Thursday, August 7, 2014

Milkweed Aphid

 Mark Bower sent me the picture above, a critter on a milkweed.  It looked like an aphid and so with a brilliant piece of detective work I Googled "milkweed aphid" and came up with the identification, the milkweed aphid, Aphis nerii.  

Like many organisms, when you dive deep enough, it has some interesting characteristics.  This aphid is an omnivorous plant sucker, feeding on sap of plants in the dogbane family such as milkweed, vinca and oleander.  It will even cross over for dessert on crassulaceae (succulents such as jade plant) and solanaceae (nightshade family).

All oleander aphids are female as males do not occur in the wild.  They are parthenogenetic (reproduce without male fertilization) and viviparous, giving birth to live nymphs instead of laying eggs.  The nymphs go through five stages all looking similar to the adult.  Some adults will develop wings when there is overcrowding or the plants are dying off, allowing the aphid to establish a new distant colony.
 Winged A. nerii - Ken Schneider
Reproducing in large numbers usually means an insect is a problem to some plants (and growers) but dinner for others.  This aphid sequesters toxic cardioglycosides from the plant sap of milkweed, making it distasteful to many species, just like the monarch butterfly.  In addition they make distasteful secretions on their cornicles (coming up next week).  When this secretion is applied to a spider's mouth parts, it will retreat and wipe its mouth (and presumably make a terrible face!)  This shows that the secretion is effective and that some entomologists have way too much time on their hands.

So what does eat these aphids?  Hover or flower flies, lacewings and lady beetles have no problem with their secretions.  A prominent source of predation are the aphid wasps such as Lysiphlebus testaceipes.   They lay their eggs on the aphid nymphs and the resulting larva enters the aphid which mummifies while the larva matures inside.

The oleander aphid can become a garden pest, injuring young shoots and producing a sticky honeydew which can develop a black sooty mold on it, not what you want on your garden flowers.  The aphids can be removed by hand and are vulnerable to insecticidal soaps and oil.  Gardeners should avoid other insecticides as they are also toxic to butterflies and other pollinators.
Large Milkweed Bug-  Mark Bower
Photo- Mark Bower
I was initially confused by pictures of another bug on the same plant, a deeper orange but co-existing with the aphids.  Kevin Firth cleared up the confusion by identifying it as a nymph of a Oncopeltus fasciatus, the Large Milkweed Bug. Seen below with the adult, it is a minor enemy of milkweed but a friend to science. It is easy to grow and to dissect, making it an ideal insect to study in the lab.

Large Milkweed Bug adult and nymphs-  Photo by Greg Hume
Like the aphids, they suck the juices of milkweed and accumulate toxic glycosides in their bodies.  This and their bright orange aposematic color serves to warn off many predators.
Details on the aphid are found at this University of Florida website.

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