Results and Discussion
The semiochemicals tested were successful at attracting pea leaf weevils in both the spring and fall activity periods, and trends were similar in the spring and fall. Baited traps captured significantly more pea leaf weevils than the blank controls (Wilcox rank sum test, p-value <0.0001; Figure 13).
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Our mixed effects model showed that pea leaf weevil trap capture is not influenced by either aggregation pheromone dose or by release tube size (Table 3; Figure 14). If neither of these lures is significantly more attractive to pea leaf weevils, the most optimal lure is the one that is the least expensive to produce: the lowest dose of pheromone in a small release tube. Although pheromone dose and release substrate did not influence the attractiveness of the lure, addition of host volatiles did. Lures that incorporate host volatiles are substantially more attractive than aggregation pheromone alone (Table 3; Figure 14). These relationships are seen in both the spring and fall. Based on the results of this study, the optimal lure for attracting pea leaf weevils in both the spring and fall is a low dose (21 mg) of aggregation pheromone loaded into a small release device (250 µL Eppendorf tube) and paired with host plant volatiles.
Figure 14. The average weekly pea leaf weevil trap catch is represented for each lure tested at our ten sites during both the spring and fall trapping periods. Our results indicated that neither the aggregation pheromone dose nor the release substrate influenced attraction of the pea leaf weevil to the lures. However, the addition of host plant volatiles significantly enhanced attraction of pea leaf weevils to the lure.
More pea leaf weevils were trapped in the fall than in the spring (Figure 15). A high rate of mortality during overwintering is the likely driver of this difference. However, we cannot rule out the possibility that pea leaf weevil adults may respond differently to these lures in the spring and fall, as adults are physiologically different during these two periods. To gain further insight into the possibility of phenotypic plasticity, the attractiveness of the pea leaf weevils to these lures could be measured in the lab. Olfactometer assays could test the behavioural response of adult pea leaf weevils to volatiles by exposing them to a combination of lures under controlled conditions. Electroantennograms, which measure nerve impulses in antennae, would further complement these studies and give us an understanding of the mechanisms behind these behaviours. I would also like to perform dissections to determine if there is a link between reproductive development and plasticity in response to the aggregation pheromone or host plant volatiles.
Pea leaf weevil captures are relatively high at the start of the spring trapping period and then decrease (Figure 14). Unfortunately, this means that we did not capture the start of peak activity. In 2014, trapping should start in late April in order to capture the entire spring activity peak. Weather conditons and crop phenology should also be recorded; correlating trap catches with possible determinants of pea leaf weevil activity will allow us to better predict the start of the spring flight.
Captures start off low in the fall but increase dramatically in mid-August (Figure 14). The fall peak of activity occurred at the end of August and coincided with harvest. The increase in pea leaf weevil catch in the fall could be explained by a number of factors. Once the pea crop has been harvested, these plants are no longer producing violatiles that compete with the semiochemical traps. The fall peak may also be caused by pea leaf weevils moving out of the pea field in response to the disruptive harvesting. Attraction of pea leaf weevils to these semiochemicals might also increase as they prepare to overwinter.
Pea leaf weevil captures are relatively high at the start of the spring trapping period and then decrease (Figure 14). Unfortunately, this means that we did not capture the start of peak activity. In 2014, trapping should start in late April in order to capture the entire spring activity peak. Weather conditons and crop phenology should also be recorded; correlating trap catches with possible determinants of pea leaf weevil activity will allow us to better predict the start of the spring flight.
Captures start off low in the fall but increase dramatically in mid-August (Figure 14). The fall peak of activity occurred at the end of August and coincided with harvest. The increase in pea leaf weevil catch in the fall could be explained by a number of factors. Once the pea crop has been harvested, these plants are no longer producing violatiles that compete with the semiochemical traps. The fall peak may also be caused by pea leaf weevils moving out of the pea field in response to the disruptive harvesting. Attraction of pea leaf weevils to these semiochemicals might also increase as they prepare to overwinter.
Figure 15. The average weekly pea leaf weevil catch is represented for the optimal lure identified in this study. This lure includes host volatiles paired with a low dose of aggregation pheromone in a small release tube. Peaks of adult pea leaf weevil activity were seen in the spring, when adults colonize pea and bean fields, and in the fall, when adults move to overwintering sites.
The results of this study have shown that pea leaf weevils are attracted to synthetic copies of aggregation pheromones and bean volatiles in both the spring and fall. Future field experiments will aim to capture the start of spring activity and to correlate biotic and abiotic factors with pea leaf weevil activity. I will also perform field experiments optimizing trap structure and placement and correlating real populations with trap captures. Future lab experiments will elucidate the relationship between physiology and attraction to these lures.