Abstract
Pieris brassicae L. is a serious pest of cultivated crucifers in several parts of the world. Larvae of P. brassicae also feed prolifically on garden nasturtium (Tropaeolum majus L., of the family Tropaeolaceae). Proteolytic digestion was studied in larvae feeding on multiple hosts. Fourth instars were collected from cauliflower fields before transfer onto detached, aerial tissues of selected host plants in the lab. Variable levels of midgut proteases were detected in larvae fed on different hosts using protein substrates (casein and recombinant RBCL cloned from cauliflower) and diagnostic, synthetic substrates. Qualitative changes in midgut trypsin activities and quantitative changes in midgut chymotrypsin activities were implicated in physiological adaptation of larvae transferred to T. majus. Midgut proteolytic activities were inhibited to different extents by serine protease inhibitors, including putative trypsin inhibitors isolated from herbivore-attacked and herbivore-free leaves of cauliflower (CfTI) and T. majus (TpTI). Transfer of larvae to T. majus significantly influenced feeding parameters but not necessarily when transferred to different tissues of the same host. Results obtained are relevant for devising sustainable pest management strategies, including transgenic approaches using genes encoding plant protease inhibitors.
Highlights
P. brassicae of the order Pieridae is purported to have a Palearctic distribution with reports from Asia, Europe, and North Africa of host plants belonging to families Cruciferae, Resedaceae, Papilionaceae, Umbelliferae, and Trapaeolaceae (Feltwell, 1978)
Rubiscolytic activities measured in midgut samples of larvae found feeding on mustard (MT-ctrl), radish (RD-ctrl), and T. majus (Tp-ctrl) were significantly higher at p ≤ 0.05, as compared to the total midgut proteolytic activities detected in larvae feeding on cauliflower and cabbage
Multiple trypsin substrates were employed as fourth instar P. brassicae larvae fed on cauliflower have multiple pH optima for total midgut protease activities detected using protein substrates viz. casein and rRBCL (Bhardwaj et al, 2014)
Summary
P. brassicae (the Large Cabbage Butterfly) of the order Pieridae is purported to have a Palearctic distribution with reports from Asia, Europe, and North Africa of host plants belonging to families Cruciferae, Resedaceae, Papilionaceae, Umbelliferae, and Trapaeolaceae (Feltwell, 1978). Broods of P. brassicae frequently defoliate host plants and cause farmers to spray insecticides including highly hazardous class 1b organo-phosphates (Sharma and Gupta, 2009; Weinberger and Srinivasan, 2009). Farms in the region are generally small-holdings where different crucifers are planted in adjacent fields as winter crops (Weinberger and Srinivasan, 2009). In years of high pest density, P. brassicae larvae are highly mobile and move from an exhausted food source to a proximal, alternative host plant of the same or different species. Larvae have been reported to move from and between various crucifers
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