Jan 11, 2023Green Peach Aphids and the Red Morph
With days getting longer and temperatures a bit warmer, aphids are likely to be more common. Experience has also shown us that aphid population growth begins to increase significantly once the weather warms in January. Our local weather forecast suggests that temperatures will be in the upper 60’s with moderate nighttime lows in the high-40’s for the next two weeks. These conditions may be ideal for aphid population growth, particularly green peach aphid. To date, green peach aphid is the only aphid species that I’ve observed, or reported by PCAs, in local produce crops. However, there’s been a little confusion with identifying green peach aphid. Since December, PCA’s have been reporting the presence of “red” colored aphids in lettuce wondering whether they are Lettuce aphids, aka Red Aphid. In each case that I saw, the aphids have been the red morph of the Green peach aphid. We typically see two color morphs of the green peach aphid each season, and both are the same species - Myzus persicae. The green color morph is the most common, but we will often see a few individuals of the red color morph. This year the red morph appears to be occurring more often than in the past few seasons. There is no clear reason why the red morph appears. It’s been suggested that red morphs of green peach aphid are more tolerant of warmer temperatures. Not sure that applies to Yuma. Some suggest it could be insecticide resistance. We saw in the mid-1990s that the red morph was less susceptible to organophosphates, carbmates and pyrethroids than the common green morph, but red morphs were equally susceptible to imidacloprid as the green morph. You should have no problem controlling these red morphs given the array of chemistries available. Remember, green peach aphid does not over-summer in the desert but rather immigrates onto our crops each fall on N-NW winds. Thus, we don’t know the genetic history of the population that is now present in the desert, or whether this has anything to do with why red morphs are more common this winter.
There are two definitive ways of distinguishing the difference between the Red morph of the green peach aphid and Lettuce aphid (Red aphid). First, are the antennal tubercles. The tubercles in the green peach aphid converge inward away from the antennae, while in the lettuce aphid the tubercles diverge outward towards the antenna. For more information on aphid identification see the UA Aphid ID Guide. The other way to distinguish the difference in the two aphid species is where you find them on lettuce. Lettuce aphids almost always colonize the terminal growth, and can often be found in heads or hearts, while new colonies of green peach aphids are typically found on the frame leaves in high numbers before moving into the heads. Also remember that among the local produce crops we grow, lettuce aphids are only found on lettuce. In contrast, green peach aphids infest just about all crops- lettuce, Cole crops, celery, spinach, etc. If you are uncertain which aphid species is infesting your crop, don’t hesitate to drop off samples at the Yuma Ag Center and we’ll get them correctly identified for you.To contact John Palumbo go to: jpalumbo@ag.Arizona.edu