Instar IV Research Articles

Compensatory Mortality in Defoliator Population Dynamics

In one set of trials, predaceous birds and ants were excluded from trees containing cohorts of the western spruce budworm. Birds and ants contributed about 29% of the killing power (K) during the budworm generation, and about 76% between instar IV and adults. Except at high budworm densities, birds compensated for ants on trees protected from ants, and vice versa. In other trials, predaceous birds and small mammals were excluded from gypsy moth cohorts. Birds and small mammals contributed about 27% of the killing power during the gypsy moth generation, and about 47% between instar V and adults. In this life system, small mammals compensated for birds, but not the converse. In both life systems, other factors did not compensate effectively in places protected from both predator groups. These results emphasize the need to consider possible compensatory interactions among mortality-causing processes in evaluating any given life system.

Selective predation on cladocerans by common pond insects

A variety of common pond insects was presented with a mixture of different size classes of Daphnia magna or different sizes of Daphia pulex, Moina affinis, and Ceriodaphnia sp. in predation trials. Juvenile Belostoma flumineum and most instars of Notonecta undulata and Anax Junius fed at the highest rates on the largest available cladoceran prey, and late-instar Buenoa corfusa fed at the highest rates on prey between 0.8 and 2.0 mm in length. Predation rates of instar IV Chaoborus americanus larvae were highest on Daphnia < 1 mm in length, and lowest on Daphnia > 2 mm in length. First-instar Chaoborus larvae did not eat cladocerans in these trials. The size-selective feeding patterns exhibited by Notonecta adults and late-instar Anax were similar in the light and dark, although overall feeding rates were depressed in the dark. Buenoa, on the other hand, only exhibited size-selective feeding in the light. The results indicate, however, that all of these insect predators can feed in the dark. Predation rates for late-instar Chaoborus larvae were unaffected by light conditions or the presence of filamentous algae. Chaoborus larvae were readily eaten by late-instar Notonecta and Anax.

The Life Cycle and Spatial Distribution of the Aquatic Parasitoid Agriotypus armatus (Hymenoptera: Agriotypidae) and its Caddis Host Silo pallipes (Trichoptera: Goeridae)

(1) The life cycle of Silo pallipes (Fabricius) took one year in a small, stony stream. Adults were present chiefly in June and July; a few being taken in May, August, September. Oviposition occurred on large stones just below the water surface; eggs hatched within 19-35 days. The five larval instars built eleven stony-cases. Growth was rapid in the early instars; most larvae were in instar IV or V by November. Pupation commenced in spring; emerging adults were taken from May to August. (2) Changes in population density followed a similar pattern in each year-class. Density increased during egg hatching, reached a maximum in July, decreased exponentially at a fairly constant rate from July to May, and then decreased rapidly during pupation and emergence from June to August. Only 5% of the initial population in the egg stage reached the pupal stage. 67-68% of the pupal population emerged as adults, 22-23% died and 9-10% were parasitized by Agriotypus armatus Curtis. (3) The life cycle of A. armatus also took one year. Adults were present in May and June, and laid their eggs in the pupal cases of their host. The five larval instars fed externally on their living host and grew rapidly to the pupal stage by August. There was only one parasitoid per host. All parasitoids were in the adult stage by October, overwintered as resting adults within the pupal cases and emerged in spring. Only 4-7% of the initial population in the egg stage reached the adult stage in spring. (4) Three methods were used to analyse spatial distribution. The simple index of dispersion, tested by X2, was too insensitive to detect clumping in most samples. Although the negative binomial was fitted successfully to most samples, the statistic 1/ could not be used as an index of clumping because it either followed a curvilinear relationship with increasing population density (Silo larvae), or was highly variable (Silo pupae, Agriotypus). The third, and most successful, method was Taylor's power law which showed that the spatial distribution of both species was density dependent. Relative clumping remained fairly constant between sites, year-classes and larval instars of Silo, but increased markedly between the larval and pupal stages. Although the aquatic stages of Agriotypus were also clumped, the degree of relative clumping was less than that of their pupal host.

Identification of instars and species in some larval Polypedilum (Polypedilum) (Diptera: Chironomidae)

Instars II and III of Polypedilum aviceps Townes, Polypedilum convictum (Walker), and Polypedilum illinoense (Malloch) can be identified to species by associating them with instar IV because key taxonomic characters remain relatively unchanged from instar to instar. Instars I cannot be identified to species or genus unless they are associated with older, identifiable larvae reared from the same egg masses. No single character evaluated on slide material can be used to clearly separate instars in all three species. Larvae of P. aviceps can be separated into instars based on any four of seven characters; P. convictum by either of two characters; and, P. illinoense by a combination of two characters. Changes in structures of instars II, III, and IV are described for all three species. Growth ratios for some structures are compared and discussed with regard to Dyar's Rule.

THE BIOLOGY AND ECOLOGY OF THE TWOLINED CHESTNUT BORER, AGRILUS BILINEATUS (COLEOPTERA: BUPRESTIDAE), ON OAKS, QUERCUS SPP., IN WISCONSIN

AbstractAgrilus bilineatus (Weber) is univoltine in Wisconsin. Adults were present from early June through mid-September; however, peak flight and oviposition occurred during the second half of June. Adults consumed significantly more oak foliage than foliage of hardwoods other than oak. Stressed oaks attracted significantly more adults than did uninjured controls, as measured by density of captured adults on sticky traps. Four larval instars were determined; instar IV was the overwintering stage. Fourth instar larvae generally overwintered in pupal cells in the outer bark or in sapwood if the bark was thin. Larvae, especially third and fourth instars, damaged oaks by girdling the conducting xylem and phloem. Attacked oaks had low root starch content relative to those unattacked. Tree death normally resulted after 2 or 3 years of borer infestation, yet tree death may occur in a single season. Initial attack usually began in the live crown and proceeded downward in succeeding years with no apparent reinfestation of previously killed areas. Mean adult longevity in days for adults fed red oak foliage was 28.1 at 20°, 37.8 at 24°, and 8.3 at 30 °C. Mean duration of the pupal stage in days was 11.7 at 24 °and 8.5 at 30 °C.

Life Tables of the Range Caterpillar, Hemileuca oliviae (Lepidoptera: Saturniidae) 1

Field data for life tables of the range caterpillar, Hemileuca oliviae Cockerell, were collected weekly from two sites in Union County, N.M. The wandering-quarter method, a plotless technique, was used to estimate densities of the gregarious early instars. Densities of older life stages were estimated by counting all individuals within randomly established quadrats. Development from egg eclosion (middle of May) to adult took 130 to 160 days. Original populations were reduced at least by 60% by instar IV. Principle factors contributing to mortality included heavy rainfall, poor nutritional quality of grass due to drought, predation, and parasitism by the tachinid Exorista mella (Walker).

ESTIMATES OF POPULATION SIZE AND SURVIVAL OF IMMATURE STAGES OF FOUR SPECIES OF AEDES (DIPTERA: CULICIDAE) IN A TEMPORARY POND

AbstractA temporary pond close to Calgary, Alberta, was studied for 2 years. In the spring of the first year (1974) the pond filled with water to a much higher level than normal and it did not dry and reflood until August. The first flooding hatched eggs of Aedes euedes, A. cataphylla, A. excrucians, A. fitchii, A. flavescens, and A. mercurator; the second flooding hatched eggs of A. vexans and A. cinereus. Estimates of larval population size were made for the latter two species and little larval mortality occurred until the pond was reduced to a few small puddles, when larvae became stranded. The pond dried completely before the pupal stage was attained. Fifty per cent of surviving A. vexans larvae attained instar II in 3.7 days, instar III in 6.6 days, and instar IV in 10.3 days. A. cinereus developed more slowly.In the second year (1975) the pond contained water from the middle of April to the beginning of June, and did not refill in the summer. Estimates of larval population sizes were made for A. euedes and A. mercurator. These larvae shared a preference for the deeper water in the centre of the pond. Their numbers declined at rates of 9% per day for A. euedes and 7% per day for A. mercurator. Larvae in the second instar survived a period of 3 days when the water surface was covered with ice and snow, but A. euedes experienced a sharp drop in numbers at this time.Fifty-three per cent of the eggs of A. vexans survived a period of 1 year in areas not flooded with water at appropriate temperatures for hatching.

Rhodnius prolixus and its symbiotic actinomycete: a microbiological, physiological and behavioural study.

The rôle of the symbiont in this blood-sucking insect has been reinvestigated by a novel method: the substitution of wild-type symbionts by auxotrophic mutants produced in vitro , unable to synthesize particular B-group vitamins namely nicotinamide, thiamin, pyridoxine, riboflavin, p -aminobenzoic acid or biotin. Symbiont-free instar IV insects infected with a mutant of any one of these phenotypes moulted to instar V essentially as though infected with wild-type symbionts and did not exhibit the substantial developmental disturbance shown by symbiont-free controls. Though occasional overgrowth of auxotrophs by prototrophic revertants occurred it was not a significant problem. Continued growth of the nicotinamide, thiamin or pyridoxine auxotroph- and wild-type-infected instar V insects was comparable. With the remaining auxotroph-infected insects results varied between experiments, possibly because of disjunction of pre- and post-ecdysial changes in the insect’s pharyngeal pump. Overall, the results are interpreted as disproving the hypothesis that the de novo synthesis of B-vitamins by the symbiont is the sine qua non of the relationship. This contrasts with the conclusions of previous workers. It is suggested that these differences might be accounted for by the more precise experimental framework adopted here. Variations on the ‘traditional’ vitamin hypothesis are suggested: these involve the ‘conversion of metabolites’ rather than the de novo synthesis of vitamins and may help reconcile conflicting results. The symbiont showed unexpected carbon source utilization in vitro and from this it is inferred that erythrocyte-mediated glycolysis within the insect’s blood-meal continues, thus converting to lactate glucose present in the insect. As blood-meal digestion progresses glycolysis would cease, leading to a possible increase in glucose levels within the gut. Cyclical changes in glucose/lactate levels might influence the development of Trypanosoma cruzi the aetiological agent of Chagas’ disease, which undergoes part of its life cycle in the triatomid gut. Peak populations of about 10 8 symbionts per instar IV insects were found, corresponding to about 5 % of the dry increase of the instar. Symbionts survived well in the insect’s faeces and this, in conjunction with the demonstrated coprophilic behaviour of the insect, which would lead to a high probability of contact with faeces, may be of critical importance in the establishment of the symbiotic relationship.

Patterns of Gypsy Moth1 Occurrence Within a Sparse and Numerically Stable Population3

Patterns of subadult gypsy moth, Porthetria dispar (L.), occurrence within a sparse, numerically stable population that was studied in 1965 were modelled as functions of insect stage and certain components of environmental structure. Our results imply the following. (1) Once newly hatched larvae find suitable foliage, they tend to stay on or near it until after they have molted into instar III. (2) These insects begin to wander sometime before reaching instar IV, and continue to do so until suitable daytime resting locations are found. These locations are usually in the litter at the tree base, but the insects will aggregate under bark flaps on dead limbs or on the dead boles of trees in clumps in preference to the litter if these are available. (3) Most of the insects pupate in the locations used earlier for resting sites, but some eventually pupate in objects that may be many feet away from the closest living host tree. (4) Bark flaps may have provided less hazardous resting sites for the growing larvae and pupae than other potential resting locations within the environment.

Effect of starvation on molting and growth in Aedes aegypti and A. vexans.

Starvation deferred larval development and caused a reduction in size and weight of larvae of the yellow-fever mosquito, Aedes aegypti (L.), and A. vexans (Meigen). Larvae of both species, starved throughout instar IV, did not pupate. Even when larvae were given food for 8 hours out of the normal 48–50 hours spent in instar IV at 26°C, they were unable to pupate. Starved instar III larvae of A. aegypti were able to molt to instar IV, those of A. vexans were not.

Irradiation effects on spermatogenesis in the gypsy moth, Porthetria dispar (L.)

In connexion with a detailed study of the effects of gamma irradiation on the gypsy moth, histological examination was made of normal larvae in instars I to V and pupae to determine the timing and sequence of spermatogenesis. Similar examination was made of pupae that had been irradiated as instar IV and V larvae with dosages ranging from 500 to 20,000 roentgens. Spermatogenesis occurred in the larval period, and meiotic activity began in instar IV. Fully formed spermatozoa were first observed in instar V. The appearance of gonadal tissues of treated and untreated specimens in the pupal stage is described. All irradiation dosages caused some cytological damage; instar IV larvae exposed to 20,000 r died in 7–10 days. The major gross effects of irradiation were: degeneration of germinal cysts, shattering of sperm bundles within the follicles, plugging of the entrance to the vas deferens by interstitial and other tissue (this occurred with higher dosages only), thickening of testes walls and septa and of the vas deferens, and clumping of fat body cells around the testes. One or more of these morphological changes might be related to the sterile condition.