The University of Arizona Vegetable IPM team is dedicated to delivering science-based solutions that help desert vegetable growers protect yield and quality while reducing reliance on broadly toxic pesticides. This mission comes to life each year through the Lettuce Insect Losses & Insecticide Use Workshop, where PCA surveys provide a snapshot of pest management practices across the desert.

At this year’s Lettuce Insect Losses & Insecticide Use Workshop, survey results continue to show the industry’s steady shift toward selective, reduced-risk products, while still relying on broad-spectrum tools when necessary. Paired with intensive scouting—nearly four visits per week—this approach allows PCAs to make smarter, more sustainable decisions that protect both crops and the environment.

Bottom Line: Twenty years of lettuce IPM research have given us stronger tools and smarter choices. Advances in chemistry, combined with the leadership of experts like Dr. John C. Palumbo have built grower confidence to lead with selective, reduced-risk products—protecting today’s lettuce crops while sustaining the tools we depend on for the future.

Figure 1. Survey results from the Lettuce Insect Losses & Insecticide Use Workshop
show total desert lettuce acreage treated with insecticides across three broad categories
of toxicity and selectivity: reduced-risk/selective, broad-spectrum, and broadly toxic.

CEU Event Coming 9/3
Don’t let the season slip by without meeting your CA and AZ PCA license requirements—UA Extension will host one more major session before the season heats up.The meeting will cover the highly desired Laws and Regs CA DPR category, giving you the chance to finish your renewals before the first pest outbreak of the season (hopefully). Details below:

2025 Laws & Regulations Update
Date: Tuesday, September 3
Location: Yuma Agricultural Center
Credits: 4 CA DPR Laws (requested) | AZDA hours pending
Time: 10:30a-3:30p

Agenda can be found HERE. We will need a headcount for lunch, please RSVP to Macey via email (maceyw@arizona.edu) or text (928-580-5785). 

If your schedule doesn’t allow for in-person attendance, UA Extension also offers online, on-demand CEU videos so you can earn credits at your convenience. You can find them here: Continuing Education Units | UA Cooperative Extension

Identifying important stages of crop growth and development (crop phenology) for in-season management and harvest dates is important with melon (Cucumis melo ‘reticulatus’ L.) crop production. To monitor and predict plant development it is best to measure actual thermal conditions in the plant’s environment. Plant growth is a direct response to temperature and environmental conditions. Measuring “thermal time” is a more reliable measure and predictive tool for plant development as opposed to a calendar.

Thus far, 2026 has been an abnormally warm year for Arizona and the desert Southwest. Accordingly, crop development has been accelerated by conventional calendars but closely following the thermal calendars that plants respond to.

Various forms of temperature measurements and units commonly referred to as heat units (HU), growing degree units (GDU), or growing degree days (GDD) have been utilized in numerous studies to predict phenological events for many crop plants (Baskerville and Emin, 1969; Brown, 1989; Baker and Reddy, 2001; and Soto, 2012). A graphical depiction of HU computation using the single sine curve procedure is presented in Figure 1 (Brown, 1989).

A little over twenty-five years ago we began working on the development and testing of a phenology model for desert cantaloupe production for Arizona conditions. The basic cantaloupe phenology model is shown in Figure 2 (Silvertooth, 2003; Soto et al., 2006; and Soto, 2012). Since cantaloupes are a warm season crop, we use the 86/55 ºF thresholds for phenological tracking.

This melon crop phenology model was developed under fully irrigated and well-managed conditions, primarily in the lower Colorado River Valley. That is important since non-irrigated fields are more likely to experience water stress, which significantly disrupts crop development patterns.

Key stages of growth or “guideposts” indicated in Figure 2 represent general average or “target” values that are subject to a slight degree of natural variation, which is normal.

Referring to the data from the Arizona Meteorological Network (AZMET) and several locations in the Yuma area, the HU accumulations (86/55 ºF thresholds) from 1 January 2026 to a set of four possible 2026 planting dates are listed in Table 1. The HU accumulations from 1 January to 1 April 2026 are listed in Table 2.

The HU accumulations after planting (HUAP) for these four possible planting dates for three Yuma area locations to 1 April 2026 are shown in Table 3. The HUAP values in Table 3 are simply the difference between the values in Tables 1 and 2. An example for the Yuma Valley (UA Yuma Ag Center), 15 January 2026 planting date is: HU 1318- 95 HU = 1223 HUAP.

It is rather easy to test and evaluate this crop phenology model in the field under various planting dates, varieties, and conditions. Tracking HUs and reference to this phenology model (Figure 2) can serve as a check for melon crop development in the field. Based on the estimates in Table 3, the earliest planting dates are ready or close to crown fruit harvest and later planting dates are for small golf-ball and slightly large sized melons. Calendar wise, this is rather early but right on track based on seasonal HU accumulations.

Note: Yuma AZMET site is at the University of Arizona Yuma Agricultural Center. The North Gila AZMET site is located west of the Laguna Dam Road near County 3E. The Roll site is located near E. County 8th Street and S. Avenue 35E.

References:
Baker, J.T., and V.R. Reddy. 2001. Temperature effects on phenological development and yield of muskmelon. Annals of Botany. 87:605-613.

Baskerville, G.L., and P. Emin. 1969. Rapid estimation of heat accumulation from maximum and minimum temperatures. Ecology 50:514-517.

Brown, P. W. 1989. Heat units. Ariz. Coop. Ext. Bull. 8915. Univ. of Arizona, Tucson, AZ.

Silvertooth, J.C. 2003. Nutrient uptake in irrigated cantaloupes. Annual meeting, ASA-CSSA-SSSA, Denver, CO.

Simonne, A., E. Simonne, R. Boozer, and J. Pitts. 1998. A matter of taste: Consumer preferences studies identify favorite small melon varieties. Highlights of Agricultural Research. 45(2):7-9.

Soto, R. O. 2012. Crop phenology and dry matter accumulation and portioning for irrigated spring cantaloupes in the desert Southwest.  Ph.D. Dissertation, Department of Soil, Water and Environmental Science, University of Arizona.

Soto-Ortiz, R., J.C. Silvertooth, and A. Galadima. 2006. Nutrient uptake patterns in irrigated melons (Cucumis melo L.). Annual Meetings, ASA-CSSA-SSSA, Indianapolis, IN. 

Table 1. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2026 on four
possible 2026 planting dates utilizing Arizona Meteorological Network (AZMET) data for
each representative site.


Table 2. Heat unit accumulations (86/55 ºF thresholds) after 1 January 2025 on 12 April
2026 utilizing Arizona Meteorological Network (AZMET) data for each representative site
in the Yuma Valley, North Gila Valley, and Roll.

Table 3. Heat unit accumulations (86/55 ºF thresholds) after planting (HUAP) as of 10
April 2026 from four possible 2026 planting dates and three sites in the Yuma area
utilizing Arizona Meteorological Network (AZMET) data for each representative site.
Note: the values in Table 3 are determined by taking the difference between the HUs for each representative site and four planting dates in Tables 1 and 2.

Yuma Valley: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az02
Yuma North Gila: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az14
Roll: https://azmet.arizona.edu/application-areas/heat-units/station-level-summaries/az24

Figure 1. Graphical depiction of heat unit computation using the single sine curve procedure. A sine curve is fit through the daily maximum and minimum temperatures to recreate the daily temperature cycle. The upper and lower temperature thresholds for growth and development are then superimposed on the figure. Mathematical integration is then used to measure the area bounded by the sine cure and the two temperature thresholds (grey area). (Brown, 1989)


Figure 2. Heat Units Accumulated After Planting (HUAP, 86/55 oF)

 

I’m excited to share an upcoming free webinar series from the Emerging Viruses in Cucurbits Working Group (EVCWG). For those readers who may be unfamiliar with this organization, the EVCWG is a collaborative partnership that works to promote virus research and communication across the cucurbit industry. It was established with support from the cucurbit industry and funding from the Southern Region IPM Center, a sister organization to the Western IPM Center, but works throughout the US and internationally. The EVCWG brings together members from academia, government, and private industry who work in research, production, extension, outreach, and regulation. The diversity of expertise, geographic locations, and industry connections help create more effective and practical strategies for identifying and controlling viral threats to US cucurbit production. With the lack of hard freezes this winter and pressure from key cucurbit vectors arriving early across Yuma County, you won't want to miss these upcoming informational sessions!

 

Upcoming Webinars
Aphid-, Beetle-, and Thrips-transmitted Viruses in Cucurbits
Wednesday, April 22, 2026 11:00 AM – 12:30 PM (Central Time)

This 1 hour and 20 minute session will cover key insect-vectored viruses affecting cucurbit production, including identification, transmission, and management strategies. Registration link: https://bit.ly/47DlDPi

Other Vectored and Mechanically- and Seed-transmitted Viruses and Viruses with Unknown Modes of Transmission in Cucurbits
Tuesday, May 5, 2026 11:00 AM – 12:00 PM (Central Time)

This one-hour webinar will explore additional virus transmission pathways and discuss emerging challenges in cucurbit virus management. Registration link: https://bit.ly/4rHHpbM

To learn more about the EVCWG and the resources they make available, please visit their homepage at: 
https://ecucurbitviruses.org/

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

 

Christopher Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

It is with mixed emotions that I write to inform you that this will be my last University of Arizona Vegetable IPM Update. The reason – I am retiring. Thank you for your support over the years. It’s been an honor and a privilege working with you and serving this ag community.

Figure 1. Automated Thinning & Weeding Technologies Field Day.
(Photo credits: Rosa Bevington)

As we wrapped up our chemigation trial, I would like to extend our sincere thanks to all the participating partners and to every grower, PCA, and representative who took time from a busy schedule to visit and observe the trial.

With spring well underway across the Yuma valley, weed proliferation is accelerating rapidly. Both in winter vegetable crops approaching harvest and in newly planted, emerging warm-season crops. Proactive and effective weed management remains essential for crop protection, harvest efficiency, and long-term field productivity within this desert production system.

The Yuma Integrated Weed Management Program supports growers and pest control advisors by providing locally relevant, research-based guidance to address current and emerging weed challenges.

Crops We Typically Support
Our program works closely with the Yuma production system to address weed management challenges in:

• Leafy Greens: Lettuce, spinach, and other leafy greens
• Cole Crops: Broccoli, cauliflower, and cabbage
• Spring and Warm-Season Crops: Watermelon and cantaloupe

 

Common Weed Challenges This Time of Year

During April in Yuma Valley, we typically observe:

• Heavy emergence of summer annual weeds as temperatures rise
• Residual winter weeds impacting harvest operations and field cleanliness
• Questions about herbicide efficacy relative to irrigation timing and soil moisture
• Crop safety concerns as production cycles overlap between winter and warm-season crops
• Early signs of herbicide resistance or shifts in weed populations

Timely management decisions made now can significantly reduce weed pressure throughout the season and help prevent seed accumulation for future crops.

How the Yuma Weed Management Program Help
We provide direct, practical support to growers and pest control advisors, including:

• Precise identification of weeds at both early and advanced growth stages
• Evaluation and optimization of pre- and post-emergence herbicide programs
• Guidance on weed resistance management and herbicide rotational strategies
• Integration of chemical, cultural, and mechanical control options
• Label interpretation, application timing, and crop safety considerations
• Applied on-farm research trials conducted under Yuma-area conditions

Our focus is on solving the real-world, field-level weed problems that are unique to the Yuma region.

Get in Touch — We Are Here to Help

If you are experiencing weed problems in your fields, have concerns regarding herbicide efficacy or crop response, or need assistance planning current or future weed management programs, we encourage you to reach out.

Growers — Contact us for weed identification, control options, and management strategies tailored to the current season and your specific crops.

PCAs — We welcome collaboration and field observations. Working together allows us to address weed challenges more effectively across the entire Yuma production system.

Early communication leads to better outcomes. The Yuma Weed Management Program, operating within the broader Integrated Pest Management framework, is available throughout the season to help you manage weeds effectively and sustainably.

Cole crops (broccoli, cauliflower, cabbage, and leafy brassicas) are highly susceptible to green peach aphid and whiteflies. These pests not only reduce plant vigor but can also contaminate harvestable portions and transmit viruses, leading to direct market losses. Azadirachtin-based insecticides (neem-derived products) can be a valuable tool for early-season and rotational use in organic cole crop production.

How do Azadirachtin-Based Insecticides Work?

• Slows feeding and reduces plant damage
• Disrupts development of immature stages (aphids and whiteflies)
• Limits the buildup in protected plant areas

These effects are particularly important for whitefly management, where targeting immature stages is critical for breaking the life cycle.

Timing for Success

• Start applications early, before populations build
• Focus on early vegetative stages
• Continue monitoring through crop development

Application Tips for Cole Crops
Cole crops present unique spray challenges due to canopy structure:

• Prioritize coverage
• Use higher spray volumes to penetrate dense foliage
• Target leaf undersides and inner canopy
• Use an appropriate nozzle setup to improve coverage
• Repeat applications may be necessary (short residual)
• Avoid long spray intervals when pest pressure is building

Key Limitations

• Not a quick knockdown product
• Less effective once pests are well established inside the heads
• Requires consistent scouting and timely applications

Research-Based Evidence
Field trials conducted in Arizona demonstrate that azadirachtin-based insecticides can suppress populations of green peach aphid and whiteflies on broccoli when applied early and with good coverage.

Figure 1. Green peach aphid numbers on broccoli as affected by azadirachtin-based
insecticides and M-Pede. Treatments with azadirachtin reduced aphid populations
compared to the untreated control. (Yuma, AZ, Spring 2026)

Figure 2. Whitefly numbers on broccoli as affected by azadirachtin-based insecticides.
Treatments with azadirachtin reduced the number of whiteflies compared to the
untreated control. (Yuma, AZ, Fall 2025)

 

Take-Home Message for Cole Crop GrowersAzadirachtin-based insecticides are a practical option for managing the green peach aphid and whiteflies in organic cole crops. Their effectiveness depends on early application, thorough coverage, and consistent application timing.

Two weeks ago, we discussed how unusually high maximum and minimum air temperatures in Yuma Valley were likely accelerating crop development and shortening the vegetable production window. Mean relative humidity (RH) provides an additional perspective on this spring’s weather pattern because it affects crops through a different pathway than temperature. While temperature strongly influences crop development rate, respiration, and time to maturity, RH is more closely related to atmospheric drying, crop water relations, canopy microclimate, and some aspects of pest management and spray application conditions.

Mean RH at the AZMET Yuma Valley station from January 1 through March 29, 2026, was more variable than the temperature pattern discussed in the earlier blog (Figure 1). Unlike maximum and minimum air temperature, which showed a clearer and more sustained departure from the 2020–2025 pattern, RH fluctuated more over time. Even so, several periods during March 2026 appear to have had relatively lower RH than the recent historical pattern, including part of the late-March period highlighted in the figure. This distinction is important because lower RH under warm conditions can increase the drying power of the air and contribute to greater atmospheric demand for water.

Figure 1. Mean relative humidity at the AZMET Yuma Valley station from January 1
through March 29, 2026, compared with the 2020–2025 pattern.

This RH pattern should be interpreted differently from the maximum and minimum temperature trends. Higher maximum temperatures can accelerate heat-unit accumulation and move crops more quickly toward maturity. Higher minimum temperatures can increase nighttime respiration and reduce carbon-use efficiency by increasing the fraction of assimilated carbon used for maintenance. Lower RH, in contrast, does not directly speed crop development or increase respiration in the same way. Instead, it can increase evaporative demand and strengthen the gradient that drives water loss from the crop and soil surface. When relatively lower RH occurs during an already warm period, crop water demand may increase further, especially in actively growing fields.

From an agronomic standpoint, this means that RH may add to the stress associated with the temperature pattern rather than duplicate it. In fields where irrigation timing, soil moisture, or root-zone conditions are already marginal, periods of lower RH may increase the likelihood of transient midday stress, reduced leaf turgor, and lower growth efficiency. This does not mean that lower RH alone caused crop stress or yield loss, but it may have increased atmospheric pressure on crops that were already developing under unusually warm conditions. For this reason, RH can help explain why water management may have become more challenging during March.

The RH pattern may also have relevance for IPM. Hot and relatively dry conditions can favor some stress-associated arthropod pests in certain crop systems, particularly when the crop is already under heat or water stress. At the same time, relatively drier air may make conditions less favorable for some moisture-dependent foliar diseases. However, these responses are highly dependent on the crop, pest, pathogen, irrigation method, canopy structure, and leaf wetness duration. Therefore, this figure should not be interpreted as showing a universal increase or decrease in pest or disease pressure. Rather, it suggests that the field environment may have shifted in ways that could influence pest dynamics and disease favorability.

Relative humidity may also matter operationally for crop protection. Under hot and relatively dry conditions, spray droplets can evaporate more rapidly, especially during the warmest part of the day. This can make application conditions less forgiving and may increase the importance of timing and coverage. Again, this does not necessarily mean reduced efficacy, but it does mean that lower RH can add practical challenges to foliar application under already warm spring conditions.

Overall, the mean RH does not replace the temperature story described in the blog from two weeks ago. Instead, it adds another layer to it. The earlier temperature data suggested that crops in Yuma Valley may have been developing more rapidly than normal because of unusually warm days and nights. The RH pattern presented here suggests that, during some periods in March, the atmosphere may also have been relatively drier than normal, potentially increasing evaporative demand and adding to crop water-management and IPM complexity. In that sense, this figure helps explain how the spring environment may have become more demanding even beyond temperature alone.

What this may mean going forward
Going forward, this pattern suggests that unusually warm periods combined with lower RH could increase the risk of faster crop development, higher water demand, and more challenging IPM timing in Yuma Valley. If these conditions become more frequent, growers may need to place even greater emphasis on weather-based irrigation scheduling, close field scouting, and timely management decisions.

VegIPM Update Vol. 17, Num. 8
April 15, 2026

Results of pheromone and sticky trap catches below!!

Corn earworm:  CEW numbers remained steady at most locations over the last two weeks, high numbers continue to be seen at Welton and Tacna areas. CEW is active throughout the summer and high numbers are expected leading into the next produce season.

Beet armyworm: BAW activity remained steady over the last two weeks. Historically adult BAW is collected through the summer, although numbers are lower compared to Mar-Apr seasonal highs.

Cabbage looper:  Cabbage looper moth activity continued to decrease over the last two weeks. Numbers are expected to continue decreasing as temperature gets warmer.    

Diamondback moth: Adult DBM numbers decreased across most locations. Based on other years data, DBM numbers are expected to continue decreasing as cole crops acres decrease.  

Whitefly: Adult remains steady across all locations, about average for this time of year. Expect numbers to continue increasing as temperatures warm up.  

Thrips: Thrips activity remains steady across locations. Historically, WFT numbers peak in May.  

Aphids: Winged aphid numbers continued to decrease at most sites in the last two weeks. Winged aphids disappear from our traps after April as temperatures become too hot for aphids.  

Leafminers: Adult leafminer activity decreased over the last collection. Leafminers are collected in our traps throughout the summer.