for late instars while similar between the 2nd and 1st instars. Both egg-to-host mummification time and egg-to-adulthood time were longer for the 1st instar than for the late instars. Realised brood size was smaller for the 1st instar host than for the late instars, and there were no differences among the late instars. Sex ratio of realised brood was not significantly different among host instars and was female-biased. Body size of offspring male and female adults at emergence was not significantly different among host instars. These results suggest that middle instars of C. septempunctata are of higher quality for offspring development of H. eytelweinii.

Population age structure of ostracods in lake sediment and its implication for within-lake transport of microfossils

In palaeolimnology, microfossils play an important role as ecological indicators of lake history. A reliable interpretation of microfossil data requires a comprehensive understanding of transport processes as well as determining whether microfossil assemblages are representative of their living communities. The occurrence of transport in ostracod taphocoenoses can be identified from the population age structure of valves. In this study, 23 surface sediment samples from Dalongchi Lake, northwestern China, were analysed for species composition, and 16 were also analysed for the population age structure of Fabaeformiscandona gyirongensis. Ostracod valves are dominated by F. gyirongensis, with frequently occurring Ilyocypris sp. in the surface sediments. The distribution and abundance of ostracods are significantly correlated with the sampling depth and grain size, implying that hydrological conditions are the major controlling factor. The dominant species F. gyirongensis seems to prefer relatively deep and stable lacustrine conditions, while Ilyocypris sp. seems to prefer shallow conditions with rich submerged macrophytes and relatively coarse substrate in Dalongchi Lake. The spatial changes in the population age structure of F. gyirongensis match the different hydrodynamic regimes within the lake, with abundant early and middle instars in deeper waters and abundant middle and late instars in shallower waters, denoting that downslope transport of valves may be responsible for the spatial changes in the population structure. Our findings, together with previous works, indicate that downslope transport of instars and subsequent depth differentiation of the population structure of ostracods can occur for different species in both marine and lacustrine settings, which potentially leads to some biases based on the “in situ” inference of microfossils in palaeoenvironmental reconstruction. Hence, careful transport process assessment of fossil assemblages based on population age structure and other characteristics, such as the ratios of adults to juveniles and carapaces to valves, is a valuable prerequisite for palaeoenvironmental analysis.

Location of Helicoverpa zea (Lepidoptera: Noctuidae) larvae on different plant parts of determinate and indeterminate soybean.

Helicoverpa zea (Boddie) is a damaging pest of many crops including soybean, Glycine max (L.), especially in the southern United States. Previous studies have concluded that oviposition and development of H. zea larvae mirror the phenology of soybean, with oviposition occurring during full bloom, younger larvae developing on blooms and leaves, intermediate aged larvae developing on varying tissue types, and older larvae developing on flowers and pods. In a field trial, we investigated the presence of natural infestations of H. zea larvae by instar in determinate and indeterminate soybean varieties. In complementary experiments, we artificially infested H. zea and allowed them to oviposit on plants within replicated cages (one with a determinate variety and two with an indeterminate variety). Plants were sampled weekly during the time larvae were present. In the natural infestation experiment, most larvae were found on blooms during R3 and were early to middle instars; by R4, most larvae were found on leaves and were middle to late instars. In contrast, in the cage study, most larvae were found on leaves regardless of soybean growth stage or larval stage. Determinate and indeterminate growth habit did not impact larval preference for different soybean tissue types. Our studies suggest H. zea larvae prefer specific tissue types, but also provide evidence that experimental design can influence the results. Finally, our finding of larval preference for leaves contrasts with findings from previous studies.

Variability of photosensitive period and voltinism among populations of a butterfly, Ypthima multistriata, inhabiting similar latitudes and altitudes

AbstractIn Ypthima multistriata Butler (Lepidoptera: Nymphalidae), there are univoltine and bivoltine populations in adjacent areas with similar climatic conditions. A previous study revealed that larvae of both univoltine and bivoltine populations diapause under a constant short day (i.e., a constant short light period; L13:D11), but not under a constant long‐day condition (L16:D8). However, in both types of populations, adults of an overwintering generation appear and oviposit in June and soon thereafter larvae hatch. Therefore, the younger larvae (at least the first instars) of both types of populations experience a long day; nevertheless, the larvae of univoltine populations diapause in nature. To resolve this inconsistency, we set up two hypotheses: (1) the photosensitive stage of larvae is the second instar or later, and (2) the photosensitive stage of univoltine populations is later than that of bivoltine populations. To test these hypotheses, we performed rearing experiments with two univoltine populations and two bivoltine ones. The results indicated that the photosensitive stage was the second or third instar and that the photosensitive stage was later in one univoltine population than in the two bivoltine populations. Larvae of the other univoltine population diapaused under all conditions. The former result supports our hypothesis, and the latter result indicates that the response to photoperiod is different among univoltine populations. In addition, larval development was slower in one univoltine population than in the bivoltine populations, which also delays the timing of the diapause decision in this univoltine population. Larvae that experienced a long day during the first and middle instars but experienced a short day at the end of their larval stage developed faster than larvae that experienced a constant long day. This may be an adaptation to enable emergence before the start of a cold season that is unsuitable for reproduction.

Hydrologic Influences, Disturbance, and Intraspecific Competition in a Stream Caddisfly Population

In an earlier study, exploitative intraspecific competition was demonstrated for middle instars of the caddisfly Helicopsyche borealis in Big Sulphur Creek, a stream in northern coastal California. This population is univoltine with early instars first appearing in June, growing into late instars the following spring, and emerging as adults in May. Competition occurs during the summer dry season (June—September), a period of low—flow conditions without storms or increases in stream discharge. Beginning in autumn, however, larval densities decline because of wet—season storms and high discharge. Using multiple regression, we found a strong relationship between spring (i.e., post wet season) density of late—instar H. borealis and various hydrologic parameters of the preceding wet season over a 4—yr period, 1985—1988. Of eight hydrologic parameters used as indices of disturbance, total wet—season rainfall and peak discharge together accounted for 42% (P < .0001) of the variation in H. borealis density. In contrast, densities of middle instars of the next generation in summer were unrelated either to the preceding wet—season hydrologic regime of late—instar densities of the previous generation. Field experiments conducted in enclosures using larval densities of H. borealis that correspond to harsh, moderate, and mild wet—season hydrologic regimes (low, intermediate, and high densities, respectively) indicated that: (1) increases in larval density had a strong negative effect on algal biomass (as chlorophyll a); (2) larval density was negatively correlated with the proportion of larvae that pupated, and with pupal size; and (3) the high density (mild hydrologic regime) treatment produced smaller adult males and females, and lower fecundities than intermediate and low densities (moderate and harsh regimes, respectively). In most years, storms and associated disturbance reduce late—instar H. borealis density below levels of larval competition; this counteracts earlier competition among middle instars that occurs during low—flow conditions. In drought years, however, densities may not be reduced as severely by wet—season storms and competition may continue until emergence. In this case, competition can reduce individual fitness, but because of high adult densities during these mild—disturbance conditions the reproductive capacity of the population is unaffected.

Insecticide Resistance in the Light Brown Apple Moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae): Larval Response to Azinphosnlethyl

Azinphosmethyl resistance in the light brown apple moth Epiphyas postvittana (Walker) was confirmed with four methods: topical application of larvae, a Potter tower direct-spray method, and two residue exposure methods using middle instars and newly hatched larvae. The Mariri population exhibited a 140-fold resistance factor, using topical application of larvae. A 20-fold resistance factor was obtained by using a Potter tower to spray the insects directly. The first residue exposure method, using a Potter tower to treat apple leaf discs with the 50 WP formulation, produced a 13-fold resistance factor when third- and fourth-instar larvae were caged on the discs for 48 h. In the second residue exposure method, when first-instar larvae were caged onto leaf discs for 16 h, the resistance factor was 5-fold. The Potter tower, acetone-spray method was found to be the most satisfactory technique for evaluation of azinphosmethyl resistance in larvae. The application of this technique for monitoring resistance in the field is suggested.

The ontogeny of multiple hemoglobins in Chironomus thummi (Diptera): The effects of a compound with juvenile hormone activity

The multiple hemoglobins (Hbs) of Chironomus thummi show distinct and significant ontogenetic changes during development from the third instar through the fourth instar and metamorphosis into the pupa. A total of nine Hbs are resolved by 12.7% acrylamide gel electrophoresis (pH 8.65). Hbs 2 and 3, which are stage specific for the fourth instar, are first detected on the fourth day of this stage by electrophoresis and immunoprecipitation. Hb 4 is the predominant Hb species in the early and middle fourth instar, but during the late fourth instar and prepupa, Hb 1 predominates. The concentrations of Hbs 5–9 remain relatively constant in middle instars and decrease during later development. The Hb content of larval hemolymph exhibits changes that coincide with developmental stages; molting is characterized by low Hb content, whereas, the hemolymph of intermolt animals contains relatively high levels of Hbs. Treatment of fourth instars with a juvenile hormone analog, Altosid, prolongs this stage and inhibits the progress of normal development resulting in the formation of larval-pupal intermediates. Altosid also appears specifically to inhibit the accumulation of soluble hemolymph proteins related to pupation and metamorphosis, without affecting the concentration of Hb. Most significantly, it induces the precocious appearance of Hbs 2 and 3, which remain elevated above control levels in the late larval and prepupal stages. The present results strongly suggest that Altosid stimulates the appearance and accumulation of larval-specific proteins in vivo, while it inhibits the appearance of pupation-related proteins.

Growth and development of geographic and crossbred strains of colonized Simulium decorum (Diptera: Simuliidae)

The developmental times from larval hatch to adult emergence were computed for strains of Simulium decorum from Warm Springs, Georgia (GA), and Dryden Lake, New York (DL). Rearings were conducted at several constant temperatures. Differences between geographic strains and between sexes within strains were significant in nearly every instance (P < 0.01). At cool temperatures (13 to 17 °C), the GA strain developed more quickly; however, this trend was reversed at intermediate temperatures (18 to 20 °C), and at 21 °C only GA males developed faster than DL males. With few exceptions, both sexes of the GA strain required more cumulative degree-days for development than their northern counterparts.The number of instars was determined for each strain by measuring the length of the postgena of head capsules collected during warm (21 °C) and cool (13 °C) rearings. Although both strains pass through seven stadia, differences between strains were noted in the size of middle instars. In all rearings sexual dimorphism was evident in the morphology of last-instar head capsules; in rearings conducted at cool temperatures (13 °C), sexes could also be separated by the size of last-instar head capsules.Strains were crossed and their progeny reared. High values for net reproductive rate (R0), fecundity, and adult survival suggest that heterosis resulted from crossbreeding the two geographic strains.

An energy budget for Rhodnius prolixus (Hemiptera: Reduviidae) under laboratory conditions.

An energy budget was constructed for Rhodnius prolixus feeding on laboratory mice. The amount of blood ingested, assimilation efficiency, and the apportionment of energy into growth and metabolism were estimated for each instar. A total intake of 1915 joules was required by the 5 instars. Of this, 27.2% was used in growth, 38.4% in metabolism, 31.1% lost in feces and 3.3% lost in exuviae. The greatest expenditures for metabolism occurred in instars I, IV and V, with a pronounced reduction in instars II and III; these middle instars concomitantly showed the greatest growth. By combining data from the literature on egg production with our estimates of energy contents, the efficiency of producing eggs from energy input was estimated to be 35%. It is estimated on the basis of energy budget considerations that for enough energy to be available to begin producing eggs, females must feed once every 12–13 days. Finally, the energy budget of Rhodnius prolixus is compared with that of the much larger Triatoma phyllosoma.

Population structures of two species of Caprellid amphipods (Crustacea)

Population structures of Caprella laeviuscula Mayer and Deutella californica Mayer are compared and factors structuring the populations are discussed. Instars were determined by the number of articles in the antennules. The number of D. californica per instar continuously decreased after emergence from the brood pouch. Populations of Caprella laeviuscula had 1. 1) an initial decrease in numbers, 2. 2) an increase in cohort size at middle instars, and 3. 3) decreasing population size. For both species population declines were caused by molting-related deaths. Population structures of C. laeviuscula were also caused by selective predation, intraspecific conflict during amplexus, and by rapidly molting juveniles becoming slow molting as adults. Deviations from a 1 : 1 sex ratio in C. laeviuscula were attributed to selective predation on females. Differences in the age of becoming sexually mature reduce the probability that a chance predator would remove all mature females.

Influence of photoperiod and temperature on developmental time and number of molts in nymphs of two species of Odonata

Nymphs of the damselflies Enallagma hageni and Enallagma aspersum were collected in North Carolina from August to March and subjected to 11- and 14-h photoperiods at 16 and 21 °C. Development was generally completed more rapidly under long-day conditions. Under all day-length and temperature regimes, developmental time from the middle instars to emergence decreased from a maximum for those nymphs collected in August to a minimum for those collected in March. The two species had similar developmental times under long photoperiods at 21 °C; short photoperiods, however, were more inhibitory to the growth of E. hageni than to E. aspersum. The photoperiodic responses of E. hageni were similar at both temperatures, the developmental times being only slightly slower at 16 °C. In contrast, nymphs of E. aspersum kept at 16 °C showed no photoperiodic response. In both species, diapause or prolonged development occurred primarily in the two instars before the final, but not in the final itself.Extra molts usually accompanied slow developmental times in both species, regardless of photoperiod. The incidence of extra molting was greatest under conditions combining high temperature and short daylength. Extra molts were apparently common in field populations of E. aspersum, but were rare in those of E. hageni.

Effects of photoperiod and temperature on abnormal wing-pad development in two species of Odonata

Nymphs of the damselflies Enallagma hageni and Enallagma aspersum were collected in North Carolina from August or September to March and subjected to long- and short-day photoperiods at 16 and 21 °C. Abnormal wing pads frequently developed in the later instars of both species under daylength–temperature combinations that induced extra molts and slow growth from the middle instars to emergence. Abnormal wing pads were formed only at the higher temperature, primarily under short-day conditions, where extra molts were generally more numerous and growth was usually slower than under long days. Abnormal wing pads occurred under long days at 21 °C only in the early experiments, when extra molts were common; growth was also slow under these conditions. Mortality was considerably higher in nymphs with abnormal wing pads than in those undergoing normal development. The absence of abnormal wing pads at 16 °C compared with 21 °C suggests that even small temperature alterations, such as those resulting from discharges of heated water, may have adverse effects on the development of some aquatic insects.

The spread of pebrine within a colony of the silkworm, Bombyx mori (Linnaeus)

The epizootic pattern of pebrine within a colony of the silkworm, Bombyx mori (Linnaeus), was investigated. The rate of infection was high in the first, the second, and the fifth instars, and low in the middle instars. This pattern corresponded to the change in the number of larvae excreting spores of Nosema bombycis Naegeli.