Switching among natal and auxiliary hosts increases vulnerability of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) to insecticides

Abstract

The role of insecticidal application and host plant resistance in managing Spodoptera exigua has been well documented, but the effect of different host plants, on which the pest cycles its population in the field, has seldom been investigated. Therefore, we have studied the vulnerability of S. exigua against commonly used insecticides (cypermethrin, chlorpyrifos, lufenuron, and emamectin benzoate) with different mode of actions when it switches its generations from natal to auxiliary hosts and vice versa. Different field populations being established on different host plants including castor, cauliflower, cotton, okra, and spinach were collected and reared in the laboratory before insecticidal bioassays. The role of larval diet and host plant switching on their response to tolerate applied insecticides was studied using leaf-dip bioassay methods. Host switching demonstrated a significant role in altering the vulnerability of S. exigua populations to tested insecticides. Spodoptera exigua sourced from castor, when switched host to okra and spinach, exhibited 50% higher mortality when treated with emamectin benzoate. This trend in mortality was consistent upon complete host switch cycle (natal-auxiliary-natal host). However, the highest increase (92%) in vulnerability was recorded when the larvae were shifted to spinach from cotton. In general, chlorpyrifos and lufenuron had highest efficacies in terms of larval mortality. The findings of present studies provide insights to a better understanding the behavior of polyphagous pests and the role of different host plants in altering the susceptibility of these pests against applied insecticides. Ultimately the results warrant that due consideration should be given to cropping patterns and time of host switching by pest population during planning and executing chemical control.

Keywords: beet armyworm management; castor; cauliflower; cotton; host switching; integrated pest management; natal versus auxiliary hosts; okra; polyphagous; spinach.

Figure 1

Detailed schema presenting major events in laboratory culturing, host switching and reversal, and bioassays

Figure 2

Comparative mortality of Spodoptera exigua due to different insecticide treatments on castor (a), cauliflower (b), cotton (c), okra (d), and spinach (e) when no switching from natal host (dark gray bars) and host switching to auxiliary hosts (light gray bars) occurred. Bars represent average percent larval mortality (%), and the error bars are 95% CI. The lowercase letters above bars indicate the outcomes of post hoc pairwise comparisons across hosts and insecticides (p < .05; Fisher's LSD with Benjamini–Hochberg correction for multiple pairwise comparisons). Bars with the same letter, for each of the comparisons outlined above, were not statistically different from one another as indicated by LSD tests

Figure 3

Comparative mortality of Spodoptera exigua due to different insecticide treatments on castor (a), cauliflower (b), cotton (c), okra (d), and spinach (e) when no switching from natal host (dark gray bars) and reverse switching from auxiliary hosts to respective natal hosts (light gray bars) occurred. Bars represent average percent larval mortality (%), and the error bars are 95% CI. The lowercase letters above bars indicate the outcomes of post hoc pairwise comparisons across hosts and insecticides (p < .05; Fisher's LSD with Benjamini–Hochberg correction for multiple pairwise comparisons). Bars with the same letter, for each of the comparisons outlined above, were not statistically different from one another as indicated by LSD tests

Figure 4

Percent change in mortality of Spodoptera exigua due to different insecticidal treatments on various natal hosts due to host shifting (a, c, e, g, i) and after a complete shift cycle (b, d, f, h, j)