Saturday, March 14, 2009
A Tale of Two Roses and their Aphids
Aphids have proliferated on the tender spring growth of two of my roses.
Rose #1 is a floribunda purchased this winter as a containerized bare root. The deep red new growth is covered with green aphids of various stages of development, a few winged forms in attendance. Many of them have a longitudinal stripe of slightly darker green, leading me to think this group is Macrosiphum euphorbiae, the potato aphid. Potato aphids are said to employ one of two overwintering strategies. When it's colder, they mate, then the females lay eggs on rose plants where they overwinter. In warmer areas or times, the parthenogenic (non-sexual reproduction--no males needed) female population just continues living and reproducing on the roses through the winter, I suppose with their numbers building up as the weather warms and especially as the plants leaf out providing food. As summer approaches they migrate to plants in the solanum family, such as tomatoes and potatoes. The rose bush has had no leaves on it to feed active aphids all winter, and I saw no aphids; our area is by no stretch of the imagination cold, so neither of these life histories is a perfect fit for this aphid occurence.
Rose #2 is a single miniature that's been in the ground for oh, maybe ten or twelve years. I did not prune the minis this winter, but this one is putting on a lot of lush new growth that is covered with (mostly) pink aphids strongly reminiscent of Macrosiphum rosae, the rose aphid and close relative of the potato aphid. Like the potato aphid, rose aphids supposedly overwinter in the egg stage on rose plants. In this case, because of all the available foliage on the unpruned plant, it's also easy to believe there were active (albeit sluggish) adult or nymph stage aphids living on the plant all winter. As mentioned, these aphid on rose #2 are almost all pink, with a green exception and a few very pale ones I noticed. Both M. rosae and M. euphorbiae occur in pink and green forms, by the way. Sometimes the color is affected by what they are eating. So, possibly these 2 aphid colonies are the same species. But I don't think so.
Sunset Western Garden Book states "the best control tactic [for aphids] is often to do nothing and leave the pests to natural controls" and many other sources concur. As soon as aphid populations build up to nearly freak-you-out levels, the predators and parasites show up. I saw parasitic wasps working the potato aphid patch this morning, for example. Still, would the use of dormant oil spray to kill any overwintering eggs or aphids be a worthwhile endeavor if it delayed the onset of aphids until after the first flush of flower buds develop? Maybe, but I also wonder if the spring flush of aphids isn't a strategy of roses to ensure a good crop of ladybirds, lacewings, syphids and wasps for the coming year.
Anyway, at this point my best course of action, if I want better looking rose plants, is to blast the aphids off with water or wipe them off with my fingers. And be glad and wonder why only these two rose plants (Are some rose varieties more susceptible to early aphids infestation than others? Or is it luck, timing or some aspect of cultural practices that's the cause of it) are aphid infested.