Survival of first instar larvae ofDanaus plexippus (Lepidoptera: Danainae) in relation to cardiac glycoside and latex content ofAsclepias humistrata (Asclepiadaceae)

Abstract

Our paper addresses field survivorship of first instar monarch butterfly larvae (Danaus plexippus L., Lep.: Danainae) in relation to the dual cardenolide and latex chemical defenses of the sand hill milkweed plant,Asclepias humistrata (Asclepiadaceae) growing naturally in north central Florida. Survival of first instar larvae in the field was 11.5% in the first experiment (15–20 April 1990), and dropped to 3.4% in the second experiment (20–30 April). About 30% of the larvae were found glued to the leaf surface by the milkweed latex. Predator exclusion of non-flying inverte-brates by applying “tanglefoot” to the plant stems suggested that the balance of the mortality was due to volant inverte-brates, or to falling and/or moving off the plants. Regression analyses to isolate some of the other variables affecting survivorship indicated that first instar mortality was correlated with (1) increasing cardiac glycoside concentration of the leaves, (2) increasing age of the plants, and (3) the temporal increase in concentration of cardiac glycosides in the leaves. The study also provided confirmatory data of previous studies that wild monarch females tend to oviposit onA. humistrata plants containing intermediate concentrations of cardiac glycosides. Cardiac glycoside concentration in the leaves was not correlated with that in the latex. The concentration of cardenolide in the latex is extremely high, constituting an average of 1.2 and 9.5% of the mass of the wet and dry latex, respectively. The data suggest that an increase in water content of the latex is compensated for by an influx of cardenolide with the result that the cardenolide concentration remains constant in the latex systems of plants that are growing naturally. We also observed first instar larvae taking their first bite of milkweed leaves in the field. In addition to confirming other workers findings that monarch larvae possess elaborate “sabotaging” behaviour of the milkweed's latex system, we discovered that several larvae on their first bite involuntarily imbided a small globule of latex and instantly became cataleptic. This catalepsis, lasting up to 10 min, may have been in response to the high concentration of cardenolide present in the latex ofA. humistrata, more than 10 times that in the leaves. The results of the present study suggest that more attention should be directed to plant chemical defenses upon initial attack by first instar insect larvae, rather than attempting correlations of plant chemistry with older larvae that have already passed the early instar gauntlet. The first bite of neonate insects may be the most critical moment for coping with the chemical defenses of many plants and may play a much more important role in the evolution of insect herbivory than has previously been recognized.