Thoughts
About Early Fall Monarch Sightings in Texas
By Dr. Bill Calvert
Overview
The main mass of migrants breezes through Texas in late September and
October. However, an early population of monarchs appears in Texas in
August and September. These early monarchs appear to be different from
the later mass migrant population. They are generally in worse condition
and they are actively breeding. We do not know how important this late
summer breeding population is in the life history scheme of the monarch
butterfly.
Discussion
Each year, during the very hot days of August and September, both adult
and larval monarchs are reported from scattered locations in Texas.
One important locus is the Texas Coast, where monarchs breed more or less continuously throughout the summer. They are able to do this because the non-native Mexican milkweed, Asclepias curassavica, presence in irrigated backyards or roadside gardens persists through the hot summer months.
Elsewhere, monarchs appear to vacate the state sometime in June and don't reappear until sometime in August.
This cycle closely follows the milkweed cycle. Milkweeds die back or disappear during the hot summer months. But widely scattered thundershowers of August apparently can bring up milkweeds sequestered underground thus providing food for monarchs.
The numbers of adults spotted pick up by mid-September apparently brought down by the rare weak fronts that occur during that season.
This population of monarchs seems very different from the main mass of migrants that will breeze through in late September and October. They are often found feeding and mating in low riparian (streamside) areas. Unlike the later migrant masses that always seem to be in pristine condition, they are sometimes very ragged and worn looking. This is especially true of the males. Often the ragged males are mating with not so ragged females. It is likely that these ragged monarchs will not make it to the overwintering areas and will be killed by some Texas frost.
It is tempting to speculate that the ragged males are using their last chance to cast their genes into the future by mating with a "healthier" female that may make it to the overwintering colonies deep in Mexico. But in order for this to work for the males, the females must be reproductive - that is they must have developing eggs in their ovarioles. At this time it is not known whether or not this is true.
There are 3 important milkweed species which monarch larvae feed on here. What happens here in Texas to the larval food plants depends on the species.
Two milkweed species (A. viridis and A. asperula) have a strong growing and maturing spurt in the spring--at the same time that monarchs are present in the spring. Both species seem to die back in June and July, essentially disappearing during the summer.
A 3rd milkweed species (A. Oenotheroides) is different. It grows in the spring and some of it does flower, but at this latitude most of the flowering occurs in the fall. It also seems to senesce or aestivate during the hot summer. Thus, when the milkweeds reappear, so do the monarchs.
The reasons behind this appearance of monarchs are controversial. Zaluki and Malcolm (monarch biologists) think that monarchs can't survive the heat. That is, that it’s the heat per se that kills them. But there are so many reports of monarchs present in high heat that it doesn't seem possible that heat kills them.
It may be a simple matter of food plant senescence during the hot summer months. ("Senescence" is when plants age or die back. )
"As I mentioned, monarchs reappear here in early September or even late August. It is still extremely hot in Texas then, and yet there is a flourishing monarch population.
This is the principal reason that I think the system is host plant - not temperature-- driven. (Allowing for the possibility that heat plays a strong role in host plant cycles.)"
