The light-brown apple moth,Epiphyas postvittana (Walker): 4. Studies on population dynamics and injuriousness to apples in the Australian Capital Territory

A population study of the light-brown apple moth (LBAM), Epiphyas postvittana (Walker), was pursued from 1963–64 to 1966–67 in a naturally-infested, unsprayed apple plantation near Canberra. The investigation covered the period of tree development leading up to and including, the production of the first substantial crop. The life history of LBAM and the occurrence of natural enemies were examined. The species did not produce seasonal series of identifiable generations, but individuals in all development stages tended to co-exist as cohorts of uncertain generation status. Whereas numbers of LBAM decreased gradually from 1963–64 to 1965–66, they rose again markedly in 1966–67, following the release in the study area of about 3,000 laboratory-reared moths. From periodic field surveys and complementary experiments, it was inferred that the population dynamics of LBAM in the study area consisted essentially in the production of excessive numbers of immature stages, which were reduced by natural enemies to much smaller cohorts of mature stages, determined by the ability of fruit trees to supply the LBAM's requisite food and shelter. It is argued that this ability, i.e. the carrying capacity for LBAM, decreased as the trees matured, and that the damage of LBAM in the crops of fully-bearing trees would have been negligible under the conditions prevailing in the study area. The probability of damage to fruits was directly proportional to the numbers of larvae and of fruits present, indicating that, unlike codling moth for instance, LBAM did not seek out fruits for attack, but struck them by chance encounter. The results of this life-system analysis are discussed in the light of Southwood & Comins’(1976) and Southwood's (1977a, b) generalizations on population dynamics, ecological strategies, and the status of pests. LBAM is believed to display all the characteristic features of an ‘intermediate strategist’ and an ‘intermediate pest’.     

The light-brown apple moth,Epiphyas postvittana (Walker) 2. Uneven sex ratios and a condition contributing to them in the field

Whereas populations of Epiphyas postvittana (Walker), the light-brown apple moth (LBAM), normally produced a slight preponderance of male adults in laboratory cultures, males tended consistently to be less abundant than females in samples drawn from Australian field populations. The sex ratio of the species was investigated with a view to explaining this fact. A condition, causing bisexually-reproducing females of LBAM to form viable progenies comprising mainly female individuals, was found to affect 7% of females in population samples from the south-eastern mainland of Australia, and 4% from Tasmania. A statistical model describing the distribution of the condition in field populations is given. The origin of the condition is unknown; the causative agent is believed to act by inducing the death of male embryos. The condition was only observed to be transmitted from mothers to daughters, of which some might produce apparently normal progenies, while others produced either all-female or predominantly-female progenies. The demographic performance of affected females differed from that of comparable ‘normal’ individuals only in fertility, which was significantly reduced. In a limited number of experiments, laboratory populations formed from field-collected samples containing affected females, which produced a proportion of males, could only be maintained through subsidies of males from non-affected stock. The present case was considered in relation to literature reports of similar conditions affecting a dipteron, a coleopteron, and a macrolepidopteron. In E. postvittana, it is surmised that the condition might contribute to optimizing the sex ratio of populations comprising affected individuals, thus increasing their adaptive fitness. A set of verifiable hypotheses are formulated concerning future trends in the condition's natural distribution and frequency.     

Detection of nucleopolyhedroviruses in the eggs and caterpillars of tea looper caterpillar Hyposidra infixaria (Walk.) (Lepidoptera: Geometridae) as evidence of transovarial transmission

The transovarial transmission of Hyposidra infixaria nucleopolyhedroviruses (HyinNPV) in the tea looper caterpillar H. infixaria was investigated. H. infixaria larvae reared from eggs (surface sterilised) collected from a tea garden showed typical signs of nucleopolyhedrosis virus infection. The polyhedrin gene amplified from larvae confirmed HyinNPV infection and provided clear evidence of vertical virus transmission from parent to offspring (transovarial transmission). This study reveals important aspects of HyinNPV that could be exploited in biological control programmes; HyinNPV can persist in larval populations and be passed on to the next generation, and thus it may be useful for the long-term control of H. infixaria.     

Inactivation of a nuclear polyhedrosis virus on cotton by the substances produced by the cotton leaf surface glands

Loss of viability of a nuclear polyhedrosis virus (NPV) of Spodoptera littoralis was shown to occur on cotton leaves but not on cabbage leaves or on semi-synthetic insect diet. The inactivation was rapid, occurred in the dark and required contact between the cotton leaf surface and the polyhedral inclusion bodies (PIBs). It was shown that the products of the cotton leaf glands, which exude a crystalline material onto the leaf surface, could account for the rapid inactivation observed. Inactivation did not involve loss of virus polyhedral inclusion bodies. The use of EDTA or a low pH buffer in the NPV suspension reduced the inactivation which suggested that the loss of activity was due to the presence of calcium and magnesium in the cotton gland exudate.     

Epizootiology of virus diseases in three lepidopterous insect species of alfalfa

Summary The incidence of virus infections in three lepidopterous insect species was studied from 1965 to 1968 in alfalfa fields in California. The insects were the alfalfa caterpillar,Colias eurytheme; the beet armyworm,Spodoptera exigua; and the alfalfa looper,Autographa californica. InC. eurytheme, the major virus was a nuclear polyhedrosis virus (NPV); inS. exigua, a granulosis virus (GV) and an NPV; inA. californica, a GV. Virus epizootics did not develop in very high densities ofC. eurytheme. Virus epizootics occurred in low host densities of the three insect species, especially in populations ofA. californica. The virus acted as a density-dependent factor in the regulation of the populations ofS. exigua andA. californica. Temperature, humidity and rainfall had no marked effect on the incidence of virus infections.     

Limits to the potential distribution of light brown apple moth in Arizona–California based on climate suitability and host plant availability

The highly polyphagous light brown apple moth (LBAM) (Epiphyas postvittana (Walk.): Tortricidae) is indigenous to Australia and was first found in California in 2006. It is currently found in 15 coastal counties in California, but nowhere has it reached outbreak status. The USDA projects the geographic range of LBAM will include much of Arizona and California and the southern half of the US, which together with economic estimates of potential crop losses have been used as the rationale for an eradication program in California. We report a temperature-driven demographic model to predict the likely distribution and relative abundance of LBAM using the detailed biology reported by Danthanarayana and colleagues, and climate data from 151 locations in California and Arizona for the period 1995–2006. The predictions of our model suggest that the near coastal regions of California are most favorable for LBAM, the northern Central Valley of California being less favorable, and the desert regions of Arizona and California being unfavorable. The model also predicts that LBAM populations can develop at two of the hottest locations in SE Australia where it is has long been known to occur. This reassessment of the potential distribution of LBAM in California and Arizona suggests that its likely ecological and economic impacts would be less than previously assessed by USDA and that its current pest status warrants re-evaluation.     

The infectivity and host range of Orgyia anartoides nucleopolyhedrovirus

The painted apple moth (PAM), Teia anartoides (Walker) (Lepidoptera: Lymantriidae) made a recent incursion into New Zealand. A nucleopolyhedrovirus (NPV), Orgyia anartoides NPV (OranNPV), originally isolated from PAM in Australia, was tested for its pathogenicity to PAM and a range of non‐target insect species found in New Zealand, to evaluate its suitability as a microbial control for this insect invader. Dosage‐mortality tests showed that OranNPV was highly pathogenic to PAM larvae; mean LT₅₀ values for third instars ranged from 17.9 to 8.1 days for doses from 10² to 10⁵ polyhedral inclusion bodies/larva, respectively. The cause of death in infected insects was confirmed as OranNPV. Molecular analysis established that OranNPV can be identified by PCR and restriction digestion, and this process complemented microscopic examination of infected larvae. No lymantriid species occur in New Zealand; however, the virus had no significant effects on species from five other lepidopteran families (Noctuidae, Tortricidae, Geometridae, Nymphalidae and Plutellidae) or on adult honeybees. Thus, all indications from this initial investigation are that OranNPV would be an important tool in the control of PAM in a future incursion of this species into New Zealand.     

Apis iridescent virus and "clustering disease" of Apis cerana

NPV of Spodoptera littoralis was completely inactivated in vitro following 10 min of exposure to a temperature higher than 90°C, but survived 3 weeks at ?20°C. At pH 12, some 75% of the infectivity was lost. Measurable proteolysis in vitro of the polyhedral protein by a larval midgut extract could be obtained only when the pH of the reaction mixture was raised to an unnatural level of 10.5, the natural pH of the midgut content being 8.5 or 9.5 according to different authors. The plant growth retardant Phosfon synergized mortality caused by the NPV. The virus could be cross-transmitted to two congeneric species of Spodoptera (S. exigua and S. litura), but could not infect any of four tested species belonging to other genera of the Moctuid family.     

Exploration into a mechanism of transgenerational transmission of nucleopolyhedrovirus in Lymantria dispar L. in Western Siberia

The mechanisms of nucleopolyhedrovirus (NPV)transmission through gypsy moth (Lymantriadispar L., Lepidoptera: Lymantriidae)populations in Western Siberia wereinvestigated. Field and laboratory studies werecarried out during 1992–1998 in a gypsy mothoutbreak in the Novosibirsk Area (WesternSiberia), Russia. The damaged trees were primarilytwo species of birch, Betula pendula Roth. and B. pubescensEhrh. Investigations were performed at threestages of the outbreak: population increase(1992–1993), population peak (1994–1995) andpopulation decline (1996–1998). Experiments onactivation of occult virus by different stressfactors were carried out in order to clarifythe mechanism of transgenerational transmissionof NPV. In both laboratory and fieldexperiments NPV-caused mortality levels werelow; most insects did not acquire a lethal doseof virus. Occult virus can provide an importantroute of transgenerational NPV transmission,particularly in Western Siberia where gypsymoths migrate by female flight and can moveaway from trees contaminated by virus fromprevious larval infections.     

Foraging in a pathogen reservoir can lead to local host population extinction: a case study of a Lepidoptera-virus interaction

In 1990, natural infestations of the polyphagous vapourer moth, Orgyia antiqua (Lepidoptera: Lymantriidae) in lodgepole pine plantations in northern Scotland, were studied to ascertain the role of host foraging behaviour on the prevalence of nucleopolyhedrovirus (NPV; Baculoviridae) infection in the population. Aerial dispersal of early instar larvae (L1–L3) from the tree canopy onto heather foliage at the forest understorey, with subsequent relocation back onto the tree as late-instar larvae (L4–L6) appeared to play a significant role in the development of a widespread virus epizootic in which approximately 80% of L4–L6 individuals succumbed to disease. Bioassays of foliage 1 year later showed that the distribution of NPV followed a pronounced vertical gradient through the forest canopy culminating in high concentrations of virus in the forest understorey. Experimental systems comprising potted pine trees positioned above heather bases showed that NPV infections could be acquired by early stage larvae following dispersal from the tree and feeding on the undercanopy vegetation, then translocated to the tree component for secondary transmission to susceptible tree-feeding individuals. Behavioural studies indicated that the tendency for first-, second- and third-instar larvae to disperse to the understorey was probably not influenced by larval density on the tree but was strongly dependent on larval instar. In contrast, the tendency for larvae to relocate from the understorey heather to the tree was affected by both larval density and larval instar, suggesting that both these factors may significantly affect virus acquisition, translocation and transmission in the host population. In the present study, the heather understorey appeared to act as a pathogen reservoir in which virus could persist between host generations. Spatial heterogeneity in virus distribution combined with host foraging behaviour (dispersal and feeding) resulted in the pathogen playing a major role in host population dynamics over an extended time period (3 years). The reservoir theory is supported by the observation that similar dynamics were not observed in O. antiqua populations at neighbouring sites which lacked understorey food plants. Received: 8 June 1998 / Accepted: 5 October 1998     

Insecticidal effect on a population of Spilarctia obliqua (Lepidoptera: Arctiidae)

Survivors of Spilarctia obliqua derived from 3^rd instars that had ingested LC₅₀ of imidacloprid (0.025%), dichlorvos (0.014%) and endosulfan (0.012%) were studied through the life table method. Survivorship was reduced in insecticide exposed populations as compared to 45‐day in the control groups. Egg hatching was significantly decreased for the insecticide treated populations. Total larval mortality was the highest for endosulfan (36.76%) as compared to the individuals that died among control groups (14.29%). Life expectancy (e_x) was decreased gradually over time and stage of development for insecticides tested and the control groups. Dichlorvos (0.014%) has caused a significant reduction in the potential fecundity i.e. 315 females/female/generation, while 415 in the unexposed cohorts. Net reproductive rate (R_o) was the lowest in endosulfan (118.47 females/female/generation) treatment followed by dichlorvos (141.97), imidacloprid (144.49) and the control groups (272.42). Similarly, the intrinsic rate of increase (r_m) was substantially decreased after exposure with endosulfan (0.124 females/female/day) as compared to 0.135 in the unexposed cohort. The finite rate of increase (λ) was not significantly different among the insecticide treatments and control groups. Mean generation time was significantly reduced after exposure to dichlorvos (37.19 days). However, 41.34 days were required to complete one generation by S. obliqua in the absence of insecticides. Based on these results, population of S. obliqua would double in 5.19 days under the influence of dichlorvos while requiring 5.13 days for unexposed cohorts.     

Impact of prey infected by nuclear polyhedrosis virus on a predator, Sycanus leucomesus Walk. (Hem., Reduviidae)

Abstract: An assassin bug, Sycanus leucomesus Walk. (Hem., Reduviidae) fed on nuclear polyhedrosis virus (NPV)-infected larvae of Spodoptera litura Fabr. (Lep., Noctuidae) inflicted variable effect on its developmental biology. The impact varied with the duration the predators were exposed. The predators that were fed infected prey throughout their lives appeared normal but they had significantly smaller sizes of head capsules and shorter tibial lengths than those fed healthy prey. These individuals had their overall pre-imaginal survival rate reduced by 10% from 91.25 to 81.25%, pre-oviposition period prolonged to more than 12 days, longevity reduced by 10 and 19 days in the males and females, respectively, and fecundity reduced to 41%. Even though ingestion of NPV could adversely influence their biology, the predators could serve as disseminators of the pathogen. The predators defecated infective viral polyhedral inclusion bodies (PIBs) ranging from 1.22 to 1.60 × 10¹⁰/deposit with an average of 1.42 × 10¹⁰. This NPV, when fed to its original host, caused 97% larval mortality within 10 days. Thus in nature, S. leucomesus may suppress the pest population not only by predation but also by dispersing the pathogen.     

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.     

An RNA virus in Autographa californica nuclear polyhedrosis virus preparations: Incidence and influence on baculovirus activity

A small RNA virus infectious to Trichoplusia ni larvae (TRV) was observed as a contaminant of several Autographa californica nuclear polyhedrosis virus preparations (AcMNPV). The extent of contamination in various AcMNPV preparations was studied by means of serial enrichment passages through T. ni larvae and enzyme-linked immunosorbent assay (ELISA). TRV could not be detected by ELISA in the original preparation of AcMNPV polyhedra prepared in 1968 even after five enrichment passages. Antibody inactivation offers a possible prophylactic method against TRV but temperature inactivation (55°C) does not. Although TRV reduced larval weight, it had little or no effect on bioassays of AcMNPV to T. ni and Heliothis virescens.     

The role of food plant and pathogen-induced behaviour in the persistence of a nucleopolyhedrovirus

Insect baculoviruses can survive between epidemics as infectious particles external to the host. Many pathogens persist in reservoirs, i.e., microhabitats where survival is enhanced, for example due to protection from the degrading effects of UV irradiation. However, the probability of infecting new susceptible hosts is usually reduced. Persistence of pathogens and their movement in and out of reservoirs is an important, albeit little understood, aspect of insect pathogen ecology. This study investigated interactions between the behaviour of infected insect hosts, virus distribution and plant species on the persistence of the winter moth (Operophtera brumata) nucleopolyhedrovirus. Habitat influenced the persistence of infectious baculovirus in the field: virus on Sitka spruce (Picea sitchensis) and oak (Quercus robur) in forested areas retained more infectivity than virus on heather (Calluna vulgaris) in an unshaded habitat. Plant species per se did not directly affect the persistence of virus on the foliage of potted seedlings. Virally infected insects had altered behaviour and moved down plants relative to control insects, whereas in other systems larvae show height-seeking behaviour. Consequently, the majority of virus particles were distributed on plant stems. In two experiments (one using winter moth NPV and one Mamestra brassicae NPV) virus persisted better on plant stems relative to foliage. Neonate larvae were shown to be able to acquire infections from tree stems contaminated with a low level of virus. These data suggest that plant stems may be important reservoirs for between-year persistence of this pathogen. The observed virus-induced changes in host behaviour in winter moth could enhance the viral persistence by increasing the deposition of occlusion bodies in these reservoirs.     

Contribution of a parasitoid species to multiplication and transmission of a multiple nucleopolyhedrovirus in caterpillars

Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Microplitis pallidipes are both used as biocontrol agents of the beet armyworm (Spodoptera exigua). However, it has not been determined how beet armyworms respond when these agents interact. Here, we studied the effects of M. pallidipes on virus multiplication and transmission using quantitative detection of SeMNPV. Our results indicated that parasitoids promoted virus multiplication in caterpillars (10⁵ copies per caterpillar) and that it was more advantageous when the M. pallidipes oviposited one day prior to infection with NPV. Interestingly, SeMNPV was transmitted by M. pallidipes in four ways. Transmission efficiency was higher for parasitoids whose body surfaces were contaminated with NPV, and for parasitoids ovipositing on NPV-infected caterpillars, than for those emerging from NPV-infected caterpillars, or feeding on mixtures of honey, water and NPV. Our study reveals that parasitoids do affect the proliferation and transmission of NPV in caterpillars and suggests that M. pallidipes could be used to strengthen the effectiveness of SeMNPV as a biocontrol agent.     

Sex‐specific distribution and diet of Platichthys flesus at the end of spawning in the northern Baltic Sea

This study examined the relationship of seascape structure, prey availability and sex on the post‐spawning distribution and diet of European flounder Platichthys flesus in the northern Baltic Sea. The objectives were to determine whether: (1) wave exposure and substratum affect abundance and distribution of P. flesus, (2) diet reflects the benthic prey composition and (3) sex affects the distribution or diet of P. flesus. The results showed that P. flesus was evenly spread in the archipelago with no correlation to wave exposure. The distribution was, however, sex specific; reproductive males dominated the exposed zone and mainly post‐reproductive females dominated the intermediate and sheltered zones. Platichthys flesus fed mainly on two bivalve prey species: blue mussels Mytilus edulis and Baltic tellins Macoma balthica. Hard substratum invertebrates dominated the diet in all habitats and apart from some typical soft substratum species, there was no clear link between fish feeding and the dominance structure of benthic prey. Diet was further sex specific, with females showing a broader range of diet than males. Results suggest that P. flesus is a specialist molluscivore found commonly and equally in soft‐ and hard‐substratum habitats throughout the archipelago area. Previous studies on P. flesus in the Baltic Sea have yielded inconsistent results regarding diet and it has commonly been believed that the distribution of Baltic Sea P. flesus is linked to sand and soft substrata. The present findings emphasize the importance of including the entire range of habitats when diet and regional species distributions are assessed.     

The effect of magnesium ions on the solubility of polyhedral inclusion bodies and its possible role in the inactivation of the nuclear polyhedrosis virus of Spodoptera littoralis by the cotton leaf gland exudate

Polyhedral inclusion bodies of Spodoptera littoralis nuclear polyhedrosis virus (NPV) treated with solutions of cotton leaf gland exudate (a process which has been shown to inactivate the NPV), exhibited a marked decrease in solubility in dilute alkali. This process is reversible with EDTA and can be simulated by MgCI₂. The effect of magnesium ions on solubility did not involve the enzyme, alkaline protease. It is suggested that this decrease in solubility may be involved in the mechanism by which the cotton leaf inactivates the NPV. However, since MgCI₂ alone could not be shown to inactivate NPV in vitro it must be assumed that other constituents of the leaf gland exudate also play a part.     

Influence of native ants on arthropod communities in a vineyard

• 1 Ants can have a range of effects on arthropods in crops, including suppressing herbivores such as caterpillars. However, ants can also increase hemipteran densities while reducing natural enemy numbers. In vineyard ecosystem, the effects of native ants and their interactions with other arthropods are poorly understood.
• 2 An ant‐exclusion experiment was designed to test the impact of native ants on both canopy and ground arthropods concurrently. The potential influence of ants on predation and parasitism of light brown apple moth (LBAM) eggs, a grape pest, was also examined. Adult grapevine scale insects and earwigs under bark were counted after a season of ant‐exclusion.
• 3 Among 23 ground ant species collected, six were found to forage in the canopy, with two Iridomyrmex species being the most commonly encountered.
• 4 There was no difference in the abundance of most arthropod orders and feeding groups between ant‐excluded and control vines, although ground spiders were more abundant under ant‐excluded vines, despite increased ground ant foraging pressure. LBAM egg parasitism and predation were low and probably affected by weather and other arthropods. Ant exclusion did not reduce survival of scale insects, although the distribution and abundance of scale insects were negatively associated with earwigs.
• 5 In conclusion, native ants did not consistently suppress arthropod assemblages, including natural enemies, and they did not promote the survival of scale insects. Interactions among native ant species within a vineyard might minimize their effects on other arthropods, although this needs further study.

     

An in vitro method for increasing UV-tolerance in a strain of Helicoverpa armigera (Lepidoptera: Noctuidae) nucleopolyhedrovirus

A method for increasing tolerance to ultraviolet (UV) radiation in a strain of nucleopolyhedrovirus of cotton bollworm, Helicoverpa armigera (Hübner) (HearNPV) using a solar simulator is described. The Coimbatore isolate (CBE I) of HearNPV was subjected to a five-step sequence of selection to increase its UV tolerance. Each step consisted of irradiation of wet deposits of the virus to near UV (at energy level of 300W/m²), bioassay against second instar H. armigera larvae and propagation in early fifth instar larvae. Selection steps carried out at 15, 30, 60 and 90 minutes of exposure revealed that the continuous exposure of HearNPV-CBE I at low doses of UV irradiation (270–540 KJ/m²) did not significantly affect the virus activity as measured by its biological activity against second instar larvae. Selection at higher doses (1620 KJ/m²) led to loss of viral activity in the first two exposure cycles; however, there was retention of virulence coupled with increased tolerance to UV doses from third cycle onwards. Further, studies on the persistence of UV tolerant strain of HearNPV-CBE I in comparison with original strain showed that the tolerant strain had more persistence even after 7 days of weathering both under exposed (18% original activity remaining) and shaded (26% original activity remaining) condition on potted cotton plant.