Dark-Colored Aphids on Milkweed

2015 May 6

Sandy has been planting milkweed near the house for the Monarch butterflies for some years now. On August 10, 2014, she noticed that some of the plants had an aphid infestation.

This was a bit unusual, as the species of aphid that is most commonly found on milkweed is bright yellow-orange, while these are dark colored to the point where they are nearly black (but with white legs). The things that look like lighter-colored, more spindly insects around them are shed aphid skins.

Rummaging around for non-orange aphids that feed on milkweed, it looks like there are some species that don’t have milkweed as their primary host, but have sometimes been seen feeding on it[1]. Of course, since the primary tool in identifying aphids is to see what kind of plant they are on, this means that I don’t really know which of the several candidate species it is. They do look rather a lot like cowpea aphids, though (Aphis craccivora). Particularly the way the fully mature ones are shiny like the one in the middle of this next picture, while the immatures have more of a matte finish.

Not only are cowpea aphids rather unfussy insects that will feed on a wide variety of plants, they are also an invasive from Europe (which usually indicates that they are going to be extremely common). So, while this isn’t a positive ID, I would say that it is a fairly high-probability guess.

[1] The particular species of milkweed that Sandy planted is one that was recommended as monarch food (I believe it is Swamp Milkweed), and is quite different from our most common wild species. In addition to having much more elongated leaves than the wild ones, it also doesn’t seem to have as much latex. Which on the one hand makes it more palatable to the monarch caterpillars[2], but on the other hand means that it doesn’t have as good of a defense against non-monarch insects.[3]

[2] While monarch caterpillars extract defensive chemicals from milkweed latex, they don’t necessarily like it. In fact, very young monarch caterpillars are prone to cutting the latex veins and letting a patch of leaf “bleed out” before eating it. So milkweed with lower latex levels will tend to be more attractive to them.

[3] Although it occurs to me that, as insects with piercing-sucking mouthparts, it is possible that aphids can just feed by drilling in between the latex veins, and getting only the nutritions sap while avoiding the toxic latex.

2 Responses
  1. Sandy h permalink
    May 6, 2015

    It occurs to me that these things are like bed bugs for plants. Flower bed bugs?

  2. May 7, 2015

    It would be interesting to do a chemical analysis on the sap from the plants the aphids will and won’t eat to find just what it is they avoid. Or is it more of a structural thing – they avoid plants whose outer walls are too thick?

    In any case, lovely photography again. Thank for sharing!

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