Callosamia promethea

Multiple Pairings: Impact on Egg Deposition Numbers

by Gene Morton

I wrote you asking about multiple matings in promethea moths probably two years ago. Well, in 2006, I did the experiments with multiple matings and degree of fertility and number of eggs laid. I am writing the results up now and will gladly send you a copy of the manuscript if you would like to see it. But, the upshot is that, yes, promethea females always mate every afternoon for at least 3 days, but that their fertility is not higher than a female that is allowed to mate only once. However, multiple-mated females lay signfiicantly more eggs than those only allowed to mate once!

I followed 37 females from six different sibling groups, allowing about half of the females to mate only once by keeping them inside a screened porch and the other half of each sib group to mate twice by putting them outside the porch on a twig when they began calling on their second day. I tallied the number of eggs laid in each night's clutch for each female throughout her laying period and checked each clutch upon hatching for unhatched eggs (these were checked for fertility vs a dead larva under a microscope). This was a very tedious process! But fun.

I found Gene's notes very interesting, and I immediately asked myself two questions:

1) Why would the moths pair more than once if there appeared to be no increased fertility?

2) Why should females that have paired more than once lay more eggs than those which only paired once?

There would seem an advantage to multiple pairings with regard to genetic diversity, and I would wonder why this is not a more commonly observed practice with other species. Since the promethea matings take place during the day and frequently only last a few hours, there would be no loss of night-egg-laying-time to such day-pairing females.

A practice of multiple night pairings of night-pairing species, with pairings typically lasting close to twenty hours, however, might severely diminish the time females of other species would have for egg depostion.

I am completely baffled as to why females that have paired more than once would lay more eggs than those which only paired once.

I wonder if the females kept in cages (single pairings) might be experiencing higher temperatures inside the screened porch as compared to those set out on a twig. If so the higher temperatures might influence moisture loss and shorten the life expectancy of these females, resulting in fewer eggs being deposited.

I also wondered if Gene's "outside females" were pairing with wild males which might might be carrying a more volumous "payload" of sperm, as opposed to reared, smaller males inside the cages. Simple internal pressures might stimulate ovipositing behaviour. Replenished sperm through multiple matings could also increase internal pressures.

I very much look forward to reading Gene's manuscript, and will post a summary here.

I am also very interested in learning if anyone else has experienced anything similar with promethea or other Saturniidae species.

I still have promethea cocoons (February 2008) available for sale if anyone else wants to try some experiments.

Bill Oehlke

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