Yesterday, as I walked through the garden, I immediately noticed something awry with the milkweed (Asclepias 'Silky Deep Red' and 'Silky Gold') growing in the driveway border. The flowers and seedpods were covered in bright yellow aphids that seemingly appeared overnight.
There didn't appear to be any feeding damage, but the plants were covered in hundreds (if not thousands) of these fluorescent little insects. A quick search on Google identified them as Aphis nerii, or milkweed aphids -- and, boy, are they fascinating. In this excellent article (including phenomenal macro photographs), I learned that in this species of aphid, there are no males. The females give birth to live clones of themselves. In fact, every new aphid is born with an exact clone of itself already inside. No wonder they seemed to multiply overnight. Wingless by default, if the population gets too dense or food source becomes scarce, the aphids will start to produce winged offspring that can leave and colonize a new food source.
I was wondering how I could prevent these aphids from destroying my milkweed when I looked down the stem and saw a large praying mantis on one of the leaves. Presto! Instant aphid control! I suddenly hoped for my own private episode of Monster Bug Wars where the mantis would start voraciously picking off the aphids with surgical precision.
But then I learned that Aphis nerii digest toxins within milkweed (and also oleander) as a defense mechanism against predators. Ladybugs that feed on this species are know to develop genetic deformities as a result. I'm not sure if the toxins would affect a predator as large as this mantis, but I'm certainly not counting on it for pest control anymore.
In fact, the mantis climbed up near the congregation of aphids and did little more than hang out for a while.
I checked this morning, and the mantis is still camped out on the milkweed. It seems disinterested in the aphids, as several crawled right over its rear legs with no reaction at all. The aphid population must be reaching maximum density, because a significant number of aphids (presumably born since yesterday) now have wings.
My research indicates that there shouldn't be much damage to the milkweed, so I plan to watch and learn as the colony of Aphis nerii morphs and grows. I didn't get my episode of Monster Bug Wars, but certainly received a satisfactory dose of cool science. For me the benefits of a garden stretch far past the plants. Being able to witness the wonders of nature -- the full ecology of a garden -- makes every ounce of effort worthwhile.