Density-mediated foraging behavioral responses of Rhyzopertha dominica (Coleoptera: Bostrichidae) and Tribolium castaneum (Coleoptera: Tenebrionidae)

Abstract

Tribolium castaneum and Rhyzopertha dominica are cosmopolitan, destructive postharvest pests. Although research has investigated how high densities of T. castaneum affect attraction to the aggregation pheromone by conspecifics, research into the behavioral response of both species to food cues after high density exposure has been lacking despite its importance to foraging ecology. Our goal was to manipulate and observe the effects of crowding on the behavioral response of both species to common food and pheromonal stimuli and to determine how the headspace emission patterns from grain differed under increasing densities. Densities of colonies for both species was altered (10-500 adults) on a fixed quantity of food (10 g of flour or whole wheat), then the behavioral response to common food and pheromonal cues was evaluated in a wind tunnel and release-recapture experiment, while volatiles were examined through gas chromatography coupled with mass spectrometry. Importantly, at least for T. castaneum, crowded conditions attenuate attraction to food-based stimuli, but not pheromonal stimuli. Crowding seemed to have no effect on R. dominica attraction to food and pheromonal stimuli in the wind tunnel, but exposure to high density cues did elicit 2.1-3.8-fold higher captures in traps. The relative composition and abundance of headspace volatiles emitted varied significantly with different densities of beetles and was also species-specific. Overall, our results have implications for expanding our understanding of the foraging ecology of two economically important pests.

Keywords: Integrated pest management; Lesser grain borer; Red flour beetle; Semiochemicals; Stored products; Taxis.

Figure 1

Schematic of wind tunnel experimental setup in the laboratory under controlled conditions in a walk-in growth chamber.

Figure 2

Total percent of T. castaneum reared at different densities exiting the release arena in a wind tunnel assay on the upwind stimulus edge for various food and pheromone attractants. Upper case letters represent pairwise comparisons among semiochemical treatments across beetle densities, while lower case letters represent pairwise comparisons among beetle densities within a semiochemical treatment. Bars with shared letters are not significantly different from each other (χ²-test, Bonferroni correction). Ctrl unbaited control, CF contaminated food from 500 beetle density, WG wheat germ oil, UCF uncontaminated food with no beetles, TSO Trece Storgard Oil, and SPB Stored Product Beetle Pheromone Tab from Insects Limited.

Figure 3

The total percent of R. dominica exiting the release arena in a wind tunnel assay on the upwind stimulus edge for various food and pheromone attractants. Upper case letters represent pairwise comparisons among semiochemical treatments across beetle densities, while lower case letters represent pairwise comparisons among beetle densities within a semiochemical treatment. Bars with shared letters are not significantly different from each other (χ²-test, Bonferroni correction). Bars with letters omitted are where within semiochemical treatments responses did not differ among beetle densities. Ctrl unbaited control, CF contaminated food from 500 beetle density, WG wheat germ oil, UCF uncontaminated food with no beetles, TSO Trece Storgard Oil, and SPB Stored Product Beetle Pheromone Tab from Insects Limited.

Figure 4

Percent (± SE) of T. castaneum adults reared at different densities and recaptured in pitfall traps baited with different semiochemical treatments under constant 27.5ºC and 60% RH after 24 h. A total of N = 8 replicates for each treatment combination. Upper case letters represent comparisons among semiochemical treatments across densities, while lower case letters represent comparisons within a semiochemical treatment among densities. Bars with shared letters are not significantly different from each other (Tukey HSD, α = 0.05). Ctrl unbaited control, CF contaminated food from 500 beetle density, UCF uncontaminated food with no beetles, TSO Trece Storgard Oil, and SPB Insects Limited SPB lure.

Figure 5

Percent (± SE) of R. dominica adults reared at different densities and recaptured in pitfall traps baited with different semiochemical treatments under constant 27.5ºC and 60% RH after 24 h. A total of N = 8 replicates for each treatment combination. Upper case letters represent comparisons among semiochemical treatments across densities, while lower case letters represent comparisons within a semiochemical treatment among densities. Bars with shared letters are not significantly different from each other (Tukey HSD, α = 0.05). Colony density was effectively 18 beetles/jar. Ctrl unbaited control, CF contaminated food from 100 beetle density, UCF uncontaminated food with no beetles, TSO Tréce Storgard oil, and SPB Insects Limited SPB lure.

Figure 6

Ordination (non-metric multidimensional scaling plot) of density emissions from wheat based on Bray–Curtis indices calculated in a pairwise fashion among R. dominica and T. castaneum A) reared at different densities (0—black or grey, 10—dark blue, 50—light blue, 100—medium blue, 500—green, colony density—purple) after 3–4 weeks, and B) differences in volatiles by species-specific emissions with R. dominica in green and T. castaneum in blue. Stress was less than 0.07, suggesting valid interpretation is possible. There were a total of n = 5–12 replicates per density, and n = 20–29 replicates per species.

Figure 7

Mean total volatile density emissions by T. castaneum (top panel) and R. dominica (bottom panel) after 4 weeks based on density. Bars with shared letters are not significantly different from each other (Tukey HSD, α = 0.05). To be included, compounds were required to be at least 0.7% the height of the major peak in the sample.