Insecticide selectivity to the parasitic mite Acarophenax lacunatus (Cross & Krantz) (Prostigmata: Acarophenacidae) on Rhyzopertha dominica (Fabr.) (Coleoptera: Bostrichidae)

March - April 2004 243 CROP PROTECTION Insecticide Selectivity to the Parasitic Mite Acarophenax lacunatus (Cross & Krantz) (Prostigmata: Acarophenacidae) on Rhyzopertha dominica (Fabr.) (Coleoptera: Bostrichidae) JOSÉ R. GONÇALVES1, LÊDA R.D’A. FARONI2, RAUL N.C. GUEDES1 AND CARLOS R.F. DE OLIVEIRA1 1 Depto. Biologia Animal; 2Depto. Engenharia Agrícola. Universidade Federal de Viçosa, 36571-000, Viçosa, MG e-mail: Neotropical Entomology 33(2):243-248 (2004) Seletividade de Inseticidas Para o Ácaro Parasita Acarophenax lacunatus (Cross & Krantz) (Prostigmata: Acarophenacidae) em Rhyzopertha dominica (Fabr.) (Coleoptera: Bostrichidae) RESUMO - Diante da presença de inimigos naturais em massas de grãos armazenados e a necessidade de se buscar alternativas de controle de pragas, avaliou-se o efeito de inseticidas sobre o ácaro Acarophenax lacunatus (Cross & Krantz), um parasita de ovos de Rhyzopertha dominica (Fabr.). Os inseticidas bifentrina, deltametrina, pirimifós metílico e fenitrotiom foram testados a 0,4; 0,5; 0,8 e 10 ppm. As unidades experimentais consistiram de placas de Petri contendo grãos de trigo, infestados com 25 adultos de R. dominica. Sete dias após a infestação, foram inoculados três ácaros em cada placa, sendo utilizadas quatro repetições por tratamento. O número de adultos de R. dominica, a perda de matéria seca dos grãos e a taxa instantânea de crescimento (ri) de A. lacunatus foram menores no tratamento com deltametrina. Os inseticidas bifentrina, fenitrotiom e pirimifós metílico apresentaram baixa eficácia na redução de R. dominica, mas foram seletivos em favor do ácaro, mostrando maior número de fêmeas fisogástricas e ri para A. lacunatus. Com isso, conclui-se que deltametrina é o inseticida menos seletivo em favor de A. lacunatus. Entretanto, esse ácaro é capaz de parasitar ovos de R. dominica sob grãos tratados com todos os inseticidas avaliados. PALAVRAS-CHAVE: Praga de grãos armazenados, ácaro, piretróide, organofosforado, controle biológico ABSTRACT - The presence of natural enemies in stored grains and the need of pest control alternatives led to the assessment of the effect of insecticides on the mite species Acarophenax lacunatus (Cross & Krantz), an egg parasite of the stored grain pest Rhyzopertha dominica (Fabr.). The insecticides bifenthrin, deltamethrin, pirimiphos-methyl and fenitrothion were tested at 0.4, 0.5, 0.8 and 10 ppm, respectively. Insecticide treated wheat grains were spread in petri dishes and infested with 25 adults of R. dominica. Seven days later, three mites were added to each dish, in four replicates. The number of adults of R. dominica, grain weight loss and instantaneous rate of increase (ri) of A. lacunatus were lower when the grain was treated with deltamethrin. Bifenthrin, fenitrothion, and pirimiphos-methyl were less effective in controlling the pest, but they were selective in favor of the mite, showed higher number of physogastric females and ri of A. lacunatus. Therefore, deltamethrin was less selective in favor of the mite species. Nonetheless the parasitic mite was able to parasitize eggs of R. dominica on wheat treated with all the insecticides evaluated. KEY WORDS: Stored grain pest, mite, pyrethroid, organophosphate, biological control Insecticide use is considered the only control option when a pest population in stored grains reaches the economic threshold (White & Leesch 1996). However, due to the development of resistance in many pests to the majority of the insecticides (organophosphates and pyrethroids) commonly used for their control (Collins et al. 1993, Guedes et al. 1996, Lorini & Galley 1999), insecticide use is becoming increasingly unviable (Arthur 1996). Because few insecticides are registered for controlling stored grain pests and the registration of new products requires a long time and at a high cost (White & Leesch 1996), alternative methods such as integration of natural enemies and insecticides need to be developed (King & Nordlund 1992). The majority of mites infesting stored grains are pests, 244 Gonçalves et al. Insecticide Selectivity to the Parasitic Mite Acarophenax lacunatus (Cross & Krantz) ... but some of them are natural enemies of stored grain-pests (Cross & Krantz 1964). Interest in the efficacy and applicability of mites for integrated pest management systems has been increasing. Mites are small, have a short life cycle, and high reproductive potential (Scholler et al. 1997, Faroni et al. 2001). Integrating the use of predatory/ parasitic mites and insecticides for stored-grain pest management requires knowledge of the impact of the insecticides on the natural enemies or, in other words, their selectivity to natural enemies (Baker & Arbogast 1995, Gonçalves et al. 2002). The egg parasite Acarophenax lacunatus (Cross & Krantz) is studied aiming its use as biocontrol agent of stored-grain pests. The whitish-colored small (0.14 mm x 0.09 mm) adults have a phoretic behavior (uses the pests to spread in the grain mass). This mite develops over a wide temperature range, from 18°C to 41°C, with an optimum at about 30°C. The female adults are responsible for egg parasitism. They suck the egg contents and distend to a bright, spherical form. The body size increases by about five times, which characterizes progeny development (physogastry). The mite progeny emerge as adults ready to parasitize new host eggs. The mite A. lacunatus significantly reduces the population of Rhyzopertha dominica (Fabr.) (Faroni et al. 2000), which is an important pest of stored products (Rees 1996). The following study was carried out to assess the insecticide selectivity to A. lacunatus and to determine the possibility of integrating the use of this mite species with some insecticides commonly used for storedgrain pest control. Material and Methods Mass Rearing of R. dominica. R. dominica was reared on non-treated whole wheat grain in petri dishes in an environmental chamber at 30 ± 1°C, 65 ± 5% relative humidity (RH) and 24h scotophase. The insects originated from a laboratory colony maintained at the same conditions for four consecutive years without insecticide exposure. Each petri dish containing the whole wheat was infested with 50 adults of R. dominica and seven days later the eggs were collected by sieving through an 1-mm aperture sieve. The collected eggs were placed on the fresh diet in petri dishes. This period was sufficient for the pest to lay its eggs. Eggs of a known age were periodically collected in a similar way (Faroni et al. 2000). Mass Rearing of A. lacunatus. Individuals of A. lacunatus were obtained from cultures of R. dominica infested with this parasite for more than four years. The mites were reared on the R. dominica at 30 ± 1°C, 65 ± 5% RH, but in a different room from uninfested insects (Faroni et al. 2000). Insecticide Application. The insecticides used were bifenthrin (Prostore 25 CE), deltamethrin (K-Obiol 25 CE), pirimiphosmethyl (Actellic 500 CE) and feni (...truncated)