Pigeon pea crop stage strongly influences plant susceptibility to Helicoverpa armigera (Lepidoptera: Noctuidae)

Abstract

Helicoverpa armigera Hübner (Lepidoptera: Noctuidae; Hübner) is the major insect pest of pigeon pea [Cajanus cajan; Fabales: Fabaceae; (L.) Millspaugh] worldwide. Research to develop pest management strategies for H. armigera in pigeon pea has focused heavily on developing less susceptible cultivars, with limited practical success. We examined how pigeon pea crop stage influences plant susceptibility to H. armigera using a combination of glasshouse and laboratory experiments. Plant phenology significantly affected oviposition with moths laying more eggs on flowering and podding plants but only a few on vegetative plants. Larval survival was greatest on flowering and vegetative plants, wherein larvae mostly chose to feed inside flowers on flowering plants and on the adaxial surface of expanding leaves on vegetative plants. Larval survival was poor on podding plants despite moths laying many eggs on plants of this stage. When left to feed without restriction on plants for 7 days, larvae feeding on flowering plants were >10 times the weight of larvae feeding on plants of other phenological stages. On whole plants, unrestricted larvae preferred to feed on pigeon pea flowers and on expanding leaves, but in no-choice Petri dish assays H. armigera larvae could feed and survive on all pigeon pea reproductive structures. Our results show that crop stage and the availability of flowers strongly influence pigeon pea susceptibility to H. armigera. An increased understanding of H. armigera-pigeon pea ecology will be useful in guiding the development of resistant varieties and other management tactics.

Keywords: herbivory; host-plant resistance; phenology; preference–performance hypothesis.

Fig. 1.

Mean total eggs laid on plants (3 plants per cage) of different crop phenological stages from the oviposition no-choice experiment. Bars represent the means, and the error bars represent standard errors. Different letters indicate a difference according to Fisher’s LSD test.

Fig. 2.

Mean number of eggs laid on different plant structures for the flowering and reproductive crop stages in the no-choice oviposition experiment. Floral structures are summed up for clarity; they include bud initials, buds, flowers, and spent flowers. Bars represent means and error bars represent standard errors. Different letters indicate a difference according to Fisher’s LSD test.

Fig. 3.

Female moth dissection data from the no-choice oviposition experiment—the proportion of moths fertilized per cage (n = 3 females per cage) and the average spermatophore count per female. Values are means ± standard errors. Different letters indicate differences in the means according to Fisher’s LSD test.

Fig. 4.

Larval survival when restricted to different locations on vegetative, flowering, and podding pigeon pea plants in the caged early instar establishment experiment. Bars represent means and error bars represent standard errors. Different letters indicate a difference according to Fisher’s LSD test. There is no error bar for the flowering raceme treatment because all replicates had 100% survival.

Fig. 5.

Larval survival and development from the uncaged early instar experiment. Values on the y-axis are mean proportions, and error bars represent the standard error of the mean. Different letters indicate a significant difference among treatments according to Fisher’s protected LSD test.

Fig. 6.

Larval distributions from the uncaged early instar experiment. Values on the y-axis are the mean proportions of larvae at plant locations after 3 days, and error bars represent the standard error of the mean. Different plant placement location treatments were pooled as placement location did not influence final larval distribution (Supplementary Fig. S1). Different letters indicate a significant difference in larval distribution within the crop stage treatments. Fisher’s protected LSD test was used to compare the distributions in the vegetative stage, and Kruskal–Wallis tests were used for the flowering and podding stages. Not all locations are present for all crop stages (i.e., vegetative plants lack flowers and pods; flowering plants lack expanding leaves, growing tips, and pods; and podding plants lack expanding leaves, growing tips, and flowers).

Fig. 7.

Larval performance measures (survival, development, and weight) from the 7 days unrestricted larvae experiment. Values on the y-axis are means and error bars represent the standard error of the mean. Different letters indicate a significant difference among treatments according to Fisher’s protected LSD test.

Fig. 8.

Proportion of larvae that survived after 96 h on different pigeon pea reproductive structures in the Petri dish assay, n = 30 replicates per treatment.