Dichotomous choice trials for <i>Callosobruchus maculatus</i> females (Indian and Brazilian strains) selecting seeds for oviposition.

<p>(A) Illustrations of the four seed conditions: Seed alone (Control), and three experimental conditions: Exp 1 seed has an egg laid by another female, Exp 2 has a living larva inside, with visual and chemical cues from the egg surface removed; Exp 3 is the same as for Exp 2 except the larva inside has been killed. (B, C) Results showing the proportion of trials where females chose the experimental condition over the control for the Indian and Brazilian strains respectively. The Indian strain avoids choosing a seed with a live larva even when external markers have been removed. Asterisks indicate significant departure from random expectation based on the adjusted <i>G</i> test (<i>P</i> < 0.05).</p

Vibrations generated by a feeding cowpea beetle larva <i>Callosobruchus maculatus</i>.

<p>(A) X-ray image of an 18-day old larva inside a mung bean seed; scale bar = 0.5 mm. (B) Power spectra of vibrations recorded from five individual larvae. (C) Bottom trace is a representative waveform recorded with a laser vibrometer from a seed containing a feeding larva (18 days old) (Scale bar: 2 sec). Top box is an expanded segment of the waveform and corresponding spectrogram showing the recurring nature of the feeding pattern and corresponding spectrogram (Scale bar: 100 ms).</p

Egg laying cowpea beetle.

<p>A female <i>Callosobruchus maculatus</i> beetle inspects a seed prior to oviposition. Arrow points to a freshly laid egg on the seed surface.</p

Dichotomous choice trials for <i>Callosobruchus maculatus</i> females choosing between vibrating and immobile seeds for oviposition.

<p>Proportion of females from two strains (Indian and Brazilian) selecting either immobile or artificially vibrating seeds. Asterisks indicate significant departure from random expectation based on the adjusted <i>G</i> test (<i>P</i> < 0.05).</p

Latency to lay eggs on seeds in adult female cowpea beetles <i>Callosobruchus maculatus</i> during dichotomous choice trials.

<p>(A) A comparison of egg-laying latencies between Indian and Brazilian strain females showing that Indian females take longer to lay; the box plots indicate the median (solid line), mean (dashed line), and range of dispersion (lower and upper quartiles, and outliers) of the latency results. (B) Correlation between latency for egg laying and body mass of adult females from two strains (Indian as diamonds and Brazilian as circles).</p

Filled contour plots exhibiting the effect of conspecific and heterospecific densities on the population growth of two species of stored grain beetles (<i>Sitophilus zeamais</i> and <i>Rhyzopertha dominica</i>) reared on maize grains that were untreated or treated with 0.7 ppm of the organophosphate insecticide fenitrothion.

<p>Filled contour plots exhibiting the effect of conspecific and heterospecific densities on the population growth of two species of stored grain beetles (<i>Sitophilus zeamais</i> and <i>Rhyzopertha dominica</i>) reared on maize grains that were untreated or treated with 0.7 ppm of the organophosphate insecticide fenitrothion.</p

Insecticide-Mediated Shift in Ecological Dominance between Two Competing Species of Grain Beetles

<div><p>Competition is a driving force regulating communities often considered an intermittent phenomenon, difficult to verify and potentially driven by environmental disturbances. Insecticides are agents of environmental disturbance that can potentially change ecological relationships and competitive outcomes, but this subject has seldom been examined. As the co-existing cereal grain beetle species <i>Sitophilus zeamais</i> Motschulsky and <i>Rhyzopertha dominica</i> F. share a common realized niche, directly competing for the same resources, they were used as models in our study. Intraspecific competition experiments were performed with increasing insect densities and insecticide doses in additive and replacement series using various density combinations of both beetle species maintained on insecticide-free or -sprayed grains. Insecticide-mediated release from competitive stress was not observed in our study of intraspecific competition in grain beetles. The insecticide enhanced the effect of insect density, particularly for the maize weevil <i>S. zeamais</i>, further impairing population growth at high densities. Therefore, insecticide susceptibility increased with intraspecific competition favoring insecticide efficacy. However, the effect of insecticide exposure on competitive interaction extends beyond intraspecific competition, affecting interspecific competition as well. <i>Sitophilus zeamais</i> was the dominant species when in interspecific competition prevailing in natural conditions (without insecticide exposure), but the dominance and species prevalence shifted from <i>S. zeamais</i> to <i>R. dominica</i> under insecticide exposure. Therefore, high conspecific densities favored insecticide efficacy, but the strength of the relationship differs with the species. In addition, the insecticide mediated a shift in species dominance and competition outcome indicating that insecticides are relevant mediators of species interaction, potentially influencing community composition and raising management concerns as potential cause of secondary pest outbreaks.</p></div

Effect of heterospecific density on the population growth of two species of stored grain beetles (<i>Sitophilus zeamais</i> and <i>Rhyzopertha dominica</i>) reared on maize grains free of insecticide residue (a) and reared on maize grains treated with 0.7 ppm of the organophosphate insecticide fenitrothion (b).

<p>Each symbol represents the results of an experimental replicate.</p

Ordination (CVA) diagram showing the behavioral divergence among males and females of populations of the maize weevil <i>Sitophilus zeamais</i> (see <b>Table 3</b>).

<p>The symbols are centroids of treatments representing the class mean canonical variates.</p

Proportion of the behavioral data variance explained by each canonical variate or factor generated from CVA or R-factor analysis, respectively.

<p>Proportion of the behavioral data variance explained by each canonical variate or factor generated from CVA or R-factor analysis, respectively.</p