Last month, Chip Taylor, Founder and Director of Monarch Watch, responded to a question related to the potential for monarch migration routes being shut down between Mexico and Texas because of the cold weather, and what this would mean for monarch population growth. The writer understood monarchs arriving early in Texas might not find nectar sources or milkweed on which to lay eggs, which could result in loss of population growth. But why would late arriving monarchs result in similar losses?

Typically this delayed arrival results in lower population numbers because the overwintering monarchs arriving late in Texas means, generationally, there may not be enough time egg to egg to increase population growth before starting the cycle all over again.

Late Arrivals

Chips response was “late arrivals mean late egg laying that produces a cohort of new adults 30+ days later that is also late relative to those that arrived earlier and matured their offspring earlier. The net effect for the population is to send a first generation north with a higher mean age to first reproduction than if all eggs were laid earlier. This later first generation cohort is likely to arrive later in the North as well as producing more late effects. The overall outcome could be a smaller fall population under some conditions.

Population growth is all about age to first reproduction – sometimes characterized as generation length. In terms of insects, I think of egg to egg intervals. I want them to be as short as possible if I want a population to grow. Degree days are one way to capture these population characteristics but degree days are not as good as direct observation for a number of reasons.

Importance of First Sightings Data

The dynamics of the growth of the population last year – when compared to all previous years – caused me to realize that the temporal and spatial distribution of eggs as well as the quantity laid by returning females – were important determinants of monarch population growth.

I’m slow to figure things out. It took me 26 years and lots and lots of First Sightings Data posted to Journey North to figure that out.

Thanks to Journey North and all those citizens who post their observations to that site.”

Note: The cold temperatures did abate last month, and monarchs had favorable conditions along both pathways out of Mexico.