Expression of Resistance in Amaranthus spp. (Caryophyllales: Amaranthaceae): Effects of Selected Accessions on the Behaviour and Biology of the Amaranth Leaf-Webber, Spoladea recurvalis (Lepidoptera: Crambidae)

Abstract

Spoladea recurvalis F. is a major pest moth of amaranth (Amaranthus spp.) flowers worldwide, with a potential of causing complete foliage loss under severe outbreaks. Chemical insecticides are uneconomical for resource-poor farmers and pose health and environmental risks. Host plant resistance (HPR) to insects is an effective, economical and environmentally friendly alternative that is poorly understood and largely unexploited among traditional leafy vegetables. A total of 35 amaranth accessions were evaluated for the expression of their antixenotic and antibiotic traits against S. recurvalis, focusing on their effects on the biology of the pest in comparison with a susceptible accession. The accession VI036227 was found to be highly resistant against the pest, exhibiting exemplary antibiosis by causing 100% larval mortality within the first 36 h, despite not being deterrent for oviposition. The accessions VI048076, VI056563 and VI047555-B demonstrated moderate resistance against the pest for specific parameters including low oviposition, moderate early stage larval mortality and reduced adult longevity. Total mortality and weight gain in these three accessions were, however, not significantly different from the susceptible control. Higher numbers of eggs were laid in no-choice compared to choice situations. The implications of these findings in the management of S. recurvalis on amaranths are discussed.

Keywords: IPM; antibiosis; antixenosis; longevity; mortality.

Figure 1

Weight gain (mg) and percentage weight gain (mean ± SE) by larvae of S. recurvalis when fed on different amaranth accessions for 48 h. (Mean weight gain (% weight gain) with the same lower (upper) case alphabet is not significantly different at p < 0.05, (Tukey’s test)). * Susceptible accession.

Figure 2

Total larval and pupal mortalities (mean ± SE) of S. recurvalis recorded on selected accessions of amaranth. (Means of total larval (pupal) mortality with the same lower (upper) case letters are not significantly different at p < 0.05, (Tukey’s test)). * Susceptible accession.

Figure 3

The mean instantaneous rate of population increase (r_i) among larvae of Spoladea recurvalis when exposed to different amaranth accessions. Positive values of r_i indicate a growing population, r_i = 0 indicates a stable population, and negative r_i values indicate a population in decline and headed toward extinction (Means with the same letter are not significantly different at p < 0.05 (Tukey’s test)). * Susceptible accession.