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The effect of cardiac glycosides (cardenolides) and nitrogen concentrations on host selection by Danaas plexippus L.
were investigated from two standpoints : (a) presence of eggs and (b) detailed observation on oviposition behaviour in relation to the host plants, Asclepias curassavica L. and A. fruticosa. L.. Two experiments were carried out, one for each approach. The first experiment involved counting the number of eggs and larvae (immatures) on the two species of milkweed and relating these counts to the plant biomass and cardenolide concentrations. A total of 323 plants (96 A. curassavica and 227 A. fruticosa) were searched from three randomly selected field plots. The 30 leaves and two flowers bearing immatures and
40 randomly selected leaves from plants without immatures were analysed for cardenolide content. The second experiment observed the actual oviposition behaviour and choice of oviposition site by D. plexippus in the field. Twelve females were followed and their behaviours recorded onto cassette tapes. Samples of plants on which they : (a) landed (n=54); (b) landed, tapped their antennae and/or dabbed their ovipositor (n=20); (c) all of (b) and laid an egg (n=31); and (d) did not alight but which were along their flight paths and/or nearest neighbour to those visited (n=16) were collected for cardenolide and
nitrogen analysis. Cardenolides concentration (in µg/0.1 gm.) varied widely between and within species and also between seasons. For A. curassavica, it varied between 104 and 804, mean 370±156 (n=34) in autumn and between 204 and 591, mean 382±.207 (n=9) in summer while for A. fruticosa it varied between 134 and 889, mean 450±193 (n=28) in autumn but between 119 and 719, mean 345±138 (n=120) in summer. An intermediate concentration level of between 200 and 300 µg/0.1 gm. seemed to be selected by D. plexippus for oviposition; 71% of eggs were laid in this concentration range. Monarchs seem to avoid plants with very low (< 200 µg/0.1 gm) and very high (> 600 µg/0.1 gm.) cardenolide concentrations; no egg was laid on plants with cardenolide content below and above these concentrations respectively. It is therefore postulated that concentrations below 200 µg/0.1 gm. are too low to stimulate oviposition while those above 600 µg/0.1 are to high (i.e deterrent). Nitrogen content seems to have no effect on host selection by D. plexippus. This is probabaly due to the fact that nitrogen is not limiting to the development of its larvae. Key words : Cardiac glycosides (cardenolides), nitrogen, concentration (content), milkweeds, Asclepias, A. curassavica. A, fruticosa. Monarch (Danaus plexippus), emetic potency, sequester, oviposition behaviour, egg laying, tapping of antennae, dabbing of ovipositor, alight, host selection, cues, visual stimuli, chemical stimuli.
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