Sex ratios and Ophryocystis elektroscirrha infection levels of Danaus plexippus during spring migration through Oklahoma, USA

Abstract

AbstractMonarch butterflies, Danaus plexippus L. (Lepidoptera: Nymphalidae), have a multiple brood migration in the spring as they move between their overwintering grounds and summer breeding grounds. In Oklahoma, USA, monarchs produce at least one generation in the spring, which develops and continues the northward migration, leaving Oklahoma without a breeding population during the hot summer months. Female monarchs leave the overwintering grounds prior to males, but it is not clear whether females re‐colonize areas along the migration route prior to, or at the same time as males. Male‐to‐female ratios are 1:1 at emergence, but studies have identified a male‐biased sex ratio in the field. Both males and females are susceptible to infection by the obligate protozoan parasite, Ophryocystis elektroscirrha McLaughlin & Myers (OE), which reduces flight abilities and life spans of infected individuals. We examine sex ratios during the spring migration through Oklahoma and whether sex ratios or OE infection estimates vary with capture technique (active or passive). Our data suggest populations are male‐biased during the 1st week of spring migration in Oklahoma, but shift to female‐biased by the 3rd week in both cool and warm springs. Therefore, males may leave southern areas prior to females or migrate longer distances per day. Active sampling (i.e., netting) did not bias sex compared to passive sampling (i.e., sticky traps). Significantly fewer OE‐carrying monarchs (with two or more spores) were captured via netting than by sticky traps which may be caused by sticky trap glue affecting tape sampling effectiveness, but there was no difference in the number of heavily infected individuals (more than 100 spores). Therefore, data from netted monarchs may underestimate OE infection rates within populations.