Dosage-mortality and time-mortality responses of the armyworm Spodoptera exempta to a nuclear polyhedrosis virus

Third-instar Spodoptera exempta larvae were fed on young maize leaves treated with 20 μl of polyhedral inclusion body (PIB) suspension of concentrations that varied from 1.6 × 10² to 1.6 × 10⁹ PIBs/ml. Daily observations were kept on mortality rates. A probit analysis on the results gave an LD₅₀ value of 48.4 PIBs/larva (lower and upper fiducial limits 39.2 and 59.4 PIBs/larva, respectively), and an LT₅₀ that varied from 146.2 to 221.3 hr, depending on the dosage. LD and LT values obtained show the high pathogenicity of S. exempta nuclear polyhedrosis virus to its host.     

Factors affecting pathogenicity of NPV preparations to the corn earworm,Heliothis armigera

Pathogenicity ofHeliothis nuclear polyhedrosis virus (HSNPV) to the corn earworm,Heliothis armigera, was studied using 3 different inoculative methods. The LD50 values of 4^th-instar larvae inoculated with corn-fed, diet-fed and inoculum-imbiding method were 1.85×10⁶, 2.55×10⁵ and 1,22×10³ PIBs/larva, respectively. The inoculum-imbiding is more sensitive and convenient for inoculatingH. armigera with HSNPV. The HSNPV product, Elcar^®, was highly pathogenic toH. armigera, the LD50 values of 2^nd-, 3^rd- and 4^th-instar larvae being 27, 83 and 1,221 PIBs/larva, respectively, as measured by the inoculum-imbiding method. The mortality of 4^th-instar larvae caused by HSNPV was increased, but the incubation period was shortened with higher incubation temperatures. However, the high temperature at 35°C caused a lower mortality, and a prolongation of the median lethal time (LT50). Stability and persistence of HSNPV preparations were better in January–February and April–May than in June–July and October–November periods when sprayed on corn silks under field conditions. The HSNPV was inactivated by weak alkaline dew (pH 8.1) collected from soybean leaves, but it remained active on those from corn, tomato and asparagus with pH 7.2–7.3. The artificial heavy rainfall of 242 mm/h for 30 min did not wash off HSNPV preparations sprayed on the corn silks.     

Comparison of in vivo infectivity of tissue-cultured nonoccluded virus and alkali-liberated occluded virus of Baculovirus heliothis

Feeding and intrahemocelic injection studies using tissue-culture-derived-nonoccluded virus (TCNOV) and occluded virus liberated by alkaline solution (ALOV) from polyhedral inclusion bodies were conducted with the single-embedded Heliothis nuclear polyhedrosis virus, Baculo-virus heliothis (H_zSEV). Comparisons of infectivity between ALOV and NOV were based upon the number of adminstered plaque-forming-units (PFU). There was little, if any, difference in infectivity between ALOV and TCNOV of H_zSEV when injected into 4th-instar larvae of Heliothis virescens. The LD₅₀, from the multiple dose injection studies, for ALOV and TCNOV was 6.5 ± 1.2 PFU per larva and 3.4 ± 0.9 PFU per larva, respectively. Injection of a single dose (5 PFU per larva) resulted in a larval mortality of 83.2 ± 3.4 and 62.6 ± 5.7% for ALOV and TCNOV of the H_zSEV, respectively. The LC₅₀ of ALOV and TCNOV, from the multiple-dose feeding tests, was 3.1 ± 0.4 PFU/cm² and 4.5 ± 0.9 PFU/cm², respectively. Feeding 24-hr-old larvae on virus-treated diets at a single dose (50.0 PFU/cm²) resulted in a 1.5-fold difference in percentage larval mortality between ALOV (91.0 ± 4.0%) and TCNOV (61.2 ± 3.0%). Counts of viral particles (VP), based upon electron microscopy, were 14.3 ± 2.6 × 10¹⁰ and 5.2 ± 1.1 × 10⁷ VP/ml for the ALOV and TCNOV, respectively. Thus, each larva ingesting or injected with one PFU received ca. 3500 × more VP of ALOV than in did of TCNOV.     

保幼激素类似物对斜纹夜蛾核多角体病毒增殖的影响

分别用5×10⁶,1×10⁷,5×10⁷,1×10⁸ PIBs/mL 4种浓度的斜纹夜蛾核多角体病毒(SpltNPV)感染斜纹夜蛾末龄幼虫（第6龄）,并于同龄期以10μg/头的保幼激素类似物(JHA) methoprene分别对各感染组进行点滴处理,以研究JHA对SpltNPV增殖的影响。研究表明,与对照组相比各不同浓度处理组病毒总产量分别提高227.06%、128.71%、52.62%、33.15%,平均单头病毒含量分别提高49.15%、48.40%、36.40%和31.11%,病毒感染死亡率分别提高119.21%、52.72%、12.64%和1.12%。其中以1×10.7 PIBs/mL感染再经JHA处理的病毒总产量和平均单头病毒含量最高,分别为1 701.8×10⁸PIBs和60.1×10⁸ PIBs。各处理组的病毒总产量和平均单头病毒含量均显著高于其对照组。在此基础上,进一步研究了JHA对染毒与未染毒宿主消化生理的影响。结果表明,JHA处理不仅延长了6龄幼虫的寿命,增加了其取食量,而且还显著提高了幼虫的食物转化率和病毒产量。     

Effect of Host Plant on the Infectivity of Sl MNPV to Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) Larvae

The susceptibility of Spodoptera litura to SlMNPV infection was markedly affected by phyto-chemicals ingested during the acquisition of viral inoculum on foliage of tomato and cauliflower. The LD₅₀ values computed for second, third and fourth instar larvae assayed on tomato leaves were 254, 819 and 23395 PIBs/larva, respectively whereas, it was 326, 1719 and 43843 PIBs/larva for respective instars when assayed on cauliflower leaves. Thus LD₅₀ values for second, third and fourth instar larvae were 1.28-, 2.09- and 1.87- fold lower, respectively in tomato leaves. Similarly, LT₅₀ values for second, third and fourth instar larvae assayed on tomato leaves were 7.1 and 7.5 days, respectively at inoculum dose of 2.7×10⁴ PIBs/larva whereas, it was 7.7 and 8.0 days for respective instars when assayed on cauliflower leaves at same inoculum. This result also showed that the S. litura were more susceptible on tomato leaves in comparison to cauliflower leaves as the time required for mortality was lower in tomato leaves. The possible biochemical bases for differential level of mortality of S. litura larvae on tomato and cauliflower crops needs to be investigated.     

Bioassay of nucleopolyhedrosis virus against larval instars of Malacosoma neustria

The relative susceptibility of third- and fourth-instar Malacosoma neustria larvae to a nucleopolyhedrosis virus was studied by bioassay in the laboratory. Larval mortality increased with increased dosage, whereas the dosage-incubation relationship was reversed. Larval age inversely affected mortality and incubation. The computed LD₅₀'s for third- and fourth-instar larvae were 1,405 and 12,320 polyhedral inclusion bodies (PIB's)/larva. The median lethal doses calculated as number of PIB's/mg body weight showed that third-instar larvae were only twice more susceptible to virus than fourths. The LT₅₀ values for 3 × 10⁵, 3 × 10⁴, and 3 × 10³ PIB's/larva were 5.9, 6.58, and 8.15 days, respectively, in third-instar assay; the corresponding figures for the two highest concentrations were 9.3 and 10.7 days in the older larvae. Lethally infected individuals died after one or, exceptionally, two molts. No correlation was found between pupal weight or adult emergence of survivors and the virus dose administered to the larvae.     

Susceptibility of Hyphantria cunea to a granulosis virus isolated from Diacrisia virginica

The dosage-mortality response of Hyphantria cunea larvae to a granulosis virus isolated from Diacrisia virginica was studied. Serial decimal dilutions of the D. virginica granulosis virus were fed to early second-instar H. cunea larvae. the LD₅₀ for this virus (7.9 × 10⁵ capsules/larva) was significantly greater than the LD₅₀ calculated for the H. cunea granulosis virus (7.06 × 10⁴ capsules/larva) against the same instar of H. cunea. Time mortality studies demonstrated that the LT₅₀ values for the D. virginica granulosis were four to six times greater than for comparable dosages of H. cunea granulosis virus. Based on the mortality response of assayed H. cunea, it is possible to distinguish between the D. virginica granulosis virus and the H. cunea granulosis virus.     

Cell Culture Derived AgMNPV Bioinsecticide: Biological Constraints and Bioprocess Issues

We have studied parameters for optimizing the Spodoptera frugiperda (Sf9) cell culture and viral infection for the production of Anticarsia gemmatalis multiple nucleopolyhedrosis virus (AgMNPV) polyhedra inclusion bodies (PIBs) in shaker-Schott or spinner bottles and bioreactors. We have assayed the k_La of the systems, initial cell seeding, cell culture volume, dissolved oxygen (DO), multiplicity of infection (MOI), nutrients consumption, and metabolites production. The medium surface oxygen transfer was shown to be higher in shaker bottles than in spinner ones, which was in direct correlation to the higher cell density obtained. Best quantitative performances of PIBs production were obtained with a SF900II medium volume/shaker-bottle volume ratio of 15% and MOI of 0.5 to 1 performed at a cell concentration at infection (CCI) of 1 to 2.5×10⁶ cells/ml in a medium containing enough glucose and glutamine. Upon infection, a decrease in the cell multiplication was observed to be dependent on the MOI used, and the μX at the exponential growth phase in infected and non-infected cultures were, respectively, of 0.2832 and 0.3914 (day⁻¹). The glucose consumption and lactate production were higher in the infected cultures (μGlucose and μLactate of, respectively, 0.0248 and 0.0089×10⁻⁸ g/cell×day in infected cultures and 0.0151 and 0.0046×10⁻⁸ g/cell×day in non infected ones). The glutamine consumption did not differ in both cultures (μGlutamine of 0.0034 and 0.0037×10⁻⁸ g/cell×day in, respectively, infected and non infected cultures). When a virus MOI of 0.1 to 1 was used for infection, a higher concentration of PIBs/ml was obtained. This was in direct correlation to a higher cell concentration present in these cultures, where a decrease in cell multiplication due to virus infection is minimized. When a MOI of 1 was used, a more effective decrease in cell multiplication was observed and a lower concentration of PIBs/ml was obtained, but with the best performance of PIBs/cell. Correlations between MOI and CCI indicate that a MOI 0.1 to 1.4 and a CCI of 10⁶ to 2×10⁶ cells/ml led to the best PIBs production performances. The virulence of PIBs produced in cultures infected at low or high MOI showed comparable DL₅₀. Culture and infection in scaling-up conditions, performed in a bioreactor, were shown to provide the cells with a better environment and be capable of potentially improving the shaker-Schott findings. For an accurate qualitative control of PIB virulence, hemolymph from AgMNPV infected Anticarsia gemmatalis was used as starting material for passages in Sf9 cells. These led to a loss of virulence among the PIBs with an increase in the DL₅₀. The loss of virulence was accompanied by a loss in budded virus titer, a decreased number of PIBs produced and an altered DNA restriction pattern, suggesting the generation of defective interference particles (DIPs). Transmission electron microscopy (TEM) studies revealed that after cell passages, PIBs lacking virions were progressively synthesized. The study described here point out the biological constraints and bioprocess issues for the preparation of AgMNPV PIBs for biological control.     

The infectivity of a nuclear polyhedrosis virus of the velvetbean caterpillar for eight noctuid hosts

Eight species of noctuid larvae were tested for susceptibility to a nuclear polyhedrosis virus of the velvetbean caterpillar, Anticarsia gemmatalis. Velvetbean caterpillar larvae were highly susceptible to crude preparations of polyhedral inclusion bodies (PIBs; LD₅₀ = 4.7 PIBs/larva), but preparations of purified polyhedra were much less effective against these larvae (LD₅₀ = 319.7 PIBs/larva). Of seven other noctuid species tested, only Heliothis virescens was as susceptible to the virus as A. gemmatalis. High dosages were required to kill Heliothis zea, Trichoplusia ni, Pseudoplusia includens, and Spodoptera ornithogalli. Plathypena scabra and Spodoptera frugiperda were not susceptible.     

Virulence of entomopathogenic fungi against Culex pipiens: Impact on biomolecules availability and life table parameters

Culex pipiens mosquitoes considered as vectors for many arboviruses such as the West Nile virus and encephalitis virus showing a global impact on human health. The natural management of the aquatic stages of this pest is crucial for maintaining an insecticide-free and sustained environment. The present work focused on studying the biological and biochemical effects of the entomopathogenic fungi: Metarhizium anisopliae, Beauveria bassiana, and Paecilomyces lilicanus, against 3^rd instar larvae of Culex pipiens laboratory colony. The results revealed that M. anisopliae showed maximum larval mortality (88%) with the lowest lethal time (LT₅₀) (22.6 hrs) at 10⁸ spores/ml followed by B. bassiana (73.33%) with LT₅₀ (38.35 hrs), while P. lilicanus showed minimum percent mortality (65%) with highest LT₅₀ (51.5 hrs). The median lethal concentration (LC₅₀) values were found to be 1.027 × 10⁵ spores/ml for M. anisopliae, 1.24 × 10⁶ spores/ml for B. bassiana, while it was 8.453 × 10⁶ spores/ml for P. lilicanus. A reduction in female fecundity, number of hatched eggs, pupation and adult emergence percentage were recorded. The biochemical analysis of the treated larvae revealed different quantitative decrease in total soluble proteins, lipids, and carbohydrate hydrolyzing enzymes compared to control. Histopathological effects of fungal infection upon insect cuticles, muscles, and midgut were investigated. Based on the obtained results, M. anisopliae proved its superior virulent effect as a bio-control agent against Cx. pipiens.     

Formulation and optimisation of various nuclear polyhedrosis virus isolates and assessment of their insecticidal activity against Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) larvae

Helicoverpa armigera (Hubner) is a pest that poses serious threat to the tomato crop in India. Larvae of this species are susceptible to the nucleopolyhedrovirus (NPV), which has attracted interest as a potential biocontrol agent. Rearing of larvae in natural diet resulted in less number of pupae, while in artificial diet more pupae and healthy adults were obtained. The NPV was tested for its insecticidal action against H. armigera in natural and artificial diets. The bioassay results of inoculated H. armigera nucleopolyhedrosis virus (HaNPV) with different concentrations indicate that the 4.0 g/l dosage caused maximum mortality (70.3% and 60.54%), and minimum mortality 46.83% and 44.08% was recorded in the 0.5 g/l dosage under laboratory and pot culture conditions, respectively. Polyhedral inclusion bodies (PIBs) were estimated using a standard haemocytometer. According to the estimate, purified PIBs were found at a concentration of 1 × 10⁹/ml. The different formulation of NPV, i.e. wettable fine powder with a hue of white, had a particle size of 100 μm with a concentration of 1 × 10⁸/ml. The encapsulated product was found to be in the form of beads and was red in colour with a concentration of 1 × 10⁸ PIBs/ml. The effectiveness of NPV against H. armigera larvae was noted at maximum LC₅₀ values at a concentration of 1 × 10⁹/ml. Qualitative analysis estimates showed that protein polyhedrin and enzyme chitinase had a molecular weight of 33 and 45 kDa, respectively. Viral DNA was isolated from NPV-infected larvae and was then separated using SDS-PAGE. The estimated polyhedrin with a molecular weight of 33 kDa, which is responsible for the increased mortality percent of larvae, was confirmed by the bioassay. Hence it was concluded that NPV is ecofriendly in the management of pests on tomato and other crops.     

Studies on the application of a nuclear polyhedrosis virus to control populations of the Egyptian cottonworm, Spodoptera littoralis

Variations in the resistance to nuclear polyhedrosis virus (NPV) were found in three populations of Spodoptera littoralis. The LD₅₀ for the most resistant population was 1.07 × 10⁴ PIB/5th instar larva as compared to 8.4 × 10² and 5.8 × 10² PIB/larva in the other two populations. The effect of NPV persisted in larvae which survived and pupated. Some of the pupae died, and those which survived produced normally shaped adults. While fecundity was sharply reduced in the less resistant populations, the effect on the most tolerant population was less pronounced. A 3-year-old inoculum, stored unprotected from daylight and without cooling, was much less effective even against the most sensitive larval population as compared to a relatively fresh and refrigerated batch. Larvae in their 6th instar proved to be approximately 10-fold more resistant to the NPV than 5th instar ones, while the difference in weight was only about twice. These variations in resistance to NPV are also discussed from the point of view of applying S. littoralis NPV in pest control schemes.     

Quantitative studies on the pathogenicity of Nosema carpocapsae,a microsporidian pathogen of the codling moth,Cydia pomonella,in New Zealand

The pathogenicity of Nosema carpocapsae for codling moth was studied using dose-infectivity experiments. The IC₅₀ (median infective concentration) was similar for the five larval instars (range 4.0 × 10³ to 6.7 × 10⁴ spores/ml). Spore loads in moths ranged from 6.0 × 10⁶ to 7.1 × 10⁷ spores per moth and varied with dose and with larval age at infection. The infection does not cause mortality but does reduce the fecundity and fertility of infected moths. Nosema carpocapsae is transmitted transovarially as well as horizontally. Infected eggs were not produced by healthy females mated with infected males, although such pairs generally produced fewer eggs than healthy pairs.     

A novel alphabaculovirus isolated from the cotton bollworm, <Emphasis Type="Italic">Helicoverpa armigera</Emphasis> (Hubner) (Lepidoptera: Noctuidae): characterization and pathogenicity

The cotton bollworm, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) is a polyphagous pest that exist all over the world. It is also very destructive in Turkey on various agricultural products. In this study, we have detected an alphabaculovirus from Helicoverpa armigera larvae collected from cotton field. The virus isolate was named as Helicoverpa armigera nucleopolyhedrovirus (HearNPV-O1) based on morphological and molecular properties. This is the first record of baculovirus from H. armigera in Eurasia region. Scanning electron microscopy examinations of polyhedral inclusion bodies (PIBs) showed their irregular morphology with average diameter of 0.85 to 1.25 μm. Transmission electron microscopy studies revealed that the nucleocapsids were multiple enveloped and bacilliform shaped. The size of nucleocapsid was found as 279 nm with a width of 56 nm. Digestion of viral genome by PstI generated 8 fragments with total size of 130.9 kbp. According to the phylogenetic analysis of the partial polyhedrin (polh), late expression factor 8 (lef8) and late expression factor 9 (lef9) genes, HearNPV-O1 isolate is very close to H. armigera MNPV Indian isolate-443. Five different concentrations of HearNPV-O1 between 10³ and 10⁷ were used in dose-response test on neonate, 3rd and 5th instars larvae of H. armigera. The highest dose (10⁷) showed 92%, 88% and 57% mortality, respectively within 14 days. LC₅₀ values of HearNPV-O1 isolate were calculated as 6?×?10⁴, 7?×?10⁴ and 8?×?10⁶ PIBs/mL^?1 against neonate, 3rd and 5th instars larval stages, respectively. These results demonstrate that HearNPV-O1 isolate may be a potential biocontrol agent to be utilized against H. armigera.     

Mass production of HzSNPV baculoviruses in immobilized<Emphasis Type="Italic">Heliothis zea</Emphasis> (HzAM1) insect cell culture

Heliothis zea (HzAM1) insect cells were immobilized in microspheres by sodium-cellulosesulfate (NaCS) and polydiallyldimethylammoniumchloride (PDADMAC). The highest HzAMI cell density was 7.5×10⁷ cells/mL in the microspheres. After infection of the immobilized cells byHeliothis zea single nuclear polyhedrosis virus (HzSNPV), the highest concentration of HzSNPV (polyhedral inclusion bodies: PIBs) produced was 2.87×10¹⁰ PIBs/mL in the microspheres.     

Further studies on the distribution of a nuclear-polyhedrosis virus of the fall webworm,Hyphantria cunea,in soil

The median infectious concentration (IC₅₀) of a nuclear-polyhedrosis virus of the fall webworm, Hyphantria cunea, in soil was 7.14×10⁷ polyhedra per gram of soil. The number of virus-killed larvae equivalent to IC₅₀ decreased with the advance of the larval instar from 22 first-instar to 0.04 seventh-instar larvae per gram of soil. The degree of virus contamination of the surface soil in an area where the virus disease had occurred naturally in host populations was estimated to be as high as IC₅₀ on the basis of soil bioassays. Nearly 1000 virus-killed seventh-instar larvae/m² were necessary to obtain such extensive soil contamination. The possibility of the occurrence of so many virus-killed cadavers in the study area was discussed in relation with the past records of epizootics and host population trends. The distribution of the fallen leaves of large trees favorable to the fall webworm, such as plane and poplar trees, correlated with that of the nuclear-polyhedrosis virus in the soil.     

Is It Feasible to Use Optical Brightener Technology with a Baculovirus Bioinsecticide for Resource-Poor Maize Farmers in Mesoamerica?

Stilbene-derived optical brighteners greatly enhance the infectivity of a number of baculoviruses. This technology has been patented for use with insect pathogenic viruses in the United States and Canada. A baculovirus is currently being tested for its potential as a biological insecticide of Spodoptera frugiperda (Lepidoptera: Noctuidae), the principal insect pest of maize in Mesoamerica. A multiply embedded nucleopolyhedrovirus isolate originally from Nicaragua was bioassayed alone and in the presence of the optical brightener Tinopal LPW (1%), using second instar S. frugiperda larvae. The LC₅₀ value of the virus alone was calculated at 82.1 polyhedral inclusion bodies (PIBs)/mm² of diet compared with 0.71 PIBs/mm² in the presence of Tinopal LPW. In contrast to other studies, the mean time to death of larvae exposed to virus and Tinopal LPW was significantly extended compared to larvae inoculated with virus alone. Analysis of the results of eight independent field trials in Mexico and Honduras revealed a significant positive relationship between log virus dose and percentage mortality observed in S. frugiperda larvae. Virus-induced mortality was approximately 50% at the highest application rate tested: 1000 larval equivalents (LE) of virus/ha. When the impact of parasitism was taken into account, larval mortality increased to 45.0–90.7% in plots treated with virus at 250 LE/ha or more. A cost analysis indicated that approximately 60% pest control can be achieved as a conservative estimate with virus application and the action of parasitoids for the price of a chemical insecticide. Formulating the virus with an optical brightener appears to be an attractive option based on laboratory findings but requires field testing. The use of optical brightener technology will probably be feasible for maize growers in Mesoamerica only if it is highly effective at very low concentrations (<0.1%) or the volume of the virus application can be reduced.     

Inhibitory effects of cadmium on carbonic anhydrase activity and ionic regulation of the estuarine crab Chasmagnathus granulata (Decapoda,Grapsidae)

This work was aimed at evaluating the gill carbonic anhydrase (CA) activity of the estuarine crab Chasmagnathus granulata exposed in vivo to cadmium, at different salinities. The in vivo effect of the specific inhibitor acetazolamide (AZ) was also assayed. Besides, the inhibition of CA activity by different heavy metals (cadmium, copper, zinc) and AZ were evaluated under in vitro conditions. For the in vivo assays, adult males were acclimated to salinities of 2.5 or 30‰. The corresponding 96-h LC₅₀ of cadmium was 2.69 mg l⁻¹ at 2.5‰, and >50 mg l⁻¹ at 30‰. Cadmium only caused a significant lower CA activity than control at 2.5‰. EC₅₀ for CA inhibition was estimated to be 1.59 mg l⁻¹ at 2.5‰. Statistical differences in Na⁺ hemolymphatic levels (P<0.05) were only detected at 2.5‰, between 0 and 1.25 mg l⁻¹ of cadmium, but no statistical differences were observed for Cl⁻ levels at any assayed salinity. As CA inhibition registered at 2.5‰ was followed by only changes in Na⁺ concentration, it is likely that cadmium exposure could differentially affect ions permeability, among others factors. The concentrations that inhibited in vitro 50% of enzymatic activity (IC₅₀) were 2.15×10⁻⁵, 1.62×10⁻⁵, 3.75×10⁻⁶ and 4.4×10⁻¹⁰ M for cadmium, copper, zinc and AZ, respectively. The comparison with IC₅₀ values of other aquatic species, indicates a higher CA sensitivity for C. granulata to pollutants.     

Effect of dose of cytoplasmic polyhedrosis virus on infection,mortality, development rate,and larval and pupal weights of the pink bollworm

Host-pathogen relationships were studied between the pink bollworm, Pectinophora gossypiella, and a cytoplasmic polyhedrosis virus (CPV). Results showed that the median effective dose (ED₅₀), the dose that infects half the test subjects, was 1.91 × 10² polyhedral inclusion bodies (PIB)/ml of diet. The median lethal dose (LD₅₀) was 1.72 × 10⁵ PIB/ml. Diagnosis for CPV infection was more reliable in adult pink bollworms than in late-instar larvae. Duration of the larval stage increased with viral dose, but duration of the pupal stage was not affected by CPV. Weights of infected male and female pupae were 23.7 and 24.0% less than those of untreated pupae, respectively. Pupal weights were not significantly influenced by increases in the viral dose. Weights of larvae of a given age decreased as dose increased. The effect of CPV on duration of the immature stages and on pupal weight was not significantly influenced by rearing temperatures between 25.0° and 32.5°C. Pupal weight of infected pink bollworms decreased as the duration of the larval stage increased.     

Susceptibility of Phthorimaea operculella Zeller (Lepidoptera; Gelechiidae) to its granulovirus PoGV with larval age

1 Granulovirus PoGV is a strong candidate to substitute for chemical insecticides in integrated pest management (IPM) of the potato tuber moth Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae). Generally, susceptibility to baculoviruses decreases with increasing larval age. For effective field applications, knowledge of the age‐related change in susceptibility is crucial. 2 The relative susceptibility of various instars of P. operculella larvae to PoGV was studied by leaf‐disc bioassays in the laboratory at 26 °C incubation temperature. The LC₅₀ increased from 4 × 10⁶ granules/mL for neonate larvae to 1.2 × 10⁷, 1.1 × 10⁸, 4.4 × 10⁸, and 4.2 × 10⁹ granules/mL, signifying resistance ratios of 3, 29, 110, and 1051, for 5, 6, 7, and 9‐day‐old larvae, respectively. 3 The relationship between log‐LC₅₀ values and log‐larval weight was significantly linear. A logistic function described significantly the relation between larval weights and physiological age measured in degree‐days (DD > 13.4 °C). Both functions allowed prediction of the activity ratio of PoGV for different larval ages by using temperature summation to express physiological age. 4 PoGV was found to be highly active against P. operculella larvae up to a physiological age of approximately 50 DD (>13.4 °C) (i.e. the first third of the total larval development time). Thereafter, the virus rapidly lost its activity against older larvae. Prospects for applying this knowledge in the field are discussed.