Sunday, July 06, 2008
Aphids on Grass
About the beginning of last month I began to see small dark aphids on a clump of Nassella tenuissima growing near the back door. The aphids for the most part lined themselves up along the flower spikes and looked remarkably like the developing seeds. Maybe this is a camoflage adaptation; maybe the flower spikes are richest in the plant juices the aphids feed on. I was not able to identify the species, but there are many aphids to choose from that prefer grains as their host plant and it is likely to be one of those: evenly grayish green in color and smallish compared to oleander or rose aphids.
The aphids were flourishing about a week later when a few adult ladybird beetles arrived. These were spotless ladybirds, Cycloneda sanguinea. They got busy and laid eggs, and just about a week later the grass was alive with beetle larvae.
It was interesting to watch both the adults and larvae hunt; it seemed challenging for them to move along the very thin grass blades. They seemed to find it easier to move upside down while hanging from their tarsi. Occasionally one would lose its footing, sometimes falling only to pick up on another grass stem and move on. There was a lot of grass to cover as in order to find and devour any one aphid the predator had to cover the stem that aphid was on. I watched for awhile and saw a lot of fruitless hunting, covering grass blades that I could see from my giant's perspective had no aphids. The beetle or larva would plug on, out to the end of a blade and back down. After about 10 days there were no aphids left on the plant, the beetle larvae had pupated and the adults had long since flown away.
So. Ladybird beetles control aphids: nothing special or unusual about that. What I think is special and noteworthy about my observation of the efficiency of ladybird/aphid predation is that it is so commonplace; that we live in an ecosystem where such specialized systems are ordinary and to be expected. Long may our ways of life prevail.
Each grass blade is about 18 inches long. In a bundle 1/2" in diameter I counted 150 stems. The entire grass clump is 3 inches in diameter. That gives an estimated 94,500 inches, or just about 1 1/2 mile of grass to cover. If two passes over each stem were required that distance would be doubled.
BTW, Mexican feather grass (n. tenuissima) is a grass native to parts of the southwest US and Mexico. I like to grow it so I can watch its brilliant fine textured leaves and golden seed awns moving through the sunlight in the slightest breeze. The particular clump in this post was a volunteer seedling. Out of the thousands of seeds the plants produce each summer very few germinate under my dryish garden conditions. The few that do grow are very easily pulled if necessary. However there is concern about n. tenuissima becoming an invasive pest in California wildlands. I'm looking into growing Purple needlegrass, nassella pulchra, (a California native) instead.