Insecticidal effect of contact insecticides against stored product beetle populations with different susceptibility to phosphine
Introduction
Phosphine is currently the most commonly used insecticide for the control of stored product insects in warehouses and food processing facilities, for a wide range of durable commodities (Daglish, 2004; Wang et al., 2006). Phosphine has several advantages, as it is cheap, easily applied, and leaves minimal residues in the treated products (Nayak and Collins, 2008). Nevertheless, the extensive use of phosphine in conjunction with poor fumigation practices, has contributed to the development of resistance by several major stored product insect species, which already threatens the future use of phosphine is some areas of the world (Benhalima et al., 2004; Daglish, 2008; Pimentel et al., 2009; Opit et al., 2012; Nayak, 2012; Nayak et al., 2013; Saglam et al., 2015).
Paradigms of resistance to phosphine are populations of the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), and the rusty grain beetle, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), which have been found strongly resistant, and can survive doses that are far higher than those recommended (Collins et al., 2005; Lorini et al., 2007; Nayak et al., 2013). More recently, other stored-product insects have been added in the list of resistant to phosphine species, such as the psocid Lipocelis bostrychophila (Badonnel) (Psocopetra: Liposcelididae) (Nayak and Collins, 2008), the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) (Holloway et al., 2016), the sawtoothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) (Pimentel and Guedes, 2010) and the tobacco or cigarette beetle, Lasioderma serricorne (F.) (Coleoptera: Anobiidae) (Saglam et al., 2015).
One of the solutions suggested as alternative to phosphine in grains is the application of grain protectants. These insecticides have the advantage of providing long residual effect for a wide spectrum of target species, while their application is cheap and easy (Arthur, 1996; Vassilakos et al., 2015; Rumbos et al., 2018). One good example of a traditional grain protectant is the organophosphorous pirimiphos-methyl, which has been proved extremely effective for the control of several species on grains (Redlinger et al., 1988; Rumbos et al., 2014, 2018). Similarly, Kljajic and Peric (2007) reported that pirimiphos-methyl was very effective for the control of the granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae). Furthermore, Rumbos et al. (2014) and Agrafioti et al. (2015) showed high mortality levels when pirimiphos-methyl was applied as a surface treatment against different stored product beetle species.
One other example is the pyrethroid alpha-cypermethrin. Agrafioti et al. (2015) reported that the application of alpha-cypermethin provided a rapid knockdown on the exposed adults of the confused flour beetle, Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae) and O. surinamensis. More recently, spinetoram, which is based on modified metabolites of the actinomycete Saccharopolyspora spinosa Mertz and Yao (Actinobacteria: Actinomycetales: Pseudonocardiacae) (Dripps et al., 2011), has been found very promising as a grain protectant (Vassilakos et al., 2014, 2015). Several studies indicate that spinetoram was effective for the control of R. dominica and the larger grain borer, Prostephanus trunctatus (Horn) (Coleoptera: Bostrychidae) (Vassilakos and Athanassiou, 2012).
There are data indicating show that strains that are resistant to phosphine may be simultaneously resistant to other active ingredients (Nayak et al., 2005; Daglish, 2008; Opit et al., 2012; Bajracharya et al., 2013). In this regard, multiple resistance, should be seriously taken into account, in order to select the active ingredient that is suitable for each scenario of target species and strain. One paradigm is spinosad that has been found effective against phosphine-resistant populations of R. dominica (Nayak et al., 2005) but not effective against phosphine-resistant populations of T. castaneum (Bajracharya et al., 2013). On the other hand, the combination of chlorpyrifos-methyl + deltamethrin was found effective for phosphine-resistant populations of both R. dominica and T. castaneum (Bajracharya et al., 2013). Resistance is directly related with a certain fitness cost, which is based in resource allocation from a basic physiological process to the protection against insecticides (Guedes et al., 2006). Fitness cost is associated with resistance to grain protectants and has received significant attention (Sousa et al., 2009). However, fitness cost of phosphine resistance has been examined only lately (Sousa et al., 2009; Daglish et al., 2015). For example, the majority of the phosphine-resistant population studies showed lower developmental and population growth rates than the susceptible to phoshine populations (Sousa et al., 2009). Under a rotation strategy, the grain protectant should be carefully selected, as insects may be also resistant to the active ingredient that is planned to be used as an alternative to phosphine. The selection of the right insecticide can be carried out easily by conducting simple laboratory bioassays of the insect populations that are to be controlled. In this context, the objective of this study was to determine the efficacy of three contact insecticides, pirimiphos-methyl, alpha-cypermethrin and spinetoram on field and laboratory populations of S. oryzae and O. surinamensis with different susceptibility to phosphine.
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Test insects
For each species, one field population and one laboratory population were used for experimentation (4 populations in total). The field populations of S. oryzae and O. surinamensis had been collected in 2015 from storage facilities in Greece, and were part of a wider surveillance of stored product insects in Greece (these data are not presented in this paper). All insects were reared in incubators set at 25 °C, 65% relative humidity (r.h.) and continuous darkness, on whole wheat kernels and oat
Evaluation of phosphine resistance
Based on the FAO protocol, for both species, the two field populations were characterized as resistant, while the laboratory populations were characterized as susceptible (Table 1). After exposure to 30 ppm of phosphine for 20 h, all adults of the laboratory populations were totally (100%) immobilized. In contrast, after the same interval, the majority of the adults of the field populations were still active.
Sitophilus oryzae
For this species, all main effects and interactions were significant (Table 2). In
Discussion
Based on our results, for S. oryzae populations, pirimiphos-mehtyl was the most effective insecticide, given that complete control was achieved even in the case of 0.1 ppm. In contrast, for O. surinamensis populations, for the same insecticide, survival was recorded even at 10 ppm. In a recent study, Rumbos et al. (2016) found that S. oryzae was very susceptible to pirimiphos-methyl among the species tested, with O. surinamensis being moderately susceptible. Agrafioti et al. (2015) noted that
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