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Monday, Jul. 13th 2015

Milkweed Insects

With all the attention to milkweeds and the decline of monarch butterflies more and more people are searching milkweeds for the monarch butterfly eggs and caterpillars.  Often what they find is something else!

Gardeners typically find at least 5 common species of insects
Monarch butterfly caterpillars may not be the most abundant insects on milkweed but they are the best known.  Monarch caterpillars start out very small after emerging from pin-head sized eggs laid on the leaves by female butterflies.  They have alternating yellow, white and black rings around the body and a pair of long, thin, black tubercles at the front and rear.  Caterpillars take 2 weeks or more to grow to 2 inches and then they perform the magic of metamorphosis, transforming from ugly caterpillar into pretty butterflies. 
Near the top of milkweed plants you may find clusters of 10 or more milkweed tussock moth larvae (photo 1).  These communal caterpillars are also orange, black and white, but where the monarch caterpillars are hairless, tussock moth caterpillars are nothing but hair.  There are several long tufts of black hair at the ends and along the lower sides of the body and 6 pairs of bright orange tufts stick awkwardly upward in the middle.  Caterpillars are fully grown at only one inch.  There are two generations each summer.
The appropriately named red milkweed beetle (photo 2) is the most common of many beetles that feed on milkweeds.  These attractive insects are slightly more than one-half inch long and red with 11 black spots scattered along the back.  The red milkweed beetle larvae live in milkweed roots from late summer until the following July.  Adults aggregate on leaves and flowers and are very common in the second half of the summer.
While examining the milkweed plant for insects, look closely at the stems and undersides of the leaves, especially at the top of the plant.  There you are likely to find one of at least 4 common species of milkweed aphids.  The most frequently observed is the yellow oleander aphid (photo 3).  Large colonies of mixed sizes of these brilliant yellow insects are common.  Look closer.  Some aphids have wings, most don’t, and there may be ants tending the aphids. 
Finally, from mid- to late summer when the milkweed seed pods are present you may find aptly-named milkweed bugs.  The adults will be orange and black (photo 4) but if you are there early in the development of the bugs you will find the orange-red nymphs (photo 5).  Milkweed bugs feed on sap from the developing seeds.  If you are saving milkweed seeds to propagate in your butterfly garden, the milkweed bugs may further reduce already-low germination rates. 
So what does all this mean to the milkweeds?  Probably not much.  Are insects on milkweed a problem?  Not usually! 
Photo 1.  Milkweed tussock moth caterpillar.
Photo 1. Milkweed tussock moth caterpillar.
Photo 2.  Red milkweed beetle
Photo 2. Red milkweed beetle
Photo 3. Oleander aphid on milkweed.
Photo 3. Oleander aphid on milkweed.
 Photo 4.  Milkweed bug adults have a distinctive orange and black coloration.Photo 4. Milkweed bug adults have a distinctive orange and black coloration.
Photo 4.  Larger milkweed bug nymphs

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University of Missouri Extension Master Gardener Program