A significant decline in insect pest activity has occurred over the past 2 weeks. These changes are consistent with cooler, seasonal temperatures. We all know that weather influences produce crop growth and ultimately can create wide fluctuations in markets. Similarly, our local weather can significantly impact pest activity on desert produce crops.    For insects, their activity and abundance are closely aligned with weather. The reason for this is that temperature is the driving force behind insect development, growth and behavior. Unlike many animals, insects are poikilothermic (“cold-blooded”); that is, they are unable to regulate their body temperature. Their internal temperature varies along with that of the ambient environmental temperature. Consequently, insect pests such as whiteflies, beet armyworms, cabbage loopers, diamondback moths, and leafminers are more active and develop rapidly when temperatures average ~85° F; in contrast, they are less active and develop much slower under cool, winter conditions.  That is one of the primary reasons these insect pests can be so abundant on produce crops in Sep-Nov.  Behavioral activities such as flight, movement, reproduction, feeding and oviposition are similarly influenced by seasonal temperatures. But temperature requirements for insects vary with species (see table below).  For example, the optimal temperature for growth and development of beet armyworms and diamondback moths is the same (86 °F), but diamondback moth can complete a generation quicker and can develop over a much broader range of temperature. Thus, they can be more problematic season long if not adequately controlled.  Then you have Bagrada bug which tend to prefer warmer weather for development.  Its lower developmental threshold of 62°F is the main reason the pest is generally hard to find during the winter months.  In contrast, green peach aphids, which we annually find on winter produce crops, thrive in cooler weather (optimal temperature for growth is 55°F.  That’s the main reason we don’t find them on summer crops; it is just too hot for them.  In general, extreme hot (> 120°F) or cold temperatures (<32 °F), can be lethal or greatly restrict insect growth and behavior.  Consequently, temperature and other weather factor such as rainfall, humidity, sunlight and wind play a major role in determining insect activity and abundance on local desert crops; but they can also suppress insect abundance under extreme conditions. For a more detailed explanation on the impact of weather on insects visit Weather Influences Desert Insect Pests.
 
 

The water shortage on the Colorado River that we are dealing with today has been accelerated by the extensive drought that began more than 20 years ago.  Climatologists have recognized this drought, extending more than 22 years, as the most extensive drought this region has experienced in more 1,200 years.  However, there are many factors that have been in operation for more than 100 years that are contributing significantly to our current water shortage dilemma.
 
To effectively address a problem, it is important to understand the situation leading to the problem in question and that includes understanding the derivation and historical background leading to the current condition.  Accordingly, I am offering a few points of historical importance in my view that have contributed to the development of the challenges we are dealing with on the Colorado River system today.
 
The Colorado River watershed and system (Figure 1) is one of the largest in the U.S.  The main channel of the Colorado River extends over 1,450 miles, covers 244,000 square miles in the drainage basin, and it has the greatest elevation fall of any watershed in North America. The Colorado River supports more than 40M people, nearly 6M acres of cropland, 30 native American tribes, and provides large amounts of electrical power generation for southwestern U.S.

Figure 1. The Colorado River watershed.  Source: USGS.

Average annual flow in the Colorado River between 1906-2017 was 14.8 million acre-feet (maf), Figure 2.  Between 2000 and 2018, the average annual flow has been ~ 12.4 maf, Figure 3.  The allocated water for diversion on the Colorado River has been 16.5 maf (15 maf in the U.S. + 1.5 maf to Mexico).  Thus, there has been a structural deficit of at least 2.0 maf for many years and during this megadrought of the early 21^st century we have been operating on a deficit of ~ 4.0 maf.  
 
Actual water use estimates for the past 30-40 years are shown in Table 1, including evaporation and actual diversions, so that the total use of the Colorado River has been estimated at ~ 16.0-17.5 maf. Collectively, as a result over the past 22 years, the great reservoirs of Lakes Mead and Powell have been significantly diminished.

Figure 2.  Colorado River Annual Flow, 1905-2010 with an overall average and 10-year
average. Source: Bureau of Reclamation.


Figure 3.  Colorado River Annual Flow, 1905-2020 with an overall average and 2-year
average. Source: Bureau of Reclamation.

Table 1. Colorado River Use Estimates. Source: Bureau of Reclamation.
MAF = million acre-feet per year.
 
One observation that can be drawn from a review of Figures 2-4 is that annual flow in the Colorado River is highly variable.  Secondly, the past 20 years indicates a distinct decline in annual flow in the river.  Comparatively, the first 20 years of the 20^th century had relatively high annual flows, particularly in contrast to the first 20 years of the 21^stcentury.  This has contributed to the difficulties being experienced today.
 
Governance of the Colorado River is commonly referenced to the “Law of the River”, which is based on the Colorado River Compact of 1922 (CRC of 1922) and a collection of laws, contracts, agreements, and definitions.  Thus, the Law of the River is not one distinct entity but rather an amalgam of various components.  Key components include the CRC of 1922 which was formally negotiated among the basin states to define how much water from the Colorado River can be used by each state on an annual basis. Other agreements contribute to the aggregate Law of the River, including a 1944 Treaty, that agreed to Colorado River water allocations to Mexico.  In 2007, basin states negotiated interim guidelines to deal with Colorado River water shortages and determine the reductions for each state depending on the elevation of water in Lake Mead.  The reductions associated with a shortage of Colorado River water was further augmented in 2019 by the Drought Contingency Plan (DCP) among the basin states, with specific guidelines for both the upper and lower-basin states.
 
The CRC of 1922 is a foundation to the “Law of the River” and was developed following a period of relatively high annual flows, Figure 4.  Based on the records associated with the transactions leading to the signing of the CRC of 1922, the hydrologists and other scientists were providing data and interpretations to the commission that was working on the CRC of 1922.  Even though these hydrologists and scientists did not have the technological capabilities that are available today, they did an amazingly good job.  There is evidence that the data was presented to commission negotiators showing that allocating 16 maf (plus the future allocation to Mexico for delivery) was more than the 10 year-average for much of the timespan of the available data.  Nevertheless, the operational base of Colorado River water allocation was defined in the CRC of 1922 with 15 maf allocated among the basin states and they have been able to make it work for 100 years.  However, as we can see from review of Figures 2-4 and considering the annual average flow of the past 20 years being ~ 12.4 maf, the original CRC of 1922 allocation budget is not sustainable.

Figure 4.  Natural annual Colorado River flow data from 1878-2013 with a running average
(gray line) and 10-year moving average (orange line) and indicating the point of CRC of 1922 signing. 

The good news is that we do have ~ 12 maf of annual flow currently to work with.  That represents about 27% less annual flow than the budgeted amount of 16.5 maf.  If all states receiving Colorado River water allocations were proportionately reduced by 27%, each state would then need to determine how that water would be allocated within their boundaries.  Arizona’s allocation would change from 2.8 to ~ 2 maf.
 
One of the good things about common attitudes for people in agriculture is the tendency to appreciate and deal with reality and the basic facts that we are faced with.  Whatever we do, we must deal with the reality of this situation on the Colorado River and come to terms with the facts of the matter the best we can.  Agriculture is responsible for ~ 70% of the total water diversions in Arizona which supports an industry worth more than $23B annually and provides food and fiber to the state, region, nation, and global exports.  
 
Some experts are advocating a review and re-negotiation of the CRC of 1922 and the basic budget of water allocations.  That is a reasonable suggestion considering the facts of the case.
 
The climate is bearing down upon us, conditions on the Colorado River are increasingly dire, and we do not have much time to act.  In agriculture, we need to be part of the process of dealing with the Colorado River water shortage and be part of the solution in a manner that is sustainable for all of Arizona and the productive agricultural systems of this state and region.

The soft-rot bacterium, Erwinia carotovora subsp. carotovora  (syn.= Pectobacterium carotovorum subsp. carotovorum), enters through growth cracks or wounds caused by cold temperatures, insects, other disease organisms, or by mechanical means. Under warm, humid conditions, uninjured tissue may become infected through natural openings.  Prolonged moisture from rain or irrigation and mild temperatures encourage disease development. Insects, tools, rain, clothing, or affected plant tissue can spread the bacteria. The bacteria survive in soil and plant debris.
Symptoms appear as small, water-soaked areas and enlarge rapidly.  Tissue becomes soft and mushy, and within a few days the affected plant part may collapse. An offensive odor usually is present.
Management: 
Cultural: 

• Set out plants in rows to allow good air drainage.
• Cultivate carefully to minimize injuring plants.
• Control frequency and source of irrigation water.Avoid frequent irrigation during head development. Time irrigation to allow the head to dry rapidly. Avoid stagnant water sources.
• Clean and spray storage walls and floors with copper sulfate solution (1 lb/5 gal water). Bactericides such as Clorox, Lysol, and quaternary ammonium products also are effective.
• In storage, use a buffering material such as straw or paper to prevent injury to the heads.
• Keep storage house humidity between 90% and 95% and the temperature between 32°F and 39°F.

Chemical control
·    Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. 4-hr reentry. 

• Cease at 3 to 6 quarts in 100 gal water. For greenhouse plants only. Preharvest interval is 0 days. 4-hr reentry. 

Interested in the latest automated weeding technologies? University of California Cooperative Extension will be hosting the 2022 Automated Technology Field Day where fifteen of the newest commercial thinning and weed control technologies will be demonstrated in the field. Featured technologies, some showcased for the first time to a general audience, include laser weeders, “smart” precision spot sprayers, autonomous in-row weeding machines, “smart” in-row cultivators, and the UC Davis steam weeder. Company representatives will be on-hand to discuss their equipment. The event will be held from 9:00 am – 12:00 noon, Wednesday, June 8^th in Salinas, CA. For additional information, see the event flyer below.

It is well known that conditions of the environment such as temperature, light, CO_2, humidity, and wind at the time of application as well as many other factors impact how herbicides perform. This could vary depending on the weed species, the compound, if the product is soil applied etc.
One of our subscribers asked specifically about Rely herbicide's response to temperature and humidity.

A study showed that plants grown at 90% relative humidity presented more translocation of glufosinate than those grown at 35%. Relative humidity had greater effect than temperature on glufosinate toxicity to Palmer amaranth, redroot pigweed, and common waterhemp ⁽¹⁾.
Additional research showed glufosinate acted slower with lower temperatures delaying injury to two weed species, but only small differences in injury existed among temperature regimens in the final evaluations 288 h after spraying. It also suggested that the activity of glufosinate ammonium on both species was significantly reduced by low relative humidity ⁽²⁾.

Barry Tickes at the The University of Arizona conducted a nutsedge trial in 2010 at the Yuma Mesa in which we sprayed Rely in June with similar temperatures to what we are having today and humidity below 20%. Then, a second application was done in August, which averaged ~40% relative humidity. The following table shows the % control was higher after the August application.  This is consistent with the studies we refer to above.

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 21, Oct 16, 2024

Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: CEW moth counts increased in most traps; about average for early October.

Beet armyworm: Trap counts increased in most areas last week, and about average for early October.

Cabbage looper: Cabbage looper trap counts still most locations, below average for this time of the season.

Diamondback moth: Adult activity increasing, particularly in the Yuma and Dome Valleys.  Above average for this time of season.

Whitefly: Adult movement increasing Tacna, Dome Valley and Yuma Valley; overall about average for early October.

Thrips: Thrips adult movement picking up in all areas, overall activity above average.

Aphids: Winged aphids beginning to show up the north Yuma and Gila Valleys; about average for early October.