Gender Identity – Milkweed Bugs

Distinguishing markings on the abdomens of a female and male milkweed bug (Lygaeidae: Oncopeltus fasciatus).

So one random factoid I encountered while researching milkweed bugs last week is that you can distinguish males and females by the markings on the underside of their abdomens.  Females have a black stripe and two black dots, while males (the smaller sex) have two black stripes.  I didn’t have a male to draw comparisons against last week, so I tracked down a neighboring lab colony of milkweed bugs and sure enough, boys and girls!  (Thus resolving any future crises I may have involving the gender identification of milkweed bugs.)

I had originally wanted to photograph a mating pair, but apparently they found being repeatedly flipped onto their backs disruptive.  I finally resorted to sticking them in the freezer for a few minutes, after which they gave a very good impression of being dead (slightly too good an impression).  Luckily they eventually perked back up and I got a few photos.

Milkweed Bugs

A female milkweed bug (Lygaeidae: Oncopeltus fasciatus)

I love Texas–when I’m not complaining about the heat–because you can find insects virtually year round.  In December I was playing with earwigs and now it’s January and already the fire ant mounds are popping up everywhere like spring flowers.  (Working with fire ants has severely warped my perceptions of this event.)  I found this large milkweed bug (Oncopeltus fasciatus) sunning itself today in the balmy 70 degree weather.  Bugguide.com lists these fellows as active only from May to October, at least in North Carolina, which is apparently because North Carolinians have an odd phenomenon known as ‘seasons.’

Ventral view of a female milkweed bug (Lygaeidae).

Milkweed bugs belong to the true bug family Lygaeidae, the seed bugs.  Like other members of the family, milkweed bugs make their living feeding on nutrient rich plant seeds, in this case usually the seeds of the eponymous milkweed plant.  (Yes, this entire previous sentence was an excuse for me to use the word ‘eponymous’.)  They use their tubular mouthparts to pierce the walls of seed pods, feeding on the seeds within.

Like that other famous milkweed feeder, the monarch butterfly, the bright, warning coloration of milkweed bugs warns predators that these bugs sequester toxic compounds from the plant in their bodies, making them distasteful.  Milkweed bugs can be fed a variety of other seeds, although interestingly they habituate to food types and it often takes several generations for them to make a switch.  Milkweed bugs in the lab are generally fed sunflower seeds, shelled or cracked since their mouthparts can’t pierce the harder husks.