Short communication

The occurrence of resistance to phosphine in adult and egg stages of strains of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae)

Ozone applied in a low-pressure system has been studied in packed grains. In the present study, we evaluated the effect of ozone applied at low-pressures for the control of egg and larval phases of Zabrotes subfasciatus and inactivation of Aspergillus flavus in beans and possible changes in product quality. Samples of 5.0 kg of beans packed in woven polypropylene bags were exposed to ozone in the hypobaric chamber of internal volume equal to 70 L. The application of ozone was preceded by the reduction of internal chamber pressure to 50000 Pa. The ozone was then injected until the chamber's internal pressure reached 100000 Pa. An inlet ozone concentration of 61.37 mg L⁻¹ was applied in 1, 4, 7, and 10 injections and a flow rate of 1.0 L min⁻¹. In the control treatment, oxygen was used. The ozonation of beans with 10 injections was efficient in controlling the egg and larval phases of Z. subfasciatus, reducing mass loss and inactivation of A. flavus in the grains. The increase in the number of injections increased the electrical conductivity of the beans. However, ozone did not affect the thousand-grain mass, percentage of whole grains after cooking, total soluble solids in the cooking broth, dimensions of the orthogonal axes of the grains, volume and length-width ratio of the cooked beans. The results suggest that the number of injections in the system at low-pressure is a determining factor for the concentration of ozone inside grain packages. In this system, the application of ozone adopting 10 injections is efficient in the control of Z. subfasciatus and in the inactivation of A. flavus, without affecting the quality of the beans.

Resistance developed by Tribolium castaneum (Herbst), a major pest of stored grain and grain products, to phosphine is a serious concern. Treatment of phosphine is given to all the stages (egg, larva, pupa and adult) of insects simultaneously in the infected commodity. Effective resistance management tactics need a proper understanding of expression of resistance in relation to dosage in immature stages including larvae. So, this study was undertaken to understand the genetic basis of phosphine resistance in larval stage of T. castaneum by crossing the resistant and susceptible parents to obtain the F₁, F₂ and the back-cross progenies. The resistant parent had a resistance ratio of 37.75 when compared to the susceptible parent. The susceptibility levels of the reciprocal F₁ hybrids did not show any significant difference indicating autosomal mode of inheritance of resistance. Level of dominance for the reciprocal F₁ hybrids was estimated (-0.70 and −0.68) which showed that the resistance is expressed as incompletely recessive in larval stage of T. castaneum. χ² analysis showed that the observed and expected mortalities differed significantly (P < 0.05) in most of the concentrations indicating that more than one gene is responsible for phosphine resistance. Two plateaus at ∼ 40 % and ∼ 90 % mortality levels for the back-cross progeny to the resistant parent further confirmed the involvement of two or more genes in imparting resistance to phosphine. Our results will aid in enhancing the knowledge on the development of resistance to phosphine in the field and thus facilitate in designing suitable resistance management tactics.

Phosphine (PH₃) resistance and tolerance contribute to the increased importance of psocids (Psocodea: Liposcelididae) as stored-product pests. This study estimated the discriminating doses (DDs) of phosphine for laboratory susceptible adults of Liposcelis bostrychophila (Badonnel), L. entomophila (Enderlein), L. decolor (Pearman), L. paeta Pearman, L. rufa Broadhead, L. obscura Broadhead, L. fusciceps Badonnel, and Lepinotus reticulatus Enderlein. Discriminating dose is defined as the upper limit of the 95% confidence interval (95% CI) of the concentration that kills 99% of the individuals in a test sample (LC₉₉). Psocids were exposed to incremental phosphine concentrations of 0.0–300.0 ppm and 0.0–150.0 ppm over a 20-h and 72-h fumigation period, respectively, at 25 °C using a modified FAO Method No.16. The estimated DDs had a range of 65.6–697.3 ppm and 18.1–194.5 ppm over a 20-h and 72-h fumigation period, respectively. For the 20-h fumigation period, the eight psocid species had higher DDs than those established for all stored-product insect pests in the FAO Method No.16 publication. Liposcelis entomophila and L. decolor were the most tolerant psocid species over a 20-h and 72-h fumigation period, respectively. Moreover, L. entomophila required approximately 9.3 and 6.7x higher phosphine concentration compared to the most susceptible L. obscura and L. reticulatus over a 20-h and 72-h fumigation period, respectively, to achieve similar levels of LC₅₀ mortality. The higher heterogeneity values in the standard 20-h fumigation period indicate the potential for increase in phosphine resistance in field populations subjected to phosphine fumigation. Discriminating doses estimated in this study can be used for detection of phosphine resistance and estimation of resistance frequencies in field-collected populations of the psocid species investigated.

Phosphine (PH₃) fumigation resulting in sub-lethal exposure has led to the development of phosphine resistance in many stored-product insect species worldwide and is a major challenge to the continued effective use of phosphine. In 2016 phosphine resistance was found in Tribolium castaneum (Herbst) and Plodia interpunctella (Hübner) collected from California dried fruit and nut processing facilities. Although Oryzaephilus surinamensis (L.) infests grain, dried fruit, and nuts in storage and processing facilities, phosphine resistance in this species has not been studied in the United States. In this study, the discriminating dose of phosphine for O. surinamensis eggs was estimated using a laboratory susceptible strain; it was found to be 28.4 ppm over a 72-h fumigation period (1 mg/L of phosphine = 714.18 ppm or 1 ppm = 0.0014 mg/L). Discriminating dose bioassays were used to determine phosphine resistance in both eggs and adults of 14 different populations collected from California and Oklahoma. Resistance to phosphine was detected in four out of 14 populations in adults and nine out of 14 populations in eggs and ranged from 2 to 100%. Phosphine percent survival values in both adults and eggs of three populations, namely, Box BR, Box BF, and OKWat were >90%. Lethal concentration values required to kill 99% of individuals in samples for adults of these three populations were predicted as 320.5, 290.7, and 263 ppm, respectively, and those for eggs were 1030.7, 1055.9, and 564.5 ppm, respectively, over a 72-h fumigation period. This study confirms that phosphine resistance is present in O. surinamensis in the United States.

Callosobruchus maculatus (Fab.) is the principal pest of the stored chickpea. The larvae of this insect feed on the grain, causing considerable damage that produces qualitative and quantitative losses. For control of this species, pesticides such as phosphine and methyl bromide have been used; the latter was withdrawn from use due to the reduction of the ozone layer. Therefore, it is important to develop novel, safe, and economical control alternatives such as Controlled Atmospheres (CA). In this study, the mortality was determined on four developmental states of C. maculatus, employing CA at different concentrations (50, 70, and 90% CO₂ with 10, 6, and 3% O₂, respectively) during 48, 72, and 120 h. 100% mortality of adult insects was observed with 50% CO₂ at 48 h. Cases of third instar exhibit higher resistance to CA than adults, with 18.9% mortality at 48 h. There were no significant differences in tests on water absorption, cooking time, texture, and color. Finally, the chickpea germination obtained was 93.5% with 90% CO₂ exposure during 120 h. In this latter treatment, significant differences were not presented between control and CA treatments. On comparison of means of vigor, the treatments are favored in terms of weight, rootlet number, plumule and radicle growth with the increase of CO₂ concentration and exposure time. In conclusion, the effect of CA was lethal on C. maculatus, did not affect chickpea quality, and it favored chickpea vigor.

Bruchid beetles are important pests of field and stored legumes, causing great economic losses. The main pest species are Callosobruchus maculatus, Acanthoscelides obtectus and Zabrotes subfasciatus. Modified atmospheres (MAs) with high carbon dioxide (CO₂) content are environmentally friendly pest control methods for stored products. They are effective in controlling a wide range of species and can be used to treat different food products without leading to an accumulation of toxic residues. The present study aimed to establish the efficacy of using MAs with high CO₂ to control all developmental stages of these three bruchid pest species. Three high CO₂ MAs (50%, 70% and 90%) were tested at 28 °C. In general, pupae and/or eggs were the most tolerant stages to hypercarbia, while adults were less tolerant. The susceptibility of eggs varied greatly according to their phase of development and the pattern was different in the three species tested. Tolerance of other stages to CO₂ also varied among the three bruchid species. To achieve total mortality of Z. subfasciatus, a maximum of 9–11 days were needed, depending on CO₂ concentration, to kill the eggs. This stage and the pupae were the most tolerant stages. However, in A. obtectus and C. maculatus, only pupae were the most tolerant and required 9 days to be killed, their eggs being more sensitive to CO₂.