Detection and characterisation of strong resistance to phosphine in Brazilian Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

Abstract

As failure to control Rhyzopertha dominica (F.) with phosphine is a common problem in the grain-growing regions of Brazil, a study was undertaken to investigate the frequency, distribution and strength of phosphine resistance in R. dominica in Brazil. Nineteen samples of R. dominica were collected between 1991 and 2003 from central storages where phosphine fumigation had failed to control this species. Insects were cultured without selection until testing in 2005. Each sample was tested for resistance to phosphine on the basis of the response of adults to discriminating concentrations of phosphine (20 and 48 h exposures) and full dose-response assays (48 h exposure). Responses of the Brazilian R. dominica samples were compared with reference susceptible, weak-resistance and strong-resistance strains from Australia in parallel assays. All Brazilian population samples showed resistance to phosphine: five were diagnosed with weak resistance and 14 with strong resistance. Five samples showed levels of resistance similar to the reference strong-resistance strain. A representative highly resistant sample was characterised by exposing mixed-age cultures to a range of constant concentrations of phosphine for various exposure periods. Time to population extinction (TPE) and time to 99.9% suppression of population (LT(99.9)) values of this sample were generally similar to those of the reference strong-resistance strain. For example, at 0.1, 0.5 and 1.0 mg L(-1), LT(99.9) values for BR33 and the reference strong-resistance strain were respectively 21, 6.4 and 3.7 days and 17, 6.2 and 3.8 days. With both strains, doubling phosphine concentrations to 2 mg L(-1) resulted in increased LT(99.9) and TPE. High level and frequency of resistance in all population samples, some of which had been cultured without selection for up to 12 years, suggest little or no fitness deficit associated with phosphine resistance. The present research indicates that widespread phosphine resistance may be developing in Brazil. Fumigation practices should be monitored and resistance management plans implemented to alleviate further resistance development.

Spread of phosphine resistance among brazilian populations of three species of stored product insects

“…Moreover, phosphine resistance in stored-product insects is documented in several neighboring South American countries (Champ & Dyte 1976. Except for the global survey undertaken by FAO (Champ & Dyte 1976) , Lorini et al 2007, Pimentel et al 2007, 2009.…”

Resistance of stored-product insects to phosphine

“…The phosphine resistance detected among the populations may be the result of previous selection pressure caused by inadequate fumigations in the storage units, from where the samples were taken. Other reasons for differences in phosphine resistance observed in the present work were: high insect populations favored by climate; little or no insect control in storage units in some farms (Collins et al, 2005;Lorini et al, 2007); problems of widespread resistance to grain protectants (Lorini & Galley, 1999); storage facilities not adequately sealed before fumigation; and fumigant concentrations not being monitored (Benhalima et al, 2004;Wang et al, 2006;Lorini et al, 2007). As consequence, some lots of grains were fumigated frequently, and since the populations did not respond to the control, very high doses of phosphine were used, increasing problems for selection for resistance (Collins et al, 2005;Lorini et al, 2007).…”

“…(Benhalima et al, 2004;Wang et al, 2006). Also, due to poor fumigation standards (Lorini et al, 2007), phosphine concentration does not reach the level required for selection of resistance at immature stages (Bengston et al, 1999;Collins et al, 2002Collins et al, , 2005Benhalima et al, 2004). The phosphine resistance detected among the populations may be the result of previous selection pressure caused by inadequate fumigations in the storage units, from where the samples were taken.…”

“…In recent years, the importance of phosphine to protect stored grain products has grown due to international agreements phasing out the fumigant methyl bromide (Bell, 2000;Zettler & Arthur, 2000;Rajendran, 2001). In addition, the lack of ideal airtight conditions for fumigation, in most storage units, increases the frequency of control failures and, consequently, increases the frequency of applications Chaudhry, 2000;Benhalima et al, 2004;Lorini et al, 2007). These factors combine to result in higher selection pressure for phosphine resistance (Bengston et al, 1999;Collins et al, 2002).…”

“…Since FAO's global report, there have been few surveys for phosphine resistance of stored-product insects in Brazil. There are only three reports showing phosphine resistance in Brazilian populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Cryptolestes ferrugineus (Stephens) (Coleoptera: Cucujidae) Sartori et al, 1990;Lorini et al, 2007). The current levels of phosphine resistance in Brazilian populations of T. castaneum, R. dominica, Sitophilus zeamais Mots.…”