RESUMEN
Selection pressure to obtain resistant genotypes can result in fitness cost. In this study, we report the effects of the selection pressure of a commercial formulation of Bacillus thuringiensis on biological aspects of a Dipel-resistant strain of velvetbean caterpillar, Anticarsia gemmatalis Hübner. Comparisons of Dipel-resistant and susceptible individuals revealed significant differences in pupal weight and larval development time. Both strains (Dipel-resistant and susceptible) were susceptible to Cry1Ac toxin expressed in foliar cotton tissues. Resistant and susceptible strains showed low survival rates of 22.5% and 51.2%, respectively, when fed with Greene diet containing Bt-cotton. Larvae bioassayed after three laboratory generations presented lower survival and less instar numbers than individuals maintained in the laboratory for more than 144 generations. Pupal weight was 9.4% lower and larval development time was 1.9 days longer in the resistant population than in the susceptible strain. Other parameters, such as duration of pupal stage, adult longevity, number of eggs per female, oviposition period, and egg fertility, remained unaffected.
A pressão de seleção para obter genótipos resistentes pode resultar em custo adaptativo. Neste estudo, relatamos os efeitos da pressão de seleção de uma formulação comercial de Bacillus thuringiensis sobre aspectos biológicos de uma raça Dipel-resistente da lagarta-da-soja, Anticarsia gemmatalis Hübner. Comparações de indivíduos resistentes e suscetíveis a Dipel revelaram diferenças significativas em peso de pupas e tempo de desenvolvimento larval. Ambas as raças (suscetível e Dipel-resistente) foram suscetíveis à toxina Cry1Ac expressa em tecidos de folhas do algodoeiro. Linhagens resistentes e suscetíveis apresentaram baixas taxas de sobrevivência de 22,5% e 51,2%, respectivamente, quando alimentadas com dieta Greene contendo algodão Bt. As larvas utilizadas no bioensaio após três gerações de laboratório apresentaram menores taxas de sobrevivência e menores números de ínstares do que os indivíduos mantidos no laboratório por mais de 144 gerações. O peso das pupas foi 9,4% menor e o tempo de desenvolvimento foi 1,9 dias mais longo na população resistente do que na raça suscetível. Outros parâmetros, como duração do estágio pupal, longevidade de adultos, número de ovos por fêmea, período de oviposição e fertilidade de ovos, não foram afetados.
RESUMEN
Effect of the dry leaves powder water suspension of the plant Furcraea selloa marginata, belonging to family Agavaceae and Bacillus thuringiensis kurstaki (Dipel-2x) was evaluated against non-infected and Schistosoma mansoni-infected Biomphalaria alexandrina snails as well as their efficacy against the free larval stages of S. mansoni. The obtained results indicated that the LC50 and LC90 values after 24 hrs exposure were 53.66 & 84.35 ppm for F. selloa marginata and 392.3 & 483.64 ppm for B. thuringiensis kurstaki against adult B. alexandrina snails, respectively. The plant F. selloa marginata and B. thuringiensis kurstaki have a larvicidal activity against S. mansoni larvae (miracidia and cercariae), the plant F. selloa marginata was more toxic against larvae than B. thuringiensis kurstaki, the miracidia were more sensitive towards the toxic action of the tested agents than cercariae and the mortality percent of miracidia and cercariae is directly proportional to the time and the tested concentrations. The results revealed that the tested sub-lethal concentrations (LCo, LC10 and LC25) reduced the survival, growth rates and egg laying capacity of both non-infected and S. mansoni-infected snails during 12 weeks of exposure in comparison with their control group. The hatchability percent of B. alexandrina eggs of one, three and six days old exposed to LCo, LC10, LC25, LC50 & LC90 concentrations of F. selloa marginata and B. thuringinesis kurstaki, significantly decreased by increasing their age and the tested concentrations. Exposing B. alexandrina snails to sub-lethal concentrations of the tested agents for 24 hours either pre-, during or post exposure of snails to S. mansoni miracidia caused a marked reduction in the infection rate and decreased the mean total number of shedding cercariae/snail. Also, elongated their prepatent period (cercarial incubation period) and shortened the duration of cercarial shedding in comparison with their control group. Under semi-field conditions the more time of exposure to the concentration (LC90= 84.35 ppm) of the plant F. selloa marginata the more mortality among snails. The mortality rates of the snails were 0%, 2%, 18% and 30% at 3, 6, 12 and 24 hrs post exposure, respectively.
RESUMEN
La polilla del tomate (Tuta absoluta Meyrick; Lepidoptera: Gelechiidae) es una de las plagas más devastadorasdel tomate en Colombia y países suramericanos, produciendo pérdidas de hasta el 100% en cultivos sin protección.En 2009, T. absoluta se detectó en España, Portugal y países del mediterráneo, además de Inglaterra,Bulgaria y Alemania. Para su control se utilizan insecticidas químicos que generan resistencia e impactoambiental y de salud. La alternativa de utilizar biopesticidas contra esta plaga es de importancia creciente. Eneste estudio se evaluaron cinco métodos de bioensayo para medir adecuadamente la toxicidad sobre larvasde T. absoluta de tres productos comerciales: Dipel®, XenTary® y Turilav®, formulaciones a base de Bacillusthuringiensis (Bt). El método de Inmersión del folíolo, con el producto Dipel®, causó el 100% de mortalidadde larvas y 96% de supervivencia del testigo; este método presentó diferencias significativas al segundo(F=0,025, p>0,05) y cuarto (F=0,0018, p>0,05) día después de la aplicación (DDA). El método de Aspersiónfoliar por aerógrafo produjo 100% de mortalidad de larvas con Dipel® al segundo DDA (F=7,94x10-10,p> 0,05), y produjo diferencias significativas también al cuarto DDA (F=3,45x10-6, p>0,05). Los métodosFoliolos sumergidos y Medio de cultivo provocaron una alta mortalidad en el control por lo que fueron rechazados.El uso de Dipel®, XenTari® y Turilav® en concentración de 1,25 g/L causó entre 80-100% demortalidad entre el segundo y octavo DDA en tres métodos evaluados válidos (1, 2, 3), además corrobora laactividad biológica de B. thuringiensis sobre este insecto plaga.
The tomato moth (Tuta absoluta Meyrick; Lepidoptera: Gelechiidae) is one of the most devastating tomatopests in Colombia and South-American countries, producing losses of up to 100% in unprotected crops. T.absoluta was detected in Spain, Portugal and Mediterranean countries in 2009, as well as England, Bulgaria andGermany. Chemical insecticides are used for controlling it; however, they produce resistance and an environmentaland human health impact. Finding an alternative to using biopesticides against this pest is becomingincreasingly important. This study evaluated five bioassay methods measuring three commercial productstoxicity on T. absoluta larvae: Dipel, XenTary and Turilav Bacillus thuringiensis (Bt) -based formulations. The leafdipping bioassay method caused 100% larvae mortality with Dipel, the control group having 95% survivalrate. The other products showed significant differences on the 2nd (F=0.025, p>0.05) and 4th (F=0.0018,p>0.05) days after application (DAA). The leaf spray airbrush method produced 100% larvae mortality withDipel on the 2nd DAA, having significant differences from the other products tested on 2nd (DAA F=7.94 x10-10, p>0.05 ), 4th (F=3.45x10-6, p>0.05 ) and 8th (F=1.07x10-5, p>0.05 ) DAA. Submerged leaflet and culturemedium methods caused high mortality in controls and were thus rejected. A variation of the leaflet immersionmethod was standardised. The three commercial products produced high mortality in Lab conditionsregarding T. absolute larvae control at 1.25 g/L concentration, thereby corroborating the biological activity ofB. thuringiensis against this insect pest.