Residual efficacy of synergized pyrethrin + methoprene aerosol against larvae of Tribolium castaneum and Tribolium confusum (Coleoptera: Tenebrionidae)

Wheat flour and different packaging surfaces (cardboard, flour bag, muslin bag, paper bag, pallet wrap, plastic overwrap, polyethylene) were exposed to aerosol formulations of either 1% active ingredient (AI) pyrethrin (synergized with piperonyl butoxide)+ 33.6% (AI) methoprene or 3% (AI) pyrethrin + 33.6% AI methoprene. The mixture was formulated as specified on the insecticide labels to give a 100 to 1 ratio of active ingredient pyrethrin to methoprene. Residual bioassays were conducted every two weeks for 16 weeks post-exposure to the aerosol by placing four-week-old larvae of the red flour beetle, Tribolium castaneum (Herbst), or the confused flour beetle, Tribolium confusum (Jacqueline duVal), on treated flour or a treated packaging surface with untreated flour added. T. castaneum was clearly the more susceptible of the two species. Less than 2% of T. castaneum larvae exposed to aerosol-treated flour or packaging surfaces emerged as normal adults, regardless of the pyrethrin concentration. Most of the T. castaneum larvae on treated flour did not advance to the pupal stage because they were either developmentally arrested or died as larvae. They were able to develop further on the treated packaging surfaces, but still could not emerge as adults. T. confusum larvae exposed to aerosol-treated flour or packaging surfaces were able to develop to the pupal or adult stage. Emergence of normal-appearing adults from T. confusum larvae exposed on the packaging surfaces treated with 1% pyrethrin + methoprene gradually increased (range of 29.7 ± 2.9 to 49.0 ± 6.7%, depending on the surface), whereas adult emergence of larvae exposed to treated flour peaked at 10 weeks post-exposure. However, when T. confusum was exposed to 3% pyrethrin + methoprene treated flour or packaging surfaces, adult emergence was reduced. Overall there were few significant differences attributable to the individual packaging surfaces.     

Toxicity of amorphous silicas and silica-pyrethroid mixtures to Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

The toxicity of some pyrogenic and precipitated amorphous silicas to Tribolium castaneum (Herbst) was assessed in admixture with wheat grain. A good correlation (r² = 0.82, n = 7) was found between the 48 h LC50 of the toxic dusts and their adsorption capacity for a saturated hydrocarbon oil. The pyrogenic silicas Cab-O-Sil M5 and Aerosil R972 were used as carriers to prepare dust formulations of permethrin, cypermethrin and deltamethrin at various concentrations, and 48 h (Cab-O-Sil M5 and Aerosil R972) and 168 h (Cab-O-Sil M5) LC50s were determined using the same bioassay system. Low and intermediate concentrations of the pyrethroids substantially reduced the 48 h LC50s of these silicas while high concentrations antagonised their toxic action due to knockdown effects. The 168 h LC50s of Cab-O-Sil/permethrin formulations were approx 15 and 150 × lower than those of talc/permethrin formulations at 2% w/w and 0.1% w/w respectively. The 168 h LC50 of the Cab-O-Sil/cypermethrin formulation was comparable with that of the talc/cypermethrin formulation, and that of the Cab-O-Sil/deltamethrin formulation was about 5 × less than a talc/deltamethrin formulation, at 2 and 0.2% w/w pyrethroid respectively on the dust carrier.     

Toxicity of three organophosphorus compounds and pyrethrins to malathion-resistant Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae)

Malathion-resistant red flour beetles, Tribolium castaneum (Herbst), were tested for susceptibility to Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl)glyoxylonitrile oxime), diazinon, and pyrethrins by topical applications. The malathion-resistant beetles showed 13·3-fold resistance to pyrethrins, 19·7-fold resistance to Bay 78182, 7·7- and 10·7-fold resistance to diazinon, and only 4·5- and 9·5-fold resistance to Bay 77488. Only about one-quarter as much Bay 77488 as malathion was required for the ₅₀ in the resistant beetles. About one-third as much Bay 78182 as malathion was required. Diazinon and pyrethrins were less toxic than malathion to the resistant insects.     

Biochemical changes induced by fenpropathrin in the sixth instar larvae of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Sixth instar larvae of Tribolium castaneum were exposed to one of four sublethal concentrations i.e. 10, 20, 200 or 400 mg/L, of a synthetic pyrethroid, fenpropathrin, for 48 hr. The lactate dehydrogenase activity decreased 44, 15 and 10% after exposure to fenpropathrin at concentrations of 20, 200 and 400 mg/L respectively, while a significant increase was recorded in glutamate oxaloacetate transaminase (15, 16, 34 and 37%) and glutamate pyruvate transaminase (6, 20, 13 and 29%) activities respectively after exposure to 10, 20, 200 and 400 mg/L. The amylase and acid phosphatase activities remained unaffected. The trehalase activity increased 42, 72 and 149%, after 20, 200 and 400 mg/L, the isocitrate dehydrogenase activity increased 28 and 67% after 10 and 20 mg/L, white alkaline phosphatase activity increased 13 and 12% after 10 and 400 mg/L respectively. The weaker (10 and 20 mg/L) and stronger (200 and 400 mg/L) doses elicited two different types of responses. A dose of 20 mg/L resulted in increased soluble proteins (14%), lipids (49%), cholesterol (57%), RNA (18%), and DNA (32%) content per larva, while the stronger dose of 400 mg/L resulted in their decrease except for lipids. The total proteins, lipids and free amino acids content per larva were not affected by either concentration, while the glucose content per insect decreased with increasing concentration of fenpropathrin.     

Toxic and antifeedant action of nutmeg oil against Tribolium castaneum (Herbst) and Sitophilus zeamais Motsch

The essential oil extracted from nutmeg seeds using steam distillation was tested against the stored product insect pests, Tribolium castaneum (Herbst) and Sitophilus zeamais Motsch., for contact toxicity, fumigant toxicity and antifeedant activity. Filter paper impregnation was employed for contact and fumigant toxicity studies, whereas a flour disc bioassay was used to investigate antifeedant effects. Adults of S. zeamais were about ten times more susceptible than T. castaneum adults to contact action (LC₅₀ values of 1.7 mg/cm² and 18 mg/cm² respectively). However, S. zeamais adults were only 1.7 times more susceptible than T. castaneum adults to fumigant action (LC₅₀ values of 4.5 mg/cm² and 7.7 mg/cm² respectively). Furthermore, the larvae (10–16 days old) of T. castaneum were more susceptible than the adults, but the susceptibility of the larvae decreased with age. Nutmeg oil also significantly (P < 0.05) affected the hatching of T. castaneum eggs and the subsequent survival of the larvae in the concentration range 1.4–3.2 mg/cm². The production of F1 progeny of both T. castaneum and S. zeamais exposed to media treated with nutmeg oil was significantly (P < 0.05) reduced at all concentrations tested. F1 progeny production was totally suppressed at nutmeg oil concentrations of 1.05 g/100 g rice for T. castaneum and 0.35 g/100 g wheat for S. zeamais. Nutritional studies showed that nutmeg oil significantly (P < 0.05) affected the growth rate and food consumption of both insect species, depending on the concentrations used, but the antifeedant activity was more pronounced against S. zeamaisthan against T. castaneum. At 20 g nutmeg oil/100 ml, the feeding deterrence index of T. castaneum was only about 7%, whereas that of S. zeamaiswas 33%. These results suggest that nutmeg oil may be useful as a grain protectant with contact, fumigant and antifeedant activities against these insects.     

Development of a PC-based automatic monitoring system for Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in a rice warehouse

Flight traps were constructed and hung 6 m from the floor of a rice warehouse, with a 5 m polyvinyl chloride (PVC) tube attached to the base of each trap. The PVC tube channelled any Tribolium castaneum (Herbst) caught into a plastic specimen vial at the bottom. A pair of traps was placed at each of four designated locations in the warehouse. One of the pair of traps at each location had the vial resting on a top pan balance. The four balances were interfaced with an IBM-compatible personal computer. The signals from the balances were recorded as the weights of the insects by the computer sequentially and continually. The other trap at each location was set 1 m from the computer-controlled trap and acted as a reference trap. The insects collected from it were weighed manually on a daily basis. Trap catches using both methods were recorded for 50 days. The system was not influenced by temperature and r.h. changes, and dust fall was negligible. The weights of individual live beetles recorded monthly for 6 months did not change significantly. Thus the weights obtained during monitoring could be converted to the number of T. castaneum. A good correlation was obtained between the computer and manual trap catches, with a mean correlation coefficient of 0.898 (P < 0.001). These results demonstrate the feasibility of using a technique which monitors flight trap catches electronically and remotely.     

Joint action of methyl bromide and phosphine on Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

When Tribolium castaneum adults were fumigated with mixtures of methyl bromide and phosphine for 5 or 10 hr in the proportions 300:1, 100:1 and 100:3 respectively, the mortality was greater than if the fumigants acted independently (P < 0.001). At low and middle-range response levels the mixtures were less potent than would be expected under additivity of doses, but at higher response levels (≥ 80%) the effect was close to, and in some conditions slightly in excess of, additivity in the sense of exchangeability of units of effective dose. In terms of these units, after 5 hr exposure the 100:1 mixture appeared to be slightly more effective than the 100:3 mixture but when the exposure time was increased to 10 hr the higher proportion of phosphine gave an advantage. The 300:1 mixture was in general less effective than the other two.     

The toxic action of phosphine in combination with some alkyl halide fumigants and carbon dioxide against the eggs of Tribolium castaneum Herbst (Coleoptera: Tenebrionidae)

The toxicity of phosphine, three alkyl halide fumigants (ethylene dibromide, methyl bromide and methyl iodide), carbon dioxide, mixtures of phosphine + alkyl halide fumigants, and phosphine + carbon dioxide, to 1-, 2-, and 3-day-old eggs of Tribolium castaneum Herbst was studied. Combinations of phosphine and alkyl halide fumigants showed antagonism as evidenced by their joint action ratios at the LD₉₀ (< 1). The mortality data indicated that mixtures of phosphine and carbon dioxide were more effective than either alone. At LD₅₀, the order of toxicity of the individual fumigants for a 24 hr exposure and on a weight basis (mg fumigant/litre of air) was: phosphine > methyl iodide > ethylene dibromide > methyl bromide. A decline in susceptibility of eggs with age for methyl bromide and ethylene dibromide and a peak tolerance at 2 days for carbon dioxide were noted.     

High temperature combined with carbon dioxide enriched or reduced oxygen atmospheres for control of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Heat combined with controlled atmospheres increased mortality of Tribolium castaneum (Herbst) larvae when temperatures of ≥38°C were combined with carbon dioxide enriched or oxygen deficient atmospheres. Preconditioning of larvae to treatment temperature for 24 or 48 hr prior to controlled atmosphere treatments at 38°C significantly reduced larval mortality relative to a 1 hr preconditioning. Heat combined with controlled atmospheres can significantly reduce treatment time. However, if heat is applied prior to the application of controlled atmospheres, the time of treatment must be extended.     

Insecticidal properties of eugenol, isoeugenol and methyleugenol and their effects on nutrition of Sitophilus zeamais Motsch. (Coleoptera: Curculionidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Eugenol, isoeugenol and methyleugenol showed similar contact toxicity to Sitophilus zeamais (LD₅₀ values approximately 30 μg/mg insect). For Tribolium castaneum, the order of potency of these chemicals compared by the LD₅₀ level was isoeugenol>eugenol>methyleugenol. Furthermore, S. zeamais was generally more susceptible than T. castaneum to these three compounds except for isoeugenol based on the LD₅₀ level. When sublethal doses (doses that produce no appreciable mortality) of eugenol, isoeugenol and methyleugenol were topically applied to adults of either species of beetles pre-treated with deltamethrin, the toxicity of deltamethrin was enhanced. A flour disc bioassay using no-choice tests was employed to assess, in terms of food consumption and growth rate, the susceptibility of S. zeamais and T. castaneum to media treated with eugenol, isoeugenol, and methyleugenol. Only eugenol significantly (P<0.05) reduced food consumption (RCR) in the adults of S. zeamais at a concentration of 13.2 mg/g food. However, it reduced the growth rate (RGR), food consumption and food utilisation (ECI) in adults and larvae of T. castaneum at concentrations of 35 and 99 mg/g food, respectively. Isoeugenol reduced the RGR and RCR in S. zeamais adults and T. castaneum adults and larvae, in a concentration-dependent manner, but only reduced ECI in the adults of T. castaneum at the concentration of 76 mg/g food. In comparison, methyleugenol reduced the RGR and RCR in the three groups of insects, and the ECI of S. zeamais adults and T. castaneum larvae.     

Influence of malathion resistance status on survival and growth of Tribolium castaneum (Coleoptera: Tenebrionidae), when fed on flour from insect-resistant and susceptible grain rice cultivars

The effect of insecticide resistance on the potential for control of stored product insect pests based on plant resistance was evaluated. Flours made from three rice cultivars (cvs.) that showed resistance or susceptibility to feeding by Tribolium castaneum were studied, using two strains, resistant and susceptible to the organophosphate insecticide, malathion. Flours made from rough, brown and milled rice were used to help identify the locus of any cultivar resistance mechanisms. The insect-resistant (Dawn) and moderately resistant (Lebonnet) cvs. had fewer eggs laid and caused greater mortality of the insects than the susceptible cultivar (cv.) (IR8). For all cvs., flour from brown rice was the most suitable for insect survival, whereas the highest larval weights were found in milled and the lowest in rough rice flours. On the other hand, the highest mortality of the susceptible strain occurred in milled rice flours. The larvae resistant to malathion survived significantly better than those of the susceptible strain, when fed on flour from the insect-resistant cv. (Dawn). The cause of this positive interaction between the insecticide resistance status of T. castaneum and the grain resistance is not known. The mechanism of malathion resistance occurring in this strain is enhanced malathion-specific carboxylesterase activity but it is not known how this can confer survival benefits, when the insect feeds on flour from the resistant cv. These results could have practical implications for the efficacy of stored product pest control using varietal resistance in situations where insecticide resistance is prevalent.     

Efficacy of chlorfenapyr against Tribolium castaneum and Tribolium confusum (Coleoptera: Tenebrionidae) adults exposed on concrete, vinyl tile, and plywood surfaces

The insecticidal pyrrole chlorfenapyr was applied to concrete, vinyl tile, and plywood surfaces, at an application rate of 1.1 g AI m⁻². Adult Tribolium castaneum (Herbst), the red flour beetle, and adult Tribolium confusum (Du Val), the confused flour beetle, were exposed for 2 and 4 h, removed, and held without food for 7 d post-exposure. All beetles survived the initial exposures, but survival of both species decreased during the 7-d holding period, with T. confusum being the more susceptible species. Survival was generally lower on concrete than on tile or plywood, and was greatly reduced on all three surfaces after 4 h of exposure compared to 2 h. Survival of T. castaneum after 2 h of exposure on concrete, tile, and plywood was 2.5±2.5%, 25.5±15.4%, and 40.0±7.1%, respectively, after 7 d. In contrast, all T. confusum exposed on concrete and tile were dead after 4 and 5 d, respectively, while survival on plywood after 7 d was 20.0±16.8%. After 4 h of exposure, all T. castaneum and T. confusum exposed on concrete and tile were dead after 2–4 d post-exposure, while survival on plywood after 7 d was 41.5±6.4% and 0 for each species, respectively. Non-linear and linear regressions were fit to the data for both species. Results show exposure to chlorfenapyr is effective against T. castaneum and T. confusum, but efficacy will vary depending on the surface substrate.     

The effect of carbon dioxide gas alone or in combinations on the mortality of Tribolium castaneum (Herbst) and T. confusum du Val (Coleoptera, Tenebrionidae)

Adults of Tribolium castaneum (Herbst) and T. confusum du Val exposed to various mixtures of N₂ or He and O₂ were killed when the O₂ concentration reached to 1·7% or below, whereas the adults exposed to CO₂ : O₂ mixtures were killed mostly due to the deleterious effects of CO₂ itself. At 26·7°C and 38 ± 6%r.h., 95 per cent mortality of T. confusum adults was obtained by an exposure of 271 hr to 45% CO₂ : 55% air mixture; 58 hr to 62% CO₂ : 38% air mixture; and 47·5 hr to 80% CO₂ : 20% air mixture, while for T. castaneum adults 95 per cent mortality required 192, 60, and 44 hr respectively. Data obtained by exposing mature and immature stages of both species to 100% CO₂ suggest that adults were the most susceptible, followed by larvae, eggs and pupae. Generally speaking, inactive stages (egg and pupal) were more tolerant to CO₂ than active stages (larval and adult). Increasing the temperature from 15·6° to 26·7°C resulted in increased insect mortalities; the degree of response varying in different stages. Increasing r.h. decreased the susceptibility of adult insects to 100% CO₂.

Under airtight conditions 200 adult T. castaneum with 8 g of food medium sealed in 1·2 1. glass flasks depleted the O₂ supply from 20·9% to a critical 1·7% level in 7 days, and adults of T. confusum depleted to the 1·6% level in 5 days of airtightness, and both species produced about 20% of carbon dioxide gas.     

A new species of Tribolium from North America previously confused with Tribolium madens (Charp.) (Coleoptera: Tenebrionidae)

The species of Tribolium from North America previously identified as T. madens is shown to be distinct from the true T. madens (Charp.) from the Old World and is described as T. audax sp.n. A lectotype is designated for T. madens. Both species have been found in stored products and associated with bees. Characters are given to distinguish the two species in the adult, pupal and larval stages. Crosses made between T. audax sp.n. and T. madens produced a few infertile F₁ hybrids.     

Response and genetic analysis of malathion-specific resistant Tribolium castaneum (Herbst) in relation to population density

Extensive use of malathion for pest control on stored cereals has resulted in worldwide resistance in red flour beetles, Tribolium castaneum. In this paper we investigate population density effects on the malathion-specific resistance in PRm, a strain from the Philippines, in an integrated resistance management framework. Two populations of malathion-specific resistant (PRm) and a malathion-susceptible strain of T. castaneum were bred at different densities: low (4 adults/g) and high (12 adults/g) density on wheat plus brewer's yeast in the laboratory. After eight generations, slopes of probit regression lines and LC₅₀ values were used to monitor the effect of insect rearing density on the progression of malathion-specific resistance. The LC₅₀ of the malathion-susceptible strain (Asm) did not change significantly during selection while LC₅₀s varied for both the high-density and low-density lines of PRm, the LC₅₀ of malathion ranged from 27.51 to 34.06 and from 21.14 to 29.39 μg malathion cm⁻² for high and low density, respectively.More than 33 generations were required to achieve a 10-fold increase of resistance for the low-density line compared to only 17 generations for the high-density line. Calculations from published formulae suggested that the malathion-specific resistance of both high- and low-density lines was under monofactorial control, with complete dominance.The data showed that environmental factors such as population density differences in insect rearing and development may influence the heritability of resistance. Furthermore, the variability in results published worldwide on resistance emphasises the need to standardize test conditions across laboratories.     

Time matters: delayed toxicity of pirimiphos-methyl on Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and its effects on efficacy estimation of residual treatments

Three time-related toxicity parameters of pirimiphos-methyl were compared using a common pest of stored cereal and grain products, Tribolium castaneum (Herbst). The effective exposure time sufficient to knock-down half of the beetles was much shorter than the median knock-down time from a continuous exposure test (8.6 min and 80 min, respectively, at the same insecticide concentration). The difference between these two measures can be attributed to delayed toxicity of pirimiphos-methyl. Pirimiphos-methyl was effective against T. castaneum at a low application rate of 0.0047 g a.i. m⁻², when beetles were exposed on the treated surface for short time intervals. The median post-exposure knock-down time for animals placed for 20 min on insecticide-treated surface and then kept on an untreated surface was 180 min. Our results suggest that actual effective exposure time might be a more realistic efficacy measure of pirimiphos-methyl at a given dose than LT50 (median lethal time for continuous exposure), and that the timing of the toxic effects should be included more often in residual toxicity testing.     

Repellent and fumigant activity of essential oil from Artemisia vulgaris to Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Repellent and fumigant activity of the essential oil of mugwort, Artemisia vulgaris, against the stored-product insect pest, Tribolium castaneum was investigated. Artemisia vulgaris oil had a very strong repellent activity to adults and was significantly repellent at a 0.6 μL/mL (v/v) and higher in a filter-paper arena test. The oil had high fumigant activity against adults and larvae with adults much more susceptible than larvae. At 8.0 μL/mL, mortality of adults reached 100%, but with 12-, 14- and 16-day larvae, mortalities were 49%, 53% and 52%, respectively. The oil also had high-fumigant activity against eggs and toxicity progressively increased with increased exposure time and concentration. At dosages of 10, 15 and 20 μL/L air and a 96 h exposure period, mortality reached 100%. Regression analysis of data on individuals fumigated in the larval stage confirmed that the percentage of larvae reaching the pupal stage and the percentage of pupae that reached the adult stage, decreased significantly with increase in dosage concentration. No larvae, pupae and adults were observed following a 60 μL/L dosage.     

The effects of atmospheres containing 5–20% oxygen with and without 10% carbon dioxide on the productivity of Tribolium castaneum (Herbst) and T. confusum Du Val (Coleoptera: Tenebrionidae)

T. castaneum (Herbst) adults were exposed to atmospheres reduced in oxygen content with and without 10% carbon dioxide for 27 days. The egg production declined with the oxygen tension but in oxygen concentrations of 10, 7.5 and 5% was about 60% higher in the presence of 10% carbon dioxide than in its absence. However, 10% carbon dioxide did not enhance productivity when oxygen comprised 20% of the atmosphere and oviposition in T. castaneum and T. confusum Du Val was then similar to that in air. Egg hatch was unaffected at 10% oxygen with or without carbon dioxide but was seriously impaired when only 5% oxygen was present. No significant adult mortality occurred in any of the gas mixtures and in all cases normal fecundity was quickly regained at the end of the experimental period.     

Effect of methoprene on the heat tolerance and cold tolerance of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Methoprene, a Juvenile Hormone analogue, was evaluated for its ability to alter heat tolerance or cold tolerance of Tribolium castaneum, the red flour beetle. Young adults and late instar larvae were exposed to a series of methoprene concentrations. They were held either at 46 °C or 0 °C for different durations, and survival of adults or the adult emergence from larvae was recorded to determine their tolerance to extreme temperatures. At 46 °C, the lethal time to kill 50% of the population (confidence intervals) for untreated adults was 10.8 (9.6-11.8) h compared to 9.3 (8.3-10.0) h for adults exposed to 3.33 ppm of methoprene for 48 h. Higher concentrations of methoprene also caused adults to be less heat tolerant. In contrast, there was no evidence that methoprene reduced the heat tolerance of larvae. At 0 °C, both unacclimated and cold-acclimated insects were tested. Methoprene did not affect the cold tolerance of adults or larvae, regardless of cold acclimation. As seen in other studies, methoprene was not toxic to adults even at 66.6 ppm, but it was highly toxic to larvae (LD₅₀ 0.015 to 0.020 ppm). Cold tolerance was slightly greater in both adults and larvae, after being held at 15 °C for two weeks. This is the first study to report that a Juvenile Hormone analogue has an impact on insect heat tolerance.     

The effect of diurnally interrupted doses of phosphine over four days on egg mortality of susceptible and resistant strains of Sitophilus oryzae (L.)

Phosphine concentrations in stored grain are uneven and can undergo cyclic fluctuations caused by diurnal changes in solar heat exchange and wind. Experiments were conducted to investigate the influence of daily concentration variations on insect mortality. Values of LT₅₀ and LT_99.9 are presented for Sitophilus oryzae eggs in response to a range of diurnally interrupted phosphine treatments over four days. Egg mortality was determined for three insect strains that varied in susceptibility to the fumigant, at concentrations that ranged from 0.75 mg/L to 3 mg/L for 32-8 h respectively, giving the same Ct product for all treatment combinations. Effects on mortality were analyzed in terms of accumulated phosphine exposure over 4 days of treatment, and in terms of the total elapsed time of treatment. Results were compared to those obtained from continuous treatments having equivalent Ct combinations. Treatments that were interrupted daily extend the total elapsed time required for a given level of mortality and that time increased as concentration increased and daily treatment time decreased. Values of LT_99.9 for a susceptible strain were 67.6 h and 103.6 h, and for a resistant strain were 133.6 h and 179.5 h at concentrations of 1 mg/L and 3 mg/L respectively. However, diurnal treatments were more efficacious than continuous treatments when only dosage time was measured. Values of LT_99.9 for the susceptible strain were 21.0 h compared with 33.8 h and 11.0 h compared with 17.7 h, and for the resistant strain were 35.2 h compared with 57.4 h and 22.2 h compared with 44.3 h at 1 mg/L and 3 mg/L respectively. The findings show that the level of mortality during interrupted treatment is mainly due to the additional time that the fumigant is absent and not a combined affect including insect recovery during the intervening periods. In fact, the dosage time is reduced by an apparent increase in phosphine toxicity.