Palumbo, based at the UA Yuma Agricultural Center, told the 14th annual Desert Crops Workshop in Holtville, Calif., earlier this month that a mild winter along with "growers getting away from using Admire" are two reasons he believes this new aphid is spreading.
It was found this season in the same area as it was discovered for the first time last year in Yuma. It likely hitchhiked to Yuma on transplants or harvesting equipment.
"These findings should serve as an indication to pest control advisers and growers that foxglove aphids could potentially be present in large numbers again this winter and spring on desert lettuce," Palumbo said.
Foxglove aphid poses a much larger threat to desert lettuce than the lettuce aphid, according to Palumbo. One reason being that it has many more hosts, including citrus. These include a wide variety of weeds including shepherd’s purse, ground cherry and pigweed, ornamentals like geraniums, gladiolas, verbena and crops (cucurbits, beans, canola, spinach, citrus, safflower, tomatoes).
"It is no coincidence that early infestations appear near citrus," said the entomologist. Weeds are other host.
Positive identification is important to differentiate the foxglove aphid from the lettuce aphid. Both aphids have short life cycles that allow populations to build up rapidly, and both tend to prefer to colonize the youngest tissue near the terminal growing point of the plant.
Wingless forms of foxglove aphid are also often confused with the green peach aphid. Both aphids are usually yellow-green to all green but the green peach aphid may also be somewhat pink or red, as is the lettuce aphid.
The foxglove aphid is slightly larger (maximum length is 3.0 mm) than the green peach aphid (maximum. length is 2.3 mm). One way to distinguish these two aphids is by the dark joints found on legs and antennae of the foxglove aphid, and the dark tips of the cornicles.
Green peach aphid
The green peach aphid also has pale-colored legs and antennae but without dark joints. Foxglove aphids are also unique in that they have a bright green or dark colored spot at the base of each cornicle. Alates have a pattern of transverse dark bars on the dorsal abdomen.
Control with foliar aphicides appears to be more difficult because the foxglove aphid’s preference for the protected terminal growth.
It is important to treat early when the aphid colonization reaches only 1 to 5 percent and use effective pesticide rates to control that population, added Palumbo. Soil applied Admire appears to be the best control method.
Light populations of foxglove aphids were first found colonizing untreated head lettuce in small experimental plots at the Yuma Agricultural Center (YAC) in the spring of 2001. Initially, the aphids were thought to be potato aphids, but were later identified as foxglove aphid.
Aphid populations in general were higher in 2003 than in the previous 10 years, based on a summary of small plot efficacy trials planted during mid-November.