Tipula iridescent virus infection in the developmental stages of Tipula oleracea

Tipula iridescent virus (TIV) is infective to all four larval instars, pupae, and adults of both sexes of Tipula oleracea, and iridescence has been observed in infected insects at all these stages. Third- and fourth-instar larvae were more resistant to ingested TIV than first and second instars. When TIV was injected into the hemocoel, the results suggested a possible decrease in resistance from the third larval instar to the pupa. Incubation periods (times from injection of TIV to appearance of iridescence) were significantly shorter in older fourth-instar larvae than in younger fourth-instar or thirdinstar larvae, but variability in incubation period was significantly greater in younger fourth-instar larvae than in the other two stages. Many insects which were inoculated with TIV in one stage developed iridescence and died in later stages. The amounts of infective TIV in two infected adults were estimated.     

Metabolic changes in diseased insects. IV. Radioautographic studies on protein changes in nuclear polyhedrosis, densonucleosis, and Tipula iridescent virus infections

Changes in glutamic acid, leucine, arginine, and tyrosine in Lepidoptera and Hymenoptera tissues infected with nuclear polyhedrosis (NPV), densonucleosis (DNV), or Tipula iridescent (TIV) viruses were studied by radioautography with a view to determining the effect of the viruses on protein metabolism.     

The mode of transmission of Tipula iridescent virus: II. Route of infection

Tipula iridescent virus (TIV) was inoculated into Tipula oleracea larvae via different routes. It was found to be much more infective when it was injected into the hemocoel than when it was ingested by the larvae.T. oleracea embryos did not become infected when eggs were laid in agar containing TIV, and there was no evidence that larvae can become infected via the spiracles or that transovum transmission occurs. It is suggested that TIV is transmitted principally by cannibalism, including killing and ingesting infected larvae, and finding dead infected larvae and ingesting them. It is proposed that a new generation becomes infected by first-instar larvae feeding upon infected fourth-instar larvae which have survived from the previous generation.     

The effect of temperature upon Tipula iridescent virus infection in Tipula oleracea

The mean incubation period (time from inoculation with virus to first appearance of iridescence) was used as an indication of the rate of replication of Tipula iridescent virus (TIV) in Tipula oleracea larvae. The mean incubation period and survival time (time from inoculation with virus to death) were compared with the mean instar duration at a series of temperatures. In most stages of the insect the optimum temperature for the replication of TIV and the temperature for the shortest mean survival time coincided with the peak temperature (the temperature for the fastest development of the insect stage). The peak temperature for T. oleracea does not appear to be the same for each stage, and the optimum temperature for TIV replication appears to be closely linked to the peak temperature of the infected stage. The optimum temperature (the temperature at which most individuals survived from hatching to the adult stage) of the insect was 20°C. Tipula iridescent virus replicated in T. oleracea larvae and pupae at 3° and 27°C, which are near the temperature limits for the insect. Incubation periods and survival times in TIV-inoculated larvae incubated in the field were much longer in winter than in summer.     

The mode of transmission of Tipula iridescent virus: I. Source of infection

A virus was isolated from a diseased tipulid larva and identified as Tipula iridescent virus (TIV) on the basis of the size and morphology of the virion, the production of iridescence in vitro and in infected tipulid larvae, and a serological reaction between antiserum against the virus and an isolate of TIV.A stock of Tipula oleracea was bred in the laboratory. Subjection of larvae to several stress factors did not result in any evidence for activation of a latent virus. Healthy T. oleracea larvae did not develop iridescence when confined in petri dishes with either live TIV-infected larvae or with large amounts of their feces, although these feces were found to contain infective virus by injecting extracts into healthy larvae. It appears that the concentration of virus in the feces of infected larvae is not high enough for them to serve as a source of infection. It was shown that the cadavers of TIV-infected larvae can serve as a source of infection for healthy first- and fourth-instar larvae.     

Nucleopolyhedrovirus infection enhances plant defences by increasing plant volatile diversity

Plant–herbivore–entomopathogen tri-trophic interactions and biodiversity are relatively understudied topics in ecology. Particularly, the effects of entomopathogens on herbivore-induced plant volatiles and plant volatile diversity on the defensive function of plants have not been studied in detail. We used soybean (Glycine max), beet armyworm larvae (Spodoptera exigua), and nucleopolyhedrovirus (NPV) as a tri-trophic system to determine whether NPV infection can promote the emission and diversity of volatiles from plants. We also investigated whether NPV infection affects the attraction of Microplitis pallidipes, an important endoparasitoid of larval S. exigua. Uninfested soybean plants released 7 detectable volatile compounds while plants fed upon by healthy and NPV-infected S. exigua larvae released 12 and 15 volatiles, respectively. Female parasitoids were more attracted to the volatiles from plants that were fed upon by NPV-infected larvae than healthy larvae, and more attracted to the volatiles from plants that were fed upon by healthy larvae than no larvae. The selective responses of parasitoids to plant odours increased as plant volatile diversity increased. Our study suggests that the NPV infection facilitates the release of plant volatiles and enhances the defensive function of plants by increasing plant volatile diversity which in turn attracts more parasitoids. Also, this work reveals that plants might accrue two indirect benefits from NPV infection, cessation of herbivore feeding and more parasitisation.     

Seasonal changes of gastrointestinal nematode populations in yearling beef cattle in Louisiana with emphasis on prevalence of inhibition in Ostertagia ostertagi

Investigation of seasonal changes in the composition of nematode populations, principally Ostertagia oslertagi, was conducted over 3 years at three locations in Louisiana. This is the most commonly occurring parasite of cattle in the state. Naturally infected yearling cattle were killed monthly over extended periods and tracer calves were grazed for monthly intervals from late autumn to summer at two locations in 1978–1979. Major objectives were to determine seasonal incidence of common gastrointestinal nematodes and for O. ostertagi, in particular, the time period during which larval inhibition was prevalent, circumstances under which larvae were conditioned to inhibition, and the duration of inhibition. Small numbers of inhibited O. ostertagi were recovered between November and February. Large numbers were found initially in March and increased numbers in April and May. Both normally developing and inhibition prone larvae were acquired during late winter-early spring, with the proportion of the latter being more prevalent in April and May. Evidence from tracer calves indicated that few O. ostertagi larvae were acquired after early June. Large burdens of inhibited larvae persisted in yearling cattle through summer; numbers of developing larvae and adults were minimal. Maturation of inhibited larvae occurs from August to October and in one instance was associated with cases of clinical parasitism. Factors responsible for inhibition were not defined, but increasing temperatures of late winter-early spring, host resistance, and density-dependence of populations were considered. Other abomasal genera were most prevalent in spring while intestinal genera were most common during autumn through spring.     

The effect of sodium butyrate on Tipula iridescent virus synthesis in insect suspension cell cultures

The effect of sodium butyrate on Tipula iridescent virus (TIV) synthesis in suspension-cultured cells of Estigmene acrea was investigated. Sodium butyrate reduces viral-induced cell fusion but this is reversible with the removal of butyrate. At 7 mM sodium butyrate, TIV replicates in cells within 8 hr, but does not replicate in this time with 10–20 mm butyrate in the cell medium; cells so treated contain large vesicles with inoculum. Upon removal of the inhibitor, TIV replication appears normal, but large inoculum vesicles can still be found in the cytoplasm, and many infected cells have highly condensed chromatin in their nuclei. Sodium butyrate causes a lag of at least 2 hr in viral DNA synthesis as detected by [³H]thymidine incorporation into viroplasmic centres and at 7 mm butyrate viral DNA synthesis is reduced by 50–60%. In comparison, butyrate at 7 and 10 mm concentration does not inhibit host DNA synthesis, but at 15 and 20 mm, nuclear DNA synthesis is markedly reduced.     

Field trials withTipula iridescent virus againstTipula spp. larvae in grassland

Field trials withTipula iridescent virus (TIV) were carried out to determine whether the infection can be introduced into populations ofTipula spp. in grassland. The virus was introduced into plots in live and deadTipula oleracea L. larvae, in a bran bait and in sprayed aqueous suspensions. Trials were conducted at 1 site in 3 successive years and at 5 further sites in the 3rd year. Tipulid larval populations in the plots were sampled at intervals of approximately 2 months. The majority of sampled larvae were not iridescent and did not become iridescent when they were incubated at 20°C for 30 days. In plots where iridescent larvae were found they generally comprised between 1 and 17% of the tipulid population. The identity of the virus infecting these insects was confirmed by the latex agglutination test. The results suggest that all the treatments introduced the virus infection into one or more of the tipulid populations; they all did so, however, with low efficiencies.

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Résumé Des essais en parcelles avec le virus irisant deTipula (TIV) ont été effectués pour déterminer si l'on peut introduire l'infection dans des populations deTipula spp. en prairie. Le virus a été utilisé sous forme de larves vivantes ou mortes deTipula oleracea L., d'appat de son, et en suspensions aqueuses. L'expérimentation a été réalisée dans un emplacement pendant 3 ans successifs et dans 5 sites complémentaires pendant la 3^e année. L'échantillonnage des populations de larves de tipules a eu lieu tous les 2 mois. La majorité des larves récoltées n'etaient pas irisantes et elles ne le sont pas devenues après un élevage à 20°C pendant 30 jours. Dans les parcelles où l'on a trouvé des larves irisantes, celles-ci représentaient 1 à 17% de la population de tipules. L'identité du virus dans ces insectes a été confirmée par agglutination au latex. Les résultats suggèrent que tous les traitements ont introduit l'infection virale dans les populations de tipules mais avec une faible efficacité.

     

Thermal ecology of western tent caterpillars Malacosoma californicum pluviale and infection by nucleopolyhedrovirus

Abstract 1. Western tent caterpillars hatch in the early spring when temperatures are cool and variable. They compensate for sub-optimal air temperatures by basking in the sun.
2. Tent caterpillars have cyclic population dynamics and infection by nucleopolyhedrovirus (NPV) often occurs in populations at high density.
3. To determine whether climatic variation might influence viral infection, the environmental determinants of larval body temperature and the effects of temperature on growth and development rates and larval susceptibility to NPV were examined.
4. In the field, larval body temperature was determined by ambient temperature, irradiance, and larval stage. The relationship between larval body temperature and ambient temperature was curvilinear, a property consistent with, but not necessarily limited to, behaviourally thermoregulating organisms.
5. Larvae were reared at seven temperatures between 18 and 36 °C. Larval growth and development increased linearly with temperature to 30 °C, increased at a lower rate to 33 °C, then decreased to 36 °C. Pupal weights were highest for larvae reared between 27 and 30 °C.
6. The pathogenicity (LD₅₀) of NPV was not influenced by temperature, but the time to death of infected larvae declined asymptotically as temperature increased.
7. Taking into account larval growth, the theoretical yield of the virus increased significantly between 18 and 21 °C then decreased slightly as temperatures increased to 36 °C.
8. Control and infected larvae showed no difference in temperature preference on a thermal gradient. The modes of temperature preference were similar to those for optimal growth and asymptotic body temperatures measured in the field on sunny days.
9. Warmer temperatures attained by basking may increase the number of infection cycles in sunny springs but do not protect larvae from viral infection.     

Epizootiology of a nuclear-polyhedrosis virus in European spruce sawfly,Gilpinia hercyniae: Birds as dispersal agents of the virus during winter

In Welsh spruce forest the relationship of birds to a nuclear polyhdedrosis virus (NPV) was studied outside the larval period of its sawfly host, Gilpinia hercyniae. Birds passed feces containing infective NPV throughout the nonlarval period (November–June). It is thought that birds acquired inoculum by feeding on the corpses of NPV-killed larvae adhering to spruce trees. The proportion of infective droppings declined in the 2 months (May and June) prior to the new larval period. Infective droppings were collected from all six species of birds netted in January. The infectivity of droppings of goldcrest was less and that of longtailed tit greater in January than in the previous September. Experimental evidence suggests that birds may carry NPV for at least 6 km.     

Transmission of NPV in uniform- and mixed-age populations ofHeliothis zea [Lep: Noctuidae] on caged soybean

Transmission of a nuclear polyhedrosis virus (NPV) was examined in uniformaged and uniform- vs mixed-age populations ofHeliothis zea (Boddie) on caged soybean. Larval collections revealed viral disease outbreaks occurred in all treatments following release of infected larvae (Primary infected larvae). Transmission of NPV in uniform-aged populations was related to the density of primary infected larvae released in the population but not to the size at death of primary infected larvae (P<0.05). In mixed-age populations horizontal transmission in the oldest larvae in the population was equal to that in uniform-aged populations, providing that primary infected larvae in the mixed-aged population were all the age of the oldest noninfected cohorts. As the mixed-age population aged, transmission increased and was generally higher than that in the uniform-aged populations. Transmission was also higher when primary infected larvae were medium sized at death than when small or large at death. The concentration of virus deposited on foliage and in soil after all larvae had died on plants was related to density of primary infected larvae released. In soil, but not on foliage, the virus concentration was related to the size at death of primary infected larvae released. This material is based upon work supported in part by the U.S. Department of Agriculture under Agreement No. CRSR-2-1000.     

Transmission of a nuclear polyhedrosis virus inHeliothis zea [Lep.: Noctuidae] larval populations on soybean

Transmission of a nuclear polyhedrosis virus (NPV) inHeliothis zea (Boddie) on soybean was examinated. Artificial infestations ofH. zea were established at densities of 6.5, 19.5 and 58.5 larvae/row-m. Additional larvae infected to die from NPV in the 2nd stage were released into subplots to simulate 5 and 25% mortality levels. Virus transmission from infected to noninfected larvae was correlated with the initial incidence of infection in the population but not the density of larvae/row-m. Deposition of virus on plants from cadavers of larvae that died of virus infection was correlated with the initial incidence of infection in the populations and the density of larvae/row-m. After pupation of larvae in the 1st population, noninfected larvae only were again released to examine transmission of viral inoculum remaining on plants and soil. The percent mortality of larvae collected from the 2nd release was low and did not differ significantly between treatments. The concentration of virus on foliage and in soil after the 2nd release was directly correlated with density of larvae/row-m but not the incidence of infection within the population in the 1st release. This material is based upon work supported in part by the U.S. Department of Agriculture under Agreement N^o 82 CRSR-2-1000.     

Susceptibility of Heliothis armiger to a commercial nuclear polyhedrosis virus

The susceptibility of Heliothis armiger larvae of different ages to a commercial nuclear polyhedrosis virus (NPV), Elcar, was determined by bioassay. The median lethal dosage (LD₅₀) increased 150-fold during the first week of larval life at 25°C, i.e., during development to early fourth instar, but daily feeding rate and thus potential virus acquisition also increased. A linear relationship was determined between log LD₅₀ and larval length, indicating that larval length constitutes a useful index for estimating the susceptibility of larval populations. Median lethal times (LT₅₀s) were similar for larvae tested at ages of 0 to 7 days and ranged from 3.6 to 8.0 days at 30°C. The amount of virus produced in a single, infected neonate was equivalent to 1.4 × 10⁶ LD₅₀s for neonates, a 900,000-fold increase on the dose supplied. The data support the practice of directing the NPV against neonates, but, on the basis of larval susceptibility alone, the age of larvae at treatment may not always be critical.     

Change in overwintering mechanism of the Cucujid beetle, Cucujus clavipes

Overwintering larvae of the Cucujid beetle, Cucujus clavipes, were freeze tolerant, able to survive the freezing of their extracellular body fluids, during the winter of 1978–1979. These larvae had high levels of polyols (glycerol and sorbitol), thermal hysteresis proteins and haemolymph ice nucleators that prevented extensive supercooling (the supercooling points of the larvae were ? 10°C), thus preventing lethal intracellular ice formation. In contrast, C. clavipes larvae were freeze suspectible, died if frozen, during the winter of 1982–1983, but supercooled to ～ ? 30°C. The absence of the ice nucleators in the 1982–1983 larvae, obviously essential in the now freeze-susceptible insects, was the major detected difference in the larvae from the 2 years. However, experiments in which the larvae were artifically seeded at ? 10°C (the temperature at which the natural haemolymph ice nucleators produced spontaneous nucleation in the 1978–1979 freeze tolerant larvae) demonstrated that the absence of the ice nucleators was not the critical factor, or at least not the only critical factor, responsible for the loss of freeze tolerance in the 1982–1983 larvae. The lower lethal temperatures for the larvae were approximately the same during the 2 winters in spite of the change in overwintering strategy.     

Histopathology of a nuclear polyhedrosis infection in Aedes epactius with observations in four additional mosquito species

Aedes epactius larvae were utilized to study the infection sequence of the nuclear polyhedrosis virus (NPV) from Aedes sollicitans. From 30 min to 6 hr postinoculation, polyhedra and many free virions were observed in the larval midgut lumen. Penetration of the midgut cells by virions was not observed. The first infected nuclei were observed 12 hr postinoculation. Nucleocapsids initially exhibited electron translucent cores which became electron dense before the nucleocapsids acquired an envelope. Envelope acquisition occurred through a process of de novo membrane morphogenesis. Occlusion of the singly embedded virions began by 18 hr postinoculation with the mature rough-surfaced polyhedra averaging approximately 1 by 2 μm. Unusually long nucleocapsids (approximately two or three times the length of other nucleocapsids) were only observed in late infection period nuclei. There was no evidence that long nucleocapsids represented an early developmental stage for nucleocapsids of standard length. Infection was restricted to midgut nuclei and gastric caecae cells. Infected early instar A. epactius larvae became moribund 36 to 40 hr postinoculation and infected midgut nuclei were observed to undergo lysis. The late stages of NPV infection were observed in larvae of A. annandalei, Wyeomyia smithii, Toxorhynchites brevipalpus, and Eretmapodites quinquevittatus. Virion development and occlusion in these species was basically identical to the sequence observed in A. epactius larvae.     

Observations on the biology and ecology of the chrysomelid beetle Gastrophysa polygoni in cereal fields

Abstract. 1. Observations were made on the biology of Gastrophysu polygoni (L.) (Coleoptera: Chrysomelidae) in cereal fields in southern England in 1977, 1978 and 1979. Adults of the overwintering generation emerged in late April/ May and there were usually two generations during the spring and summer months. In 1979 there was some evidence for at least a partial third generation.
2. In the field, the oviposition period was 44 days in the first generation and c . 25 days in the second. Fecundity varied from 586 to 1028 eggs per female and was higher in the first than in the second generation in both 1977 and 1979; in 1978 the reverse was true.
3. Every year there were Iarge losses in the numbers within a generation. However, only one parasite was bred from the developmental stages and a pathogen attacking the larvae was found only in 1977.
4. In some fields and in some years, harvesting and straw burning operations were carried out when eggs were present on the plants. Harvesting did not result in a significant reduction in the numbers of eggs. Burning reduced the numbers of egg batches. The effect was most severe when the straw was spread over the field prior to burning.
5. In the field, significantly more eggs were laid on plants of Polygonum aviculare than on P.convolvulus . In the laboratory, larval survival was higher and duration of development shorter on these two species than on other Polygonaceae found on the farm.     

幼虫密度对草地螟生长发育及繁殖的影响

为了明确幼虫密度对草地螟Loxostege sticticalis种群增长的影响, 对室内条件下（温度22±1℃, RH 70%±5%, 光周期16L∶ 8D）不同幼虫密度[1, 10, 20, 30和40头/瓶（650 mL）］饲养的草地螟生长发育及繁殖进行了研究。结果表明: 幼虫密度对草地螟幼虫体色、 发育历期和存活率, 以及蛹重和成虫生殖等有显著影响。随着幼虫密度的增加, 幼虫体色黑化程度呈增强趋势, 幼虫密度大于10头/瓶时的体色黑化值均显著大于幼虫密度为1头/瓶时的体色黑化值（P<0.05）。20头/瓶的幼虫和蛹历期最短, 且随幼虫密度的增加而显著延长（P<0.05）。幼虫存活率以10头/瓶最高, 其他幼虫密度的幼虫存活率显著较低（P<0.05）。蛹重以1头/瓶的最重, 并随幼虫密度增加而显著下降（P<0.05）。成虫产卵量和交配率分别以1和20头/瓶的幼虫密度最高, 幼虫密度升高则产卵量、 交配率逐渐降低。成虫产卵历期随着幼虫密度的增加逐渐缩短。雌、 雄蛾寿命分别以10和20头/瓶幼虫密度时最长, 幼虫密度过高时雌雄蛾寿命又显著缩短（P<0.05）。生命表分析表明, 幼虫密度对草地螟种群增长指数有显著影响, 以10头/瓶幼虫密度的种群增长指数最高, 幼虫密度过高或过低时种群增长指数下降。据此认为, 幼虫密度是影响草地螟种群增长的重要因子之一。     

Interaction between MbMNPV and the braconid parasitoid Habrobracon hebetor (Hym., Braconidae) on larvae of beet armyworm,Spodoptera exigua (Lep., Noctuidae)

The survival of a braconid parasitoid Habrobracon hebetor was investigated on nucleopolyhedrovirus (NPV)-infected Spodoptera exigua larvae. The second-instar larvae were exposed to 30, 51.4 and 180 PIB/mm² of Mamestra brassicae NPV (MbMNPV) as under-LD₅₀, LD₅₀ and over-LD₅₀ values, respectively. They were accessible to be parasitized by H. hebetor after 24, 48 and 72 h post-treatment. Infection of the larvae with MbNPV was deleterious to the survival and parasitism of H. hebetor. The survival of H. hebetor in MbNPV-infected S. exigua larvae was dependent on the interval between viral infection and parasitization, as well as on the treatment dose of MbMNPV; very few adults of parasitoid emerged from infected hosts when host larvae were exposed to 180 PIB/mm² of MbNPV on 72-h interval treatment. The inoculation dose of MbNPV and the timing of parasitoid release had significant effect on the development of H. hebetor on virus-infected hosts. Field applications of virus for biocontrol of S. exigua may lead to substantial mortality of immature parasitoids.     

An experimental study of population regulation in the salamander,Notophthalmus viridescens dorsalis (Urodela: Salamandridae)

Summary The roles of density-dependent larval survival and cannibalism of larvae as potential mechanisms of population regulation in the newt (Notophthalmus viridescens dorsalis) were evaluated in laboratory and field experiments. In laboratory containers, adults cannibalized larvae and large larvae cannibalized smaller larvae. In artificial ponds, larval survival did not depend on initial larval density. No cannibalism could be demonstrated in the complex environment, although the experiment was powerful enough to detect an ecologically relevant difference in survival. Adult growth was negatively correlated with the final biomass of larval newts, suggesting that the two life stages competed for resources. Larval growth rates were negatively correlated with final larval density, suggesting that larvae competed with each other. The proportion of larvae that became sexually mature at age 7 months (paedomorphs and adults that skipped the eft stage) varied inversely with larval density. Therefore, the potential regulatory mechanisms identified in this study are competition within and between life stages.