Efficacy of five insecticides for the control of Trogoderma granarium Everts (Coleoptera: Dermestidae) larvae on concrete

Highlights

• Chlorfenapyr caused 70% immediate mortality of small larvae.

• Chlorfenapyr caused 76% delayed mortality of small larvae and 61.5% of large larvae.

• There was a noticeable delayed effect for pirimiphos-methyl and deltamethrin.

• Large larvae were by far more tolerant than small ones in all combinations examined.

Abstract

Small and large larvae of the khapra beetle, Trogoderma granarium Everts were exposed for 1, 3, and 7 d on concrete that was treated with chlorfenapyr at 0.055 mg active ingredient (a.i.)/cm² and 0.11 mg (a.i.)/cm², deltamethrin at 0.0025 mg (a.i.)/cm² and 0.005 mg (a.i.)/cm², pirimiphos-methyl at 0.025 mg (a.i.)/cm² and 0.05 mg (a.i.)/cm², pyriproxyfen at 0.000115 mg (a.i.)/cm² and 0.00023 mg (a.i.)/cm², and spinosad at 0.05 mg (a.i.)/cm² and 0.1 mg (a.i.)/cm². Then, the survived individuals were transferred on untreated concrete for 7 more days and the delayed mortality of small or large larvae was evaluated. Concerning the immediate mortality of small larvae, chlorfenapyr was the most effective among the insecticides tested causing 70% immediate mortality after 7 d of exposure at the higher dose. The other insecticides provided similar mortality levels with the exception of pyriproxyfen which was not effective, for any of the combinations tested, as mortality did not exceed 4.4%. Delayed mortality for chlorfenapyr was also high reaching 76% at 7 d of exposure at the higher dose. Pirimiphos-methyl and deltamethrin gave similar results, indicating that for these two insecticides there is also a noticeable delayed effect. For pyriproxifen and spinosad, the delayed mortality was low. Large larvae were by far more tolerant than small ones in all dose-insecticide-exposure combinations examined for both immediate and delayed mortality counts. The results of the present study show that none of the insecticides were able to totally control T. granarium larvae, illustrating the difficulties in controlling this species, with dose rates that are usually effective for other major stored product beetle species.

Introduction

The khapra beetle, Trogoderma granarium Evertrs (Coleoptera: Dermestidae) is one of the most destructive stored product insect species globally (Myers and Hagstrum, 2012), while it has been listed in the 100 most invasive species worldwide (Lowe et al., 2000). Moreover, this species is considered the most important stored product quarantine insect in many parts of the world, such as Australia, Russia, and USA (Peacock, 1993, Bell and Wilson, 1995, Myers and Hagstrum, 2012). For instance, Australia was erroneously listed in the “khapra beetle country”, and it took more than 15 years to remove the country from this list (Emery, 1999). In the USA, T. granarium was present for decades, where it was eventually eradicated from sixties to eighties (Myers and Hagstrum, 2012). Nevertheless, during the period from 1985 to 2010, T. granarium was intercepted 559 times in entry ports of USA (Myers and Hagstrum, 2012). In addition, this species has been established in several countries of Africa, Asia and Europe (EPPO, 2013) whereas it was intercepted in countries with different climatic conditions, i.e., Australia, Austria, Bulgaria, Croatia, Czech Republic, Italy, Poland, Portugal and Slovakia (EPPO, 2015). Recently, T. granarium was detected in Greece (Athanassiou et al., 2015).

The biology of T. granarium has been examined in detail by several researches throughout the world (Nair and Desai, 1972, Karnavar, 1973, Chaudhary and Kapil, 1976, Gothi et al., 1984, Burges, 2008). The species is able to heavily infest a wide variety of durable commodities, including cocoa, cereals, oilseeds, pulses, and reproduce extremely rapidly in hot and dry areas, in conditions that are marginal for the development of other major stored-product insect species (Solomon and Adamson, 1955, Burges, 1959, Burges, 1962b, Burges, 1963, Hill, 1990, Peacock, 1993). Infestation occurs due to the activity of larvae, while adults are short-lived, do not fly despite they are winged and rarely feed (Hill, 1990). Thus, larvae are considered much more important than the adults concerning control measures (Athanassiou et al., 2015). One of the key elements that determine its wide distribution is its diapause, which occurs at the larval stage. In fact, larvae can stay in diapause for several years (Burges, 1962a, Hill, 1990), while they are able to disrupt diapause, by carrying out short “foraging excursions” (Nair and Desai, 1973). At the same time, even if the conditions are favorable for the development of this species, it has the ability to keep a proportion of the population as a “diapausing reservoir” (Aitken, 1975). Apparently, as it has been noted for several other stored product insect species, diapausing larvae of T. granarium are considered more tolerant to insecticides (Hole et al., 1976, Bell and Wilson, 1995) and also to unfavorable environmental conditions (Burges, 1962a, Hill, 1990). Therefore, control of larvae, especially when they are active, i.e., before they enter diapause or during foraging, is essential in order to effectively control this species. Nevertheless, the application of insecticides on the grain mass only, even if they are effective, does not necessarily mean that the population has been controlled, due to the fact that a percentage of the population may not be in grains, but in cracks and crevices and also in other refugia (Myers and Hagstrum, 2012). This scenario is very likely to occur in areas when low numbers of larvae exist, such as in the case of containers or in warehouses and processing facilities in temperate areas, where newly-introduced larvae are expected to start infesting the products. Hence, apart from the application of insecticides in the commodities, it is important to also apply insecticides on the surfaces or in cracks and crevices. This application is expected to reduce the initial population of T. granarium, and should be evaluated further, for purposes of quarantine and pre-shipment.

There are few studies, mostly coming from seventies and eighties, that examine the effectiveness of insecticides on surfaces against T. granarium (Girish et al., 1973, Yadav and Jha, 1985, Yadav, 1987, Chaudhry and Anwar, 1988, Yadav, 1988, Reddy and Gopala Swamy, 2006). To our knowledge, the only recent study that examined the efficacy of insecticides on surfaces for the control of T. granarium is that of Athanassiou et al. (2015). In that study, the authors tested the neonicotinoid thiamethoxam and the pyrethroid alpha-cypermetthrin for the control of adults and larvae of T. granarium on concrete and reported that larvae were much more tolerant than the adults, while alpha-cypermethrin was generally more effective than thiamethoxam. To date, although several new insecticides have been introduced in the market for surface treatment, there are no data available for their effect against T. granarium. In the present work, we evaluated five insecticides with different mode of action, against T. granarium larvae. All tested insecticides are registered for post-harvest treatments.