Defaecation timing of the Triatominae vectors of Trypanosoma cruzi directly affects the transmission probability of Chagas disease to mammal hosts. Experimental studies with fifth instar nymphs of Triatoma infestans showed that defaecation time was negatively affected by blood meal size and positively affected by starvation period and bug initial weight. Since blood meal size and starvation period are both density-dependent, low density domestic populations of T. infestans would represent a higher transmission risk than high density populations. As low density populations could occur in recently reinfested houses after control using insecticide, vigilance activities should be reinforced to protect the human population at the highest risk of disease transmission.

Los cultivos Bt ejercen alta presión de selección sobre las plagas blanco. Para mitigar la evolución de resistencia, la estrategia Alta Dosis-Refugio (AD-R) requiere: a) una concentración de toxina en la planta suficientemente elevada para provocar que los alelos resistentes sean funcionalmente recesivos; b) baja frecuencia inicial del alelo resistente; c) refugios con plantas no-Bt. Para reforzar la estrategia AD-R, se propone la acumulación de dos o más toxinas en una planta (genes “piramidados”), bajo el supuesto de que estas tienen sitios de acción diferentes, a fin de minimizar el riesgo de resistencia cruzada. Una reciente revisión respalda las fortalezas de la estrategia AD-R en la demora de la evolución de resistencia. El incumplimiento de uno o más componentes de esta estrategia explica los pocos casos de resistencia confirmados. En San Luis, Argentina, se detectaron importantes daños de Diatraea saccharalis en híbridos de maíz Bt. Entre las hipótesis para explicar estos daños, se incluyen dominancia del alelo resistente, baja concentración de toxina y bajo nivel de adopción de refugios. Esto último motiva discusiones sobre la conveniencia de implementar refugios integrados. Se recomienda un abordaje caso por caso para diseñar estrategias de manejo de resistencia de acuerdo a cada plaga.

1.  To investigate the effect of insect recolonization on the insecticide‐induced resurgence of crop pests, a modified Lotka–Volterra predator–prey model was partitioned into two areas, ‘sprayed’ and ‘unsprayed’. 2.  The unsprayed area provided a source of insects to recolonize the sprayed area, resulting in a change in pest dynamics in both areas following an insecticide application. 3.  Model sensitivity to insecticide selectivity and rates of predation, insect dispersal and pest population increase were examined. 4.  Resurgence risk in the sprayed area increased with increasing pest dispersal rate, but decreased with increasing predator dispersal rate. 5.  Pest resurgence could also occur in the unsprayed area, especially when prey dispersal rates were low. The extent of resurgence in the unsprayed area could in some circumstances be worse than in the sprayed area itself. 6.  The more efficient and longer‐lived the predators, the greater the level of pest resurgence in both areas following insecticide use. 7.  More selective insecticides, killing the pest, but not the predator, reduced resurgence provided that the average life‐span of the predators was reasonably long. Even highly selective insecticides could cause resurgence of the pest in the unsprayed area. 8.  When the prey carrying capacity of the unsprayed area was increased relative to that of the sprayed area, resurgent effects in the unsprayed area were reduced, but could still be significant under some circumstances.

Different biological parameters of the stink bugs, Nezara viridula L. and Piezodorus guildinii Westwood (Hemiptera: Pentatomidae), are affected by the developmental stage of the soybean (Glycine max Merrill) pods they feed on. These effects of the soybean on the stink bugs could represent a selection pressure leading to the ability of these species to discriminate the phenological stage of soybean pods, and, therefore, to exhibit feeding preferences. We designed three studies: (1) Distant detection of soybean pods through an olfactometer; (2) Free choice tests to evaluate preferences for soybean pods of different developmental stages; (3) No choice tests to study effects of soybean pod development on feeding time and number of probes. Stink bugs showed no differential response to olfactometer arms with or without soybean pods, suggesting an inability to detect soybean volatiles. Free choice tests showed no species effects on pods selection, but significant differences among fifth instar nymphs, adult male, and adult females. Fifth instar nymphs fed more frequently on soybean pods of advanced development stages compared to female adults, despite previous evidence showing poor development of stink bugs fed pods of the same stage. No choice tests showed significant effects of stink bug species, stink bug stage and sex, and soybean pod phenology. N. viridula expressed shorter feeding times and higher numbers of probes than P. guildinii. The highest numbers of probes of both species were observed when they were fed soybean pods in early phenological stages. When placed in direct contact with food, fifth instar nymphs prefered to feed on more developed pods, despite these pods being suboptimal food items. These results suggest that for the ecological time framework of soybean-stink bugs coexistence, around thirty-five years in Argentina, the selection pressure was not enough for stink bugs to evolve food preferences that match their performance on soybean pods of different development stages.

The spatial pattern of the Río Cuarto Corn Disease vector, Delphacodes kuscheli (Hom., Delphacidae), was analysed in oat fields within the endemic area of the disease, during the growing seasons 1993 and 1994. The spatial pattern was analysed by fitting the probabilistic models Poisson and negative binomial and estimation of single‐date and overall aggregation indices. The population of the different stage classes, sex, and wing forms showed a significant trend to aggregation as the negative binomial model fitted the observed frequency distributions in more than 78% of the cases (sampling dates) while the Poisson model fitted well in only 28% of cases or less. Single‐date aggregation index, CA, ranged from 0.3 to 0.84. Overall (whole season) aggregation index, CA*, estimated through the Bliss and Owen’s regression method, ranged from 0.18 (female adults) to 1.08 (nymphs I–II), indicating a moderate degree of aggregation compared with other planthopper species. There were no significant relationships between aggregation and population density. The minimum number of sampling units and critical lines for sequential sampling plans were calculated based on the estimation of CA* for the precision levels (D) 0.1, 0.2 and 0.3. Even low degrees of aggregation, like that of adults, demand much more sampling effort than randomly distributed populations, particularly at high densities. General implications and limitations of the proposed sampling plans for monitoring the vector population abundance are discussed.

Locusts are major intermittent threats to food security and the ecological factors determining where and when these occur remain poorly understood. For many herbivores, obtaining adequate protein from plants is a key challenge. We tested how the dietary protein : non-structural carbohydrate ratio (p : c) affects the developmental and physiological performance of 4th-5th instar nymphs of the South American locust, Schistocerca cancellata, which has recently resurged in Argentina, Bolivia and Paraguay. Field marching locusts preferred to feed on high carbohydrate foods. Field-collected juveniles transferred to the laboratory selected artificial diets or local plants with low p : c. On single artificial diets, survival rate increased as foods became more carbohydrate-biased. On single local plants, growth only occurred on the plant with the lowest p : c. Most local plants had p : c ratios substantially higher than optimal, demonstrating that field marching locusts must search for adequate carbohydrate or their survival and growth will be carbohydrate-limited. Total body lipids increased as dietary p : c decreased on both artificial and plant diets, and the low lipid contents of field-collected nymphs suggest that obtaining adequate carbohydrate may pose a strong limitation on migration for S. cancellata . Anthropogenic influences such as conversions of forests to pastures, may increase carbohydrate availability and promote outbreaks and migration of some locusts.

The influence of weather factors and the predator Doru luteipes Scudder density on Diatraea saccharalis (F.) egg predation was studied. Mortality of D. saccharalis eggs was determined by artificially infesting maize plots with egg masses at various times within each of two maize-growing seasons. Each egg cohort was monitored every 24-48 h to determine the fate of eggs, and predation rates were calculated. Doru luteipes were sampled every 7-10 d, and the mean air temperature, the minimum percentage of relative humidity, and rainfall accumulations were recorded during the egg exposure period. To test the effects of abiotic and biotic variables on egg predation, we used a generalized linear model (GLM). Diatraea saccharalis egg predation was negatively associated with rainfall, whereas D. luteipes density and mean temperature were positively correlated with mortality. The implications of these findings for the management of D. saccharalis are discussed.

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