Lethal Time Values Research Articles

Effect of nutrition on growth and virulence of the entomopathogenic fungusBeauveria bassiana

Three isolates of the entomopathogen Beauveria bassiana along with one strain of Metarhizium anisopliae were cultured on seven media with different carbon/nitrogen (C/N) ratios. The effect of nutrition on virulence of the isolates was evaluated via measurement of colony growth, spore yield, germination speed, conidial C/N ratio and Pr1 (a serine protease) activity. 'Osmotic stress' medium produced the lowest colony growth with low numbers of conidia in all isolates. However, these conidia showed a high germination rate and virulence. However, conidial Pr1 activity was low in some isolates. In most but not in all cases conidia from 1% yeast extract, 2% peptone and low (10 : 1) C/N medium had higher Pr1 activity compared with conidia from other media. However, in some instances we could not conclude that there was a relationship among germination rate, conidial Pr1 activity and virulence. C/N ratio of conidia was statistically different among various media and fungal isolates. Conidia with lower C/N ratio generally produced lower LT(50) (lowest median lethal time) values (more virulent). Insect-passaged conidia from different media had lower C/N ratio compared with similar conidia from artificial cultures. Therefore, they should be more virulent than in vitro produced conidia. As germination rate, conidial Pr1 activity and C/N ratio are independent of host, it seems that host-related determinants such as insect cuticle and physiology and environmental conditions may influence host susceptibility and therefore fungal isolate virulence towards host insects.

Use of Residue Profile Analysis to Identify Modes of Insecticide Activity Contributing to Control of Plum Curculio in Apples

Residue profile analysis techniques were developed, along with laboratory and field-based bioassays to describe the modes of insecticidal activity responsible for the control of the plum curculio, Conotrachelus nenuphar (Herbst), in apples (Malus spp.). Adult plum curculios were treated in laboratory topical bioassays to determine acute contact activity and lethal time for five insecticides. Azinphosmethyl had the highest levels of toxicity and shortest lethal time values, followed by the neonicotinoids thiacloprid, thiamethoxam, and imidacloprid, whereas indoxacarb had the highest LD50 and LT50 values for topical exposure. Field-based residual activity bioassays assessed adult mortality, and fruit and leaf injury from plum curculio exposed to 4 h, 7 d, and 14 d field-aged residues. All compounds caused significant levels of mortality to plum curculio when adults were exposed to fruit clusters 4 h post-application. Thiacloprid, thiamethoxam, and imidacloprid showed oviposition deterrence, antifeedant, and repellency effects in the 7- and/or 14 d residual bioassays and protected fruit in the absence of significant lethal activity. Indoxacarb maintained lethal activity throughout the study intervals, with the incidence of plum curculio feeding, suggesting that ingestion is an important mode of entry. For the neonicotinoids thiacloprid, thiamethoxam and imidacloprid plum curculio mortality was highly correlated with fruit and leaf surface residues. As surface residues declined, sublethal effects such as oviposition deterrence and antifeedant effect remained. The value of the plant–insect–chemistry triad model for describing the temporal dimensions of insecticidal modes of activity and understanding a compound’s critical performance characteristics is discussed.

Use of Residue Profile Analysis to Identify Modes of Insecticide Activity Contributing to Control of Plum Curculio in Apples

Residue profile analysis techniques were developed, along with laboratory and field-based bioassays to describe the modes of insecticidal activity responsible for the control of the plum curculio, Conotrachelus nenuphar (Herbst), in apples (Malus spp.). Adult plum curculios were treated in laboratory topical bioassays to determine acute contact activity and lethal time for five insecticides. Azinphosmethyl had the highest levels of toxicity and shortest lethal time values, followed by the neonicotinoids thiacloprid, thiamethoxam, and imidacloprid, whereas indoxacarb had the highest LD50 and LT50 values for topical exposure. Field-based residual activity bioassays assessed adult mortality, and fruit and leaf injury from plum curculio exposed to 4 h, 7 d, and 14 d field-aged residues. All compounds caused significant levels of mortality to plum curculio when adults were exposed to fruit clusters 4 h post-application. Thiacloprid, thiamethoxam, and imidacloprid showed oviposition deterrence, antifeedant, and repellency effects in the 7- and/or 14 d residual bioassays and protected fruit in the absence of significant lethal activity. Indoxacarb maintained lethal activity throughout the study intervals, with the incidence of plum curculio feeding, suggesting that ingestion is an important mode of entry. For the neonicotinoids thiacloprid, thiamethoxam and imidacloprid plum curculio mortality was highly correlated with fruit and leaf surface residues. As surface residues declined, sublethal effects such as oviposition deterrence and antifeedant effect remained. The value of the plant-insect-chemistry triad model for describing the temporal dimensions of insecticidal modes of activity and understanding a compound's critical performance characteristics is discussed.

Fumigant toxicity of essential oils fromLaurus nobilis andRosmarinus officinalis against all life stages ofTribolium confusum

The essential oils from rosemary (Rosmarinus officinalis L.) and laurel (Laurus nobilis L.) obtained from Mersin Province in Turkey, were tested for their fumigant toxicity against all life stages of confused flour beetle (Tribolium confusum du Val.). GC-MS analysis showed that 1,8-cineole was found to be the major component of both rosemary and laurel essential oils. Vapors of rosemary and laurel essential oils were toxic to all life stages ofT. confusum. Only 65% mortality of the eggs was achieved when exposed to a dose of 172.6 mgl −1 air of rosemary essential oil at the longest exposure period (144 h); at the same dose, the pupae were the most resistant stage, with LT90 (lethal time) value of 120.2 h. The adults were the most resistant stage to laurel essential oil, with LT90 value of 77.2 h. On the basis of LT90 values, tolerance of the life stages ofT. confusum to rosemary and laurel essential oils was, in descending order: pupa < larva < adult, and larva < adult < egg < pupa, respectively. Based on the concentration × time (Ct) products (g hl −1), rosemary essential oil was more toxic than laurel to the adults and larvae ofT. confusum. However, laurel essential oil was more toxic than rosemary to the eggs and pupae. Since these essential oils need such high Ct products to obtain complete mortality ofT. confusum compared with the most commonly used commercial fumigants, it would be impossible to use them on their own as a commercial fumigant against stored-product insects.

Ovicidal and adulticidal activities of Cinnamomum zeylanicum bark essential oil compounds and related compounds against Pediculus humanus capitis (Anoplura: Pediculicidae)

The toxicity of cinnamon, Cinnamomum zeylanicum, bark essential oil compounds against eggs and adult females of human head louse, Pediculus humanus capitis, was examined using direct contact and vapour phase toxicity bioassays and compared with the lethal activity of their related compounds, benzyl alcohol, cinnamic acid, cinnamyl acetate, 4-hydroxybenzaldehyde and salicylaldehyde, as well as two widely used pediculicides, d-phenothrin and pyrethrum. In a filter-paper contact toxicity bioassay with female lice at 0.25 mg/cm 2, benzaldehyde was 29- and 27-fold more toxic than pyrethrum and d-phenothrin, respectively, as judged by median lethal time (LT 50) values. Salicylaldehyde was nine and eight times more active than pyrethrum and d-phenothrin, respectively. Pediculicidal activity of linalool was comparable with that of d-phenothrin and pyrethrum. Cinnamomum bark essential oil was slightly less effective than either d-phenothrin or pyrethrum. Benzyl alcohol and ( E)-cinnamaldehyde exhibited moderate pediculicidal activity. After 24 h of exposure, no hatching was observed with 0.063 mg/cm 2 salicylaldehyde, 0.125 mg/cm 2 benzaldehyde, 0.5 mg/cm 2 Cinnamomum bark essential oil, 1.0 mg/cm 2 ( E)-cinnamaldehyde, and 1.0 mg/cm 2 benzyl cinnamate. Little or no ovicidal activity was observed with d-phenothrin or pyrethrum. In vapour phase toxicity tests with female lice, benzaldehyde and salicylaldehyde were much more effective in closed containers than in open ones, indicating that the mode of delivery of these compounds was largely due to action in the vapour phase. Neither d-phenothrin nor pyrethrum exhibited fumigant toxicity. Cinnamomum bark essential oil and test compounds described merit further study as potential pediculicides or ovicides for the control of P. h. capitis.

Response of mixed-age cultures of phosphine-resistant and susceptible strains of lesser grain borer, Rhyzopertha dominica, to phosphine at a range of concentrations and exposure periods

Mixed-age cultures, containing all life stages, of a highly resistant strain (Strong-R) of lesser grain borer, Rhyzopertha dominica (F.), were exposed to a series of fixed concentrations of phosphine at a range of exposure periods at 25 °C. A susceptible strain and a less-resistant strain (Weak-R) were also tested. The aim was to characterise the resistant strain and determine if it could be controlled with phosphine. Times to population extinction (TPE) were recorded and lethal time (LT 99.9) values calculated. The relationship between exposure period t and phosphine concentration C for the resistant strains were: for Strong-R LT 99.9 C 0.5457 t=3.852 and TPE C 0.6105 t=4.0404 and for Weak-R LT 99.9 C 0.3553 t=3.6521 and TPE C 0.4507 t=3.4833. The results were used to define a range of minimum exposure period ×concentration protocols for control of the Strong-R populations. For example, at 1.0, 0.3, and 0.2 mg l −1 complete control can be expected in 5, 10 and 14 days, respectively. This information will be used to recommend phosphine rates for field trial and eventual registration.

Mortality of eggs of stored-product insects held under vacuum: effects of pressure, temperature, and exposure time.

Low pressure applied to a commodity creates a low-oxygen atmosphere that can be effective to control stored-product insects. Previous work determined that eggs of several species of stored-product insects were among the most tolerant life stages to low pressure. The current study was conducted to determine the mortality of eggs in response to various pressures, temperatures, and exposure times. An initial experiment determined that the sensitivity of eggs to vacuum varied with their age. Eggs of Plodia interpunctella (Hübner) were most sensitive to low pressure when they were 3 or 48 h old, whereas those of Rhyzopertha dominica (F.) were most sensitive at 12 and 120 h of age. In subsequent experiments, eggs of Cadra cautella (Walker), P. interpunctella, R. dominica, and Tribolium castaneum (Herbst) were exposed to pressures of 50, 75, 100, 200, and 300 mmHg in glass chambers at 5, 15, 22.5, 30, and 37.5 degrees C for times ranging from 12 to 168 h. Time-mortality data were subjected to probit analyses and lethal dose ratios were computed to determine differences in lethal time values among species across the 25 low pressure-temperature combinations for each species. In all four species the mortality of eggs increased with increasing exposure time and temperature. Low temperatures and high pressures were the least effective conditions for killing eggs, compared with high temperatures combined with low pressures in all species investigated. These results provide important guidelines for developing treatment schedules for disinfestation of commodities on a commercial scale.

Mortality of Eggs of Stored-Product Insects Held Under Vacuum: Effects of Pressure, Temperature, and Exposure Time

Low pressure applied to a commodity creates a low-oxygen atmosphere that can be effective to control stored-product insects. Previous work determined that eggs of several species of stored-product insects were among the most tolerant life stages to low pressure. The current study was conducted to determine the mortality of eggs in response to various pressures, temperatures, and exposure times. An initial experiment determined that the sensitivity of eggs to vacuum varied with their age. Eggs of Plodia interpunctella (Hübner) were most sensitive to low pressure when they were 3 or 48 h old, whereas those of Rhyzopertha dominica (F.) were most sensitive at 12 and 120 h of age. In subsequent experiments, eggs of Cadra cautella (Walker), P. interpunctella, R. dominica, and Tribolium castaneum (Herbst) were exposed to pressures of 50, 75, 100, 200, and 300 mmHg in glass chambers at 5, 15, 22.5, 30, and 37.5°C for times ranging from 12 to 168 h. Time-mortality data were subjected to probit analyses and lethal dose ratios were computed to determine differences in lethal time values among species across the 25 low pressure-temperature combinations for each species. In all four species the mortality of eggs increased with increasing exposure time and temperature. Low temperatures and high pressures were the least effective conditions for killing eggs, compared with high temperatures combined with low pressures in all species investigated. These results provide important guidelines for developing treatment schedules for disinfestation of commodities on a commercial scale.

Photoperiod effects on the UV-induced toxicity of fluoranthene to freshwater mussel glochidia: absence of repair during dark periods

The effect of photoperiod on the ultraviolet radiation (UV)-induced toxicity of fluoranthene to glochidia of the freshwater mussel, Utterbackia imbecillis, was evaluated using a series of static renewal toxicity tests conducted using one of four different photoperiods (24 h light, 16 h light:8 h dark, 12 h light:12 h dark, 8 h light:16 h dark). Rates of acute mortality were dependent both upon fluoranthene dose and the photoperiod. Median lethal time (LT50) values calculated on the basis of accumulated UV exposure time (UV-LT50) were compared with LT50 values calculated from real time of exposure (R-LT50) to determine relative rates of photoactivated fluoranthene damage versus physiologic repair during periods of darkness. UV-LT50 values were only dependent on fluoranthene dose and not on photoperiod. The fact that UV-LT50 values did not increase with decreasing light cycle length indicates that physiologic repair during dark periods was not an important process in these experiments. These findings suggest that (1) species-specific and/or life history-specific factors, in part, determine the ability of an organism to repair photoactivated polycyclic aromatic hydrocarbon (PAH) damage during dark periods; and (2) predictions of the UV-induced toxic response of PAH in glochidia need only be based upon total UV dose (dose rate and duration) and PAH dose.

Effects of temperature and exposure time on mortality of stored-product insects exposed to low pressure.

The prospects of using low pressure that creates a low-oxygen atmosphere to control stored-product insects were investigated in the laboratory. Eggs, larvae, and pupae of Tribolium castaneum (Herbst), Plodia interpunctella (Hübner), and Rhyzopertha dominica (F.) were exposed to 32.5 mmHg in glass chambers at 25, 33, 37, and 40 degrees C for times ranging from 30 min to 144 h. Time-mortality data were subjected to probit analyses and lethal dose ratios were computed to determine differences in lethal time (LT) values among all species-life stage combinations across the four temperatures. Eggs of each species were the life stage most tolerant to low pressure. Pupae of T. castaneum and R. dominica were more tolerant to low pressure than larvae. In all life stages, mortality increased with increasing exposure time to low pressure and also with increasing temperature. Immature stages of R. dominica were more tolerant to low pressure than immature stages of the other two species. The LT99 for R. dominica eggs was 176.32 h at 25 degrees C and that for P. interpunctella eggs was 28.35 h at the same temperature. An increase in temperature to 33 degrees C resulted in a LT99 < of 85.98 h for R. dominica and 6.21 h for P. interpunctella. Higher temperatures resulted in further significant reduction in lethal time values. Low pressure represents a simple, nonchemical alternative to fumigants such as methyl bromide and phosphine for controlling pests of stored-products or other commodities.

Effects of environmental factors on virulence of the entomopathogenic fungus, Nomuraea rileyi, against the corn earworm, Helicoverpa armigera (Lep., Noctuidae)

The effects of environmental factors on infection of the entomopathogenic fungus, Nomuraea rileyi, isolated from the corn earworm, Helicoverpa armigera, in Taiwan, to its host insect were studied in the laboratory. The fungus caused higher larval mortality at 20°C than at 30°C when 5 × 106 conidia/ml were sprayed on the fourth instar. However, mortality of the fifth instar injected with 1 × 103 conidia/larva was not significantly different when the inoculated larvae were incubated from 15 to 30°C. The fungal development in inoculated larvae was best at 20 and 25°C after shifting from 20°C to either lower or higher temperatures. The germination rate was higher at 20 and 25°C than at 30 or 35°C. Conidial germination was better on the wash‐off of insect cuticle than on Sabouraud maltose agar with yeast extract. Sporulation on chill‐dried cadavers was maximal at 95 or 100% relative humidity than at lower levels of relative humidity. The time required for sporulation was 2 days less at 100% than at 95% relative humidity. Although photoperiod did not affect fifth instar mortality caused by N. rileyi, the median lethal time (LT50) values were shorter upon incubating under light than in darkness. Incubation of infected cadavers under 12 or 24 h light resulted in 20‐fold more conidial production than under full darkness. Therefore, illumination is necessary for development of this isolate on insect cadavers.

Choristoneura fumiferana entomopoxvirus prevents metamorphosis and modulates juvenile hormone and ecdysteroid titers

Larvae of the spruce budworm, Choristoneura fumiferana, infected with C. fumiferana entomopoxvirus (CfEPV) continue to feed and grow without undergoing metamorphosis and die as moribund larvae. The lethal dose (LD 50) and lethal time (LT 50) values for fourth instar larvae are 2.4 spheroids and 25.2 days, respectively. One hundred percent of the control fourth instar larvae, which were fed water instead of virus, pupated by 18 days post feeding (PF). Only 30% of the larvae that were fed the LD 50 dose and none of the larvae that were fed the LD 95 dose pupated by 18 days PF. Of the control larvae, 95% became adults by 24 days PF, whereas in the treated group only 2% of larvae that were fed the LD 50 dose and none of the larvae that were fed the LD 95 dose became adults by 24 days PF. Some of the virus-treated larvae died as either larval/pupal or pupal/adult intermediates. These phenotypic effects were similar to the larval/pupal and pupal/adult intermediates, resulting from treating larvae with juvenile hormone (JH) or its analogs, which suggests that EPV may cause such abnormalities by modulating JH and/or ecdysteroid titers. In untreated sixth instar larvae the JH titer decreased to low levels by 24 h after ecdysis and remained low throughout larval life. EPV-fed sixth instar larvae had 2112 pg/ml on day 0, 477 pg/ml on day 1 and 875 pg/ml on day 8 of the sixth instar. Control larvae contained 860 ng of ecdysteroids per ml hemolymph on day 8 of the sixth instar, whereas EPV-treated larvae of the same age (30 days PF) had only 107 ng of ecdysteroids per ml of hemolymph. Thus, EPV infection results in increased JH titer and decreased ecdysteroid titer. Northern hybridization analysis was performed using RNA isolated from control and EPV-fed larvae and cDNA probes for (i) juvenile hormone esterase (JHE), which is JH inducible, (ii) Choristoneura hormone receptor 3 (CHR3), which is ecdysteroid inducible, and (iii) larval specific diapause associated protein 1 (DAP1), whose expression is larval specific. EPV-treated larvae showed higher levels of JHE and DAP1 mRNA and lower levels of CHR3 mRNA, indicating that they had higher levels of JH and lower levels of ecdysteroids. Thus, our data show that EPV prevents metamorphosis by modulating ecdysteroid and JH levels.

Comparative Behavioural Studies of Larval and Adult Stages of the Phytoseiids (Acari: Mesostigmata) Typhlodromus Athiasae and Neoseiulus Californicus

We compared the behaviours of the indigenous Typhlodromus athiasae Porath and Swirski and the exotic Neoseiulus californicus (McGregor) (= Amblyseius chilenensis Dosse) relative to their persistence in apple orchards in Israel. We studied (1) larval feeding, walking, intraspecific interactions (cannibalism, touch-avoidance responses and/or touching with palps and tarsi) and tendency to aggregate (when resting), (2) predation and cannibalism on phytoseiid eggs by young females and (3) the effects of starvation for 10 days on young females relative to ambulation speed, longevity, fecundity, progeny survival and sex ratio. Larvae of T. athiasae were almost inert, did not feed and hardly walked or interacted whereas larvae of N. californicus fed, walked and interacted, mainly by touching with palps and tarsi. No cannibalism in the larval stage was observed for either species. The presence of prey increased the larval walking and intraspecific interactions of N. californicus but not of T. athiasae. Egg predation by adult females of both species was substantially higher than cannibalism, implying that both are capable of distinguishing their eggs from those of other species. Soaking eggs for 30 min in deionized water increased cannibalism in both species. During the 10 days of starvation, the ambulation speed of adult female N. californicus ranged from 1.8 to 10.1 times that of T. athiasae. The 50% lethal time value (LT50) of T. athiasae (6.0 days) was significantly lower than that of N. californicus (10.4 days). None of the starved T. athiasae recuperated following the reintroduction of prey, whereas 75% of N. californicus did and oviposited after 2 days. These traits should enable N. californicus to persist when prey is scarce; however the selective predation of N. californicus eggs by T. athiasae could prevent establishment of N. californicus. The degree of specialization of these two predators is discussed.

The effects of hot-water treatments in combination with cooling and/or storage on the physiology and disease of mango fruits (Mangifera indica Linn.)

SummaryThis paper reports three separate studies. The first involved devising an effective phased hot-water disinfestation treatment on the basis of reported lethal time (LT90) values of 5–15 min at 46°C core temperature and heat units. Phased hot-water treatment at 36.5°C for 60 min plus 46.5°C for 43 min (4185 degree minutes) attained and maintained a seed (core) temperature of 46–46.5°C for 5–12 min. The impact of phased hot water was tested against the recommended treatment (46.5°C for 90 min – 4184 degree minutes) and a control, on the basis of fruit colour, taste, flavour firmness and the incidence and severity of diseases, internal and external injury and physiological disorders. Following hot-water treatments, ‘Keitt’ and ‘Tommy Atkins’ mangoes were subjected to an array of treatments including air or hydrocooling, 10 d of storage at 13°C with or without 1 d of intermittent warming at 34°C (sixth day) and 2 d ripening at 22°C respectively as a simulation of possible handling procedures. Phased hot-water treatment and 1 d of intermittent warming resulted in a significantly low incidence and severity of internal and external injury and diseases and low off-flavours compared with control and hot water at 46.5°C for 90 min. The impact of the above hot-water treatment at half heat units previously tested (2092.5 degree minutes) in combination with intermittent warming (34°C) during 12 d of cool storage (13°C) showed that treatment with hot water at 46.5°C for 45 min and 2 d of intermittent warming resulted in a significantly low incidence and severity of external injury and diseases, softer fruits, higher °Brix and better general appearance. The incidence of disease was influenced by pre-harvest factors and the effects of hot-water treatments on fruit quality was dependent on time/temperature combinations and not on total heat units received. Intermittent warming advanced fruit ripening.

In vivo recombination between two strains of the genus Nucleopolyhedrovirus in its natural host, Spodoptera exigua.

The DNA restriction map for the enzymes BamHI, BglII, PstI, and XbaI of SeMNPV-US (Se-US), the best-studied Nucleopolyhedrovirus strain isolated from Spodoptera exigua in California, was used as a reference to construct that of SeMNPV-SP2 (Se-SP2), a closely related Spanish strain of the same virus. After coinfection of S. exigua with both the Se-US and Se-SP2 strains per os, a recombinant virus (SeMNPV-SUR1 [Se-SUR1]) was detected after one passage which quickly replaced the parental viruses. A physical map of Se-SUR1 DNA was constructed for BamHI, BglII, PstI, and XbaI and compared to that of the parental viruses, Se-US and Se-SP2. Se-SUR1 is the result of at least four crossover events between Se-US and Se-SP2 and not of selection of a minor variant in one of the parental viruses. Bioassays of the recombinant and parental strains against L2 beet armyworm larvae demonstrated that their 50% lethal dose values were not significantly different. The differences in median lethal time values are too small to explain the replacement of the parental viruses by the recombinant virus upon successive passage in vivo, although it cannot be ruled out as an explanation for the selective advantage of the recombinant strain, Se-SUR1. The consequences of the release of nonindigenous or recombinant baculovirus strains in agro-ecosystems are discussed.

Probit Analysis of Correlated Data: Multiple Observations Over Time at One Concentration

We present a method to analyze serial time–mortality data from bioassay experiments where successive observations are made on the same group of organisms exposed to 1 concentration of a stimulus (for example, a pesticide). Standard probit techniques are not applicable because such data are correlated. Methods are presented for calculating statistics for the time–mortality line by regressing complementary log-log, logit, or probit transformations of proportion of responders on untransformed time or logarithmic transformations of time; and for calculating confidence limits on lethal time values corresponding to given mortality levels. As a result we have developed a computer program to facilitate use of the method.

Toxicity of different fungicides to adult Cryptolaemus montrouzieri mulsant (Coleoptera: Coccinellidae)

Toxicity and persistence of eight fungicides (carbendazim, tridemorph, mancozeb, captafol, triadimefon, Bordeaux mixture, sulphur w.p. and sulphur dust) to Cryptolaemus montrouzieri Mulsant adults was assessed by calculating the median lethal time (LT) values in days, following probit analysis. While tridemorph 0·05% was the safest, triadimefon 0·05% was most toxic with greater persistence. Hence the spraying of tridemorph 0·05% and the release of the predator could be simultaneous.

Evaluation of Three Methods for Detecting Chlorpyrifos Resistance in German Cockroach (Orthoptera: Blattellidae) Populations

Three methods for measuring German cockroach, Blattella germanica L., susceptibility to formulated chlorpyrifos (Dursban LO) were compared. Adult progeny from a cross between two field-derived colonies were compared with a susceptible laboratory strain through exposure to chlorpyrifos and xylene applied topically, and in glass jars at single and multiple doses. Resistance ratios (LX50 field ÷ LX50 susceptible) were 14.3, 7.8, and 2.0 for the topical application, multiple, and single concentration exposures, respectively. The single dose method failed to detect any differences in susceptibility (resistance ratio < 3.0). Topical application should be considered when surveying for resistance to organophosphate insecticides or when lethal time values obtained from jar test do not agree with empirical observations.

Residual toxicity of different insecticides to the adultCryptolaemus montrouzieriMulsant (Coccinellidae: Coleoptera)

Abstract Studies on the toxicity and persistence of eight insecticides and one acaricide to adult Cryptolaemus montrouzieri Mulsant are presented. Dichlorvos 0.1% was found to be the safest insecticide; synthetic pyrethroids were highly toxic and more persistent The adults of the predator exhibited toxicity to dicofol, an acaricide. Hence, the spray of dichlorvos 0.1% can be integrated with the release of the predator. The safety of the insecticides was assessed by calculating the median lethal time (LT) values in days following probit analysis.

Photoinduced toxicity of anthracene to juvenile bluegill sunfish (Lepomis Macrochirus Rafinesque): Photoperiod effects and predictive hazard evaluation

AbstractThe effect of daily light‐cycle duration (photoperiod) on the solar ultraviolet radiation (SUVR) ‐induced toxicity of anthracene to juvenile bluegill sunfish (Lepomis macrochirus R.) in a laboratory system under simulated sunlight was examined. Rates of acute mortality were dependent on both anthracene concentration and photoperiod. Median lethal time values calculated on the basis of accumulated SUVR exposure time (UV‐LT50) were compared with LT50 values calculated from real time of exposure (R‐LT50) to determine relative rates of phototoxic damage versus physiologic repair during periods of darkness. The comparison of these LT50 values indicated that the photoinduced toxicity of anthracene to fish was slowly repaired during periods of darkness but that enough damage accumulated over several SUVR cycles to cause acute mortality.The results from these experiments were incorporated into a relationship to predict no‐effect anthracene concentrations from the daily light‐cycle duration at one SUVR intensity. Information from acute toxicity tests was used to extrapolate to chronic no‐effect values. No‐effect anthracene concentrations in water were predicted to range from 1.2 μg/L for 24 h light:0 h dark photoperiod to 13.5 μg/L for a 6 h light: 18 h dark photoperiod. A no‐effect anthracene body burden of 131 μg/kg has been calculated for juvenile bluegill sunfish for a 16 h light:8 h dark photoperiod at an equivalent depth of 3.0 m in a typical eutrophic system. Thus, considering current natural polycyclic aromatic hydrocarbon concentrations in water and in fish tissue, there exists natural waters in which photoinduced PAH toxicity may occur.