January 27, 2003
Vol. 1; Issue 7

Production Update:
Harvest of leafy vegetables continues at a steady pace. Based on the completion of harvest and plant stages of plants, the harvest should last for 7-8 more weeks. Unusually warm weather has accelerated plant growth and maturity since early January (Table 1). Temperatures during January have been about 10 degrees above those last year (5 º above nighttime lows and 5 º above daytime highs). Winter wheat planting has increased as lettuce and cole crops are harvested. Big vein (picture) (more info.) is evident in some fields, but overall disease pressure appears to be very low. Quality of head and leaf lettuce, cole crops and spinach remains good to excellent. Spring melon planting continues in the Yuma Valley. Stands planted under plastic (mid-bed trench) are emerging, and melon planting on Yuma beds has begun.

Pest Update:
We are still seeing a bit of powdery mildew (picture) (more info. ) on both head lettuce and romaine in the Yuma Valley. Insect activity has increased in the past week or so due in part to warmer weather (Table 1 and 2). We continue to find small colonies of foxglove, green peach and potato aphids (Figure 3), and turnip aphids in cole crops throughout the area. Lettuce planted during the November planting windows appears to be the hardest hit. We anticipate aphid populations to begin to significantly increase, particularly if temperatures remain at or above normal.

Lettuce Aphid Found in Gila Valley and at Yuma Ag Center:
The Lettuce Aphid, Nasonovia ribisnigri, has been found on commercial head lettuce in the Gila Valley and on head lettuce at the Yuma Ag Center this week. In both cases, Admire had not been applied to the crop. The first reported incidence of Lettuce aphid in Yuma was on a lettuce field (not treated with Admire) in the Gila Valley in 1999 and untreated lettuce at the Yuma Ag Center in March of 1999. The lettuce aphid has been found throughout our area in head and leaf lettuces every season since then.

The species is quite different from the green peach aphid and potato aphid we find on leafy vegetables. First, unlike other aphids, the lettuce aphids deposit their young near the growing points or "crown" of the plant. The aphids we found in head lettuce were predominantly found on the cap leaf and within the head near leaf margins surrounding the "butt". Similarly, aphids found in romaine were deep within the terminal growth. Of course, this causes problems in controlling the pest with foliar spray (Figure 1). Secondly, the aphid looks distinctively different than any other aphid found locally on lettuce (Figure 2-4). My observations have been that the mature, winged (alate) forms of the lettuce aphid appear to be dark brown to orange in color, have a dark thorax , conspicuous dark bands across their abdomen, and the cornicles are completely dark. The mature, wingless (apterous) forms are brown- orange in color with dark bands across their abdomen and are black on the joints of their legs, and the tips of their antennae. Their legs are quite spindly, giving them almost a spider-like appearance. The small, newly deposited immature forms are often less distinctive, but have generally been red to pink in color. This is why lettuce aphid is often referred to as the "red" aphid. Unlike other aphids, lettuce aphids are quite mobile and walk rapidly on leaf surfaces. If you suspect you may have found lettuce aphid, please don't hesitate to bring it in to the Yuma Ag Center or Imperial Valley Cooperative Extension Office to be identified. In addition, this aphid also reproduces very rapidly. The optimal temperature for lettuce aphid biological development on lettuce is around 66 º F. This is consistent with our average temperatures during February and March.

For additional information on Lettuce Aphid visit the following Link on ACIS:
http://cals.arizona.edu/crops/vegetables/insects/veginspub.html#aphid

Table 1. Temperature recorded from AZMET stations at three Yuma growing locations.

Figure 1. Distribution of lettuce aphid on lettuce plants relative to green peach aphid, Yuma Agricultural Center, Spring 2000. From: http://cals.arizona.edu/pubs/crops/az1177/az1177-1j.pdf

Figure 2. Mature wingless lettuce aphid (on head lettuce leaf, Yuma, Arizona,
Jan 15, 2002

Figure 3. Lettuce aphid colonizing infesting cap leaf of head lettuce, Yuma, Arizona,
Feb 2, 2001

Figure 4. Mature wingless lettuce aphids colonizing and depositing live aphids on romaine lettuce, Yuma, AZ, Feb 2002.

AZMETWeather Data

Disclaimers--