Keep the monarch eggs, boot the aphids
QI planted a milkweed plant a few weeks back. It’s growing well but it has developed an infestation of small creatures, maybe one-sixteenth of an inch long, who also leave a shiny reflective spotty surface to the leaves. What treatment would you recommend to get rid of them that at the same time will not prove harmful to monarch caterpillars, which hopefully will come in mass in the future?
AThe creatures you describe are oleander aphids, also known as plant lice. They are bright yellow-orange with black legs. The easiest way of dispatching them is with a strong jet of water. You will probably not knock off monarch eggs, since they are more strongly attached to their milkweed host than the offending aphids. Alternatively, you can just rub them off the plant and squash them between thumb and forefinger.
If you do spray the aphids with neem oil (made from seeds of a tropical tree), you will want to wash down the milkweed immediately afterward since monarch caterpillars would be killed if they chomped on stems or leaves coated with neem.
Detergent sprays are commonly recommended for aphid control, the simplest concoction being 1 tablespoon of dish soap dissolved in a quart of water. However, according to Karen Oberhauser of the University of Minnesota, who has studied aphids, monarchs and milkweeds for decades, “Detergent treatments will kill any live insects on aphid-infested plants, including monarch eggs, monarch larvae and aphid predators like syrphid fly larvae, ladybug larvae and lacewing larvae. There is really no good way to kill aphids without killing everything else, except by trying to lower the population by carefully killing them by hand.”
Alas, bringing in large populations of predatory insects such as ladybugs or lacewings also is not a good idea since, even while chomping on aphids, these generally beneficial insects (in larval form) will munch monarch eggs as well.
Aphids are fascinating insects, and oleander aphids all the more so. They are both viviparous and parthenogenetic. “Viviparous” means they give birth to live young, just like mammals. The difference is that these aphid offspring, also known as nymphs, are clones of the mother. They will molt four times over the course of their lifetime, which lasts twenty-five days, and will give birth to as many as 12 aphids in a single day.
The fact that oleander aphids are parthenogenetic means that mating is not needed for reproduction and, in fact, no male oleander aphids have ever been found. If a milkweed plant is totally overcome with aphids, some of them will sprout wings in order to fly to another plant and continue reproducing there.
What appear to be white flakes are a common sight on an aphid-infested plant. These are the skins aphids shed when molting. Dead aphids — puffed up or bloated, and brown to black — are frequently spotted during an aphid infestation. These are aphids that have been cannibalized by parasitic wasp larvae. Tiny wasps, no bigger than a pencil tip, pierce the bodies of aphids with an ovipositor through which a wasp egg is passed. As the wasp larva hatches from the egg, it consumes the inside of its aphid host before pupating and emerging as an adult wasp.
It is advisable to plant native milkweeds (Asclepias fascicularis and Asclepias speciosa) as opposed to the more commonly seen tropical species (Aesclepias curassavica), the type sold in nurseries that is recognizable by either yellow and orange or pure yellow flowers.
The reason is tropical milkweed is evergreen and will disrupt the natural life cycle of monarchs by enticing them to lay eggs in winter, which monarchs do not normally do, since their native host milkweeds are dormant in winter. Monarchs that develop from eggs laid in winter have impaired navigation systems and will have difficulty migrating to their overwintering sites.
If your milkweed is evergreen, keep it cut back during winter so monarchs will not be able to lay their eggs on it.
You will see oleander aphids not only on milkweeds and oleanders but on vincas and hoyas, which are oleander relatives, too.
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There are two perennial oxalis species that, in my humble opinion, no garden should be without. Both may appear delicate when you first set eyes on them, but both are quite the opposite when it comes to perseverance and longevity in shady to semi-sunny garden spots.
Based on my experience of more than two decades, during which time I have witnessed both species thriving with little care, I cannot find a reason not to have them around.
Purple shamrock (Oxalis regnellii) not only excels in shade gardens but thrives as a houseplant as well. It should only be noted that when you bring a plant indoors that appreciates a shady garden exposure outdoors, it will demand a place next to the brightest window in your home in its new environment. That is because the shadiest outdoor exposure is still much brighter than what you will find in most indoor locations except those that are exceptionally sunny.
Purple shamrock’s dramatic appearance features large leaves divided into three triangular sections, with pinkish-white flowers almost constantly in bloom.
Both the flowers and leaves of purple shamrock are subject to photonasty, a term that refers to movement of plant parts in response to light or darkness. At night, purple shamrock flowers and leaves close up. Upon doing so, the folded leaves take on the appearance of purple butterflies at rest.
If you suddenly see this plant shrivel up as though dead, be patient. It is only going through a dormant period, and in four to six weeks it will start to send out new growth. This is because purple shamrock grows from tubers. Dormancy is the best time to dig up the plant and distribute these tubers to other shady parts of the garden or gift them to friends.
Volcanic sorrel (Oxalis vulcanicola “Zinfandel”) has become my favorite ground cover. Clover-like, burgundy red foliage is complimented perfectly by yellow flowers. Volcanic sorrel spreads by tuberous rhizomes, which you can use in its propagation. A small clump will eventually take over a shady flower bed.
Both of the above plants are fairly drought-tolerant once established, although they will perform at their best when provided one good soaking per week during the summer. They are marginally frost-tolerant, but both can be overwintered indoors.
Are you growing an unusual or seldom-encountered plant that you would like more people to know about? If so, please send me an account of your experience growing it for possible publication here.