December 17, 2003
Vol. 2; Issue 3

Production Update:

Planting of most lettuce, broccoli and cauliflower is beginning to slow considerably as we are approaching the end of the traditional planting window. Harvest activity for these crops appears to be primarily focused in the Yuma Valley where temperatures are the most moderate (Table 1). Baby leaf mixes and spinach plantings appear to be continuing at a steady pace. Soil temperatures are presently quite low (Table 2) and based on observations at the Yuma Ag Center stand establishment of lettuce and cole crops has been requiring about 10-12 days. Furthermore, low nighttime temperatures have been low for the past week or so and lettuce ice has been present area-wide during the early morning. We have had no measurable rainfall since mid-November. Markets appear to still be strong and overall produce quality remains good. No reports of major mildew or disease problems. Melon harvest is complete and fields are being prepared for spring plantings.

Pest Update:

Area-wide Trapping Network: Worm activity has declined significantly. This is to be expected given our low temperatures that influence moth ovipositional activity and larval development. Based on our trap catches in the Yuma Valley, insect activity appears to be at or approaching seasonal lows for whiteflies, leafminers and thrips (Fig 1). However, aphid flight activity is on the increase (Fig 1), and is consistent with observations from last year (Fig 2). A large proportion of the alates captured has been green peach aphid, and at a higher number than what we observed last year during this time.

We are also aware of the unusually widespread incidence of green peach aphids beginning to colonize commercial lettuce and broccoli, particularly in the Yuma Valley & Bard-Winterhaven areas. Based on samples taken from a comparative study examining seasonal aphid population growth on untreated lettuce at the Yuma Ag Center, numbers of apterous (wingless) adult and nymph green peach aphids appear to be slightly higher than what we have observed in recent years (Fig 3). If data from 2000-2001 may is any indication we could experience higher green peach aphid populations this spring (Fig 4). Although aphids in general were at a 10 year high last season (mainly comprised of foxglove aphid and Acyrthosiphum lactauca; http://cals.arizona.edu/pubs/crops/az1323/), green peach aphid numbers last season (2002-2003) were very low during (Fig 4).

Over the past 3 weeks several PCA’s have brought wingless aphid specimens found colonizing on produce crops (8 visits). In each case, the aphids were identified as green peach aphid. We have not found any foxglove aphid colonizing lettuce to date. However, we are still finding alate (winged) foxglove at the Ag Center and on traps. To assist PCAs and growers in properly identifying aphids we have included a practical pictorial key (PDF file, 224KB) for aphids on lettuce that can be used in the field. The key is in the draft stages of development and is presently being validated by several participating PCAs. We plan to improve this key by collecting better images of some of these aphid species and will produce a published hard copy some time this summer. If you would like to participate in the key validation process and would like a laminated draft copy of the aphid keys for use in the field please contact me at 928-782-3836 or jpalumbo@ag.arizona.edu

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

Table 2. December averages (Dec 1-15) recorded from AZMET stations at
three Yuma growing locations

The low temperatures recorded all occurred on December 10th . See AZMET for details. http://cals.arizona.edu/azmet .

Fig 1. Sticky Trap Catches for the Yuma Valley, 2003-2004

Figure 2. Seasonal aphid flight activity as measured by yellow sticky traps, Yuma 2002-2003.

Fig 3. Five year comparison of apterous aphid populations sampled on untreated head lettuce plants in two plantings during November and December 1999-2003 at the Yuma Agricultural Center. 2003-2004.

Fig 4. Four year comparison of Seasonal apterous aphid populations sampled on untreated head lettuce plants in two plantings during November and December 199-2003 at the Yuma Agricultural Center. 2003-2004.

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