Status of Key Insect Pests in Desert Produce and Melons
Last fall, pest pressure in the desert was historically low for some pests, while unusually heavy for other pests. Through September, insect pressure has been all over the board. Some pests like beet armyworm have been heavy, and other like whiteflies have been uncharacteristically light. Of course, the Yuma growing area is quite large and pest populations can vary according to location. Here’s how I see it so far this season.
Beet armyworm/Cabbage looper/Corn earworm: Last fall was a pretty average worm year in produce. This fall, beet armyworm pressure has been exceptionally heavy in some areas (Dome Valley, Yuma Valley). They also appeared a bit earlier than normal. For instance, a review of trials here at YAC over the past 10 years has shown that larvae typically show up about 10 -11 days after wet date. This year, I observed larvae on lettuce at 8-9 days and had several reports from PCAs of them consistently showing up at 6-7 days on cotyledons and 1 true leaf stage lettuce. They appeared to slow down a bit in the past week or so. With cooler weather in the immediate forecast, armyworm pressure should decrease. Corn earworm appeared early in September, but are much lower now. Cabbage looper are starting to show up on lettuce and cole crops, as expected with cooler weather. For information on insecticides available for Lep management see Lep Control Chart, 2018.
Diamondback moth: To date, DBM have been very light. An occasional report of light larval populations on transplants in Roll and Dome Valley, but no control issues reported. DBM larvae have been almost non-existent on plots at YAC, as opposed to last fall when DBM larval population on direct-seed broccoli were higher than what we normally see; a result of migration. However, nothing comparable to the fall of 2016. Trap catches have overall been lighter compared with last season, but moth numbers increased this past week (see DBM Trap Network). It will be interesting to see if we observe an increase in trap captures following Hurricane Rosa’s track through Northern Baja California and Sonora. DBM have been shown to move long distances in storms.
Bagrada bug: The pest has been almost non-existent the last three years, and this fall doesn’t appear to be any different. There have been a few reports of adults showing up, but numbers have been light and feeding damage negligible. Chemigation of pyrethroids have been effective.
Whitefly/CYSDV: Areawide, whiteflies in fall melons have been at very low levels relative to years past. Whitefly adult numbers on sticky traps placed next to melon crops have been very light so far; lighter than last year in most areas. Accordingly, CYSDV infection is also very light in all fields from Texas Hill to Dome Valley. Virus symptoms (yellow interveinal chlorosis) are patchy, and mainly on the edges of fields suggesting PCAs and growers are doing a great job of preventing secondary infection. Areawide, whiteflies were similarly light in produce crops with an occasional flare-up in some isolated areas. Trap catches are down with the exception of locations near recently defoliated cotton (see Areawide Trap Network).
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Lettuce was seeded, then sprinkler-irrigated to germinate seed on Nov 28, 2023 on double rows 12 in. apart on beds with 42 in. between bed centers. All other water was supplied by furrow irrigation or rainfall. Treatments were replicated five times in a randomized complete block design. Each replicate plot consisted of 25 ft of bed, which contained two 25 ft rows of lettuce. Plants were thinned Jan 4, 2024 at the 3-4 leaf stage to a 12-inch spacing. Treatment beds were separated by single nontreated beds. Treatments were applied with a tractor-mounted boom sprayer that delivered 50 gal/acre at 100 psi to flat-fan nozzles spaced 12 in apart.
Month
MaxTemp(°F)
Min Temp (°F)
Average Temp (°F)
Rainfall
November
80
51
65
0.08 in
December
71
44
57
0.82 in
January
68
42
54
1.14 in
February
73
47
59
0.50 in
Downy mildew (caused by Bremia lactucae) rating was done on variety Eblin, Bobcat, and 180 (partially funded by AILRC grant). Disease was first seen on 1-30-24. Please see attached excel file for chemicals application date. Disease rating was done on February 29, 2024. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 0.5 = one to a few very small downy mildew colonies on bottom leaves; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Most of the treatments exhibited activity against the disease to some extent. Latitude, Amara, Eject, Previcure flex exhibited good control in the variety Eblin (highly susceptible variety). Whereas Cevya, Stargus, Latitude, Amara, Revus, Thrive 4 M, Actigard, Instigo+Carbose+intereact showed activity against the pathogen in variety Bobcat. Please see excel file for the full list of chemicals and their efficacy. The lettuce variety 180 was resistant to the disease and no downy mildew was observed on the particular variety. No phytotoxicity was observed in this field.
Controlling Fusarium Wilt of Lettuce Using Steam Heat – Trial Initiated
Earlier this week, we initiated a trial examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae, the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). Preliminary results were encouraging. The device was able to increase the temperature of the top 3” of soil to over 180°F at a travel speed of 0.5 mph as shown in this video of the machine in action (shown below). These temperatures exceed that of those known to control pathogens responsible for causing Fusarium wilt of lettuce (> 140°F for 20 minutes).
Stay tuned for final trial results and reports on the efficacy of using steam heat to control Fusarium wilt of lettuce.
If you are interested in evaluating the technique on your farm, please contact me. We are seeking additional sites with a known history of Fusarium wilt of lettuce disease incidence to test the efficacy and performance of the device.
References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.
Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
A special thank you is extended to Cory Mellon and Mellon Farms for allowing us to conduct this research on their farm.
Weeds are one of the most visible of all agricultural pests. They can’t move or hide and once established often stick up over the crop. Just one weed in a 10 acre field is annoying to look at. With insects and diseases, the damage is often more visible than the pest. That is not the case with weeds. A moderate weed infestation is approximately 10 weeds per square foot. If a herbicide produces 90% control, that leaves 1 weed per square foot or 43 weeds per acre. Without an untreated check, this can look like the herbicide failed! It is easy to leave an untreated spot in a field and it is well worth doing. Many applicators do so unintentionally because of skips, powerlines and other causes. They help determine crop injury and weed control. Here are some examples of what various levels of control looked like from one of our cole crop trials:
Corn earworm:First significant CEW moth activity since mid-November; particularly active in Dome/Wellton/Tacna areas.
Beet armyworm: Moth counts remain very low consistent with seasonal temperatures, but below average for this point in the season.
Cabbage looper: Slight increase in activity, but moth counts remain unusually low for this time of season.
Whitefly: Adult movement is at seasonal low consistent with temperatures and lack of melons or cotton.
Thrips: Thrips activity beginning to pick up, particularly in Tacna and Yuma Valley. Movement is still below average for February.
Aphids: Seasonal aphid counts peaked in early February and tending down last week. Counts remain high in Gila Valley and Wellton. Above average for this time of year.
Leafminers: Adult activity remains light in most trap locations. Trap counts increasing slightly in the South Gila Valley.