Fall melon production is underway and transplanting/direct seeding of produce crops is close behind. PCAs will soon be faced with several important insect management decisions.  As crops begin to emerge, you can expect to encounter several insect species that can potentially cause serious economic losses to crop stands.  These seedling pests include flea beetles, crickets, grasshoppers, darkling beetles, earwigs, and saltmarsh caterpillars (‘woolly worms’). These insects all have chewing mouthparts, and most are capable of consuming large amounts of leaf tissue in a short period of time. Seedling crops at the cotyledon stage are most susceptible; these pests can devour entire cotyledons or outright kill small seedlings.  If left unprotected, transplants and larger seedling plants can sustain significant feeding damage on the terminal growing points or newly emerged leaves.  Not only can this feeding stunt plant growth but can result in lack of stand uniformity and ultimately, lack of uniform maturity at harvest. Host crop sources for flea beetle, crickets and "woolly worm" infestations include several summer crops (e.g., Sudan grass, cotton, and alfalfa), volunteer melons, and weeds (e.g., purslane, pigweed). 
Flea beetles are notorious for building up on weeds, particularly purslane, and alfalfa this time of the year. Fortunately, growers have been vigilant about field sanitation, and weeds are few and far between throughout the valleys. Be aware of beetle migration from recently harvested hay adjacent to your germinating crops. Crickets have also been especially abundant this summer, so keep a sharp eye for them, particularly under sprinkler pipe. Also be on the lookout of grasshoppers this fall. They can easily sneak up on you especially if we have an active monsoon.  Salt marsh caterpillars have not been seen in years but are known to disperse from alfalfa and Pima cotton (non-Bt).  Experience suggests that melon fields planted next to these crops and weedy areas are at a high risk from these seedling pests, particularly flea beetles. As cotton and Sudan grass are harvested or terminated during the next several weeks, these seedling pests typically move to the next available host crop; lettuce, cole crops and melons.  
Fortunately, there are many registered insecticide alternatives available that can be applied via sprinkler chemigation (i.e., pyrethroids) or foliar sprays (i.e., pyrethroids, methomyl, neonicotinoids) that can cost-effectively reduce their abundance and damage to susceptible produce and melon seedlings. Additionally, insecticide seed treatments are available for lettuce and broccoli that will protect stands from flea beetles and bagrada bug (i.e., Nipsit) for 14–21 days.  For more information on insect pests of leafy vegetables and melons at stand establishment please see:

1) Management of Insect Pests at Stand Establishment on Desert Produce and Melons
2) Palestriped Flea Beetles in Leafy Vegetables and Cole Crops

Soils with excess soluble salts, saline soils, and/or excess sodium (Na⁺) concentrations (sodic) are a natural feature of desert soils and common in arid land agriculture.  This is primarily due to an accumulation of soluble salts near the soil surface as water evaporates from the soil surface.
 
Saline soils are a problem in crop production systems because of the sensitivity to salinity of crop plants, although plants vary in their degree of sensitivity.  The period of greatest plant sensitivity to salinity is in the early stages of development, during germination and stand establishment.
 
Sodic soils are a problem in crop production systems because of the adverse effects of excess sodium on soil structure, causing a dispersion of soil particles and the breakdown of soil aggregates.  This leads to poor water infiltration and percolation in the soil profile.
 
Some basic points associated with saline and/or sodic soils and management are outlined in the following sections.
 

1. Saline soils have an electrical conductivity of the soil solution (extract, EC_e) of EC_c > 4 dS/m.  Functionally, saline soils have sufficient soluble salts to interfere with and diminish plant growth and development.
   1. Soil salinity is a relative term or condition based on the critical salt sensitivity levels of the crop plant in question.
   2. Plants vary tremendously in their degree of salt tolerance.
   3. Reclamation and management of soil salinity requires an understanding of the specific level of sensitivity of the target plant to salinity.
   4. Saline soils commonly have good soil aggregation and structure.
   5. Reclamation of saline soils requires sufficient water to accomplish leaching and the removal of soluble salts.  Thus, good internal soil drainage is important.

 

1. Sodic soils have a high level of exchangeable sodium (Na⁺) on the soil cation exchange complex (CEC).  Sodic soils have an Exchangeable Sodium Percentage, ESP > 15 of the soil CEC or a sodium adsorption ratio, SAR > 13 from the soil extract.
   1. Sodic soils commonly have poor soil structure and aggregation due to the dispersed condition of soil particles caused by high concentrations of Na on the CEC.
   2. In practice, finer textured soils, e.g. clay, clay loam, silty clay loam, etc. with high clay content can develop sufficient dispersion and poor soil structure at ESP or SAR levels ~ 6-10.  So, the ESP and SAR designations are general and not absolute.

https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download/?cid=nrcseprd589210&ext=pdf
Sodic soil reclamation does require an amendment that will facilitate the chemical exchange of Na⁺, usually from a calcium (Ca²⁺) source, such as gypsum (CaSO₄).
 

1. Reclamation for saline soils requires additional irrigation water for leaching.  Chemical amendments are not necessary for soluble salt removal.

An effective and straightforward method of calculating a leaching requirement (LR) can be calculated with the following equation that was presented by the USDA Salinity Laboratory (Ayers and Westcot, 1989).
Leaching Requirement (LR) Calculation:

Where: 
ECw = salinity of the irrigation water, electrical conductivity (dS/m)
ECe = critical plant salinity tolerance, electrical conductivity (dS/m)

1. Saline soils do NOT need amendments for reclamation or management.  Only leaching is needed.

 

1. Sodic soil reclamation involves a two-step process:

1. Exchange of Na with Ca and 2) leaching of soluble Na⁺ from the crop root zone.

 

1. Adequate soil leaching is required in both cases of saline and sodic soils.
   1. In the case of sodic soil reclamation, the leaching needs to occur after appropriate amendment applications for Na⁺ exchange with a suitable cation such as Ca²⁺.

 

1. Adequate drainage is necessary to accomplish sufficient leaching of solutes through the soil profile and below the root zone in the reclamation of both saline and sodic soils.

 

1. Irrigation systems capable of delivering sufficient water for leaching are necessary.
   1. Level basin flood irrigation best serves salinity leaching requirements.
   2. Sufficient salinity leaching can be accomplished with furrow flood or overhead sprinkler irrigation.
   3. Drip irrigation systems much be designed to provide sufficient volumes of water to accomplish adequate soil leaching.

i.Drip irrigation systems are commonly very limited in this respect.
 

1. Crop rotation systems are important in leaching and salinity management.
   1. For example: Crops planted on the flat and irrigated by level basin irrigation serve to facilitate good salinity leaching in a field.

i.Examples: wheat, alfalfa, sudangrass, etc.
 

1. Good field observations of crop growth and development in conjunction with regular soil and water sample analysis are key elements to the management of salinity and sodicity in agricultural fields.  

For example, young plants in the germination and seedling stages of development are most susceptible to salinity and water stress.  Abnormal amounts of seedling damage and/or difficulties in germination can often be early signs of increasing soil salinity.
Also, early signs of increasing salinity are often observed with plants demonstrating water stress with plant-available water levels in the soil that should seemingly be adequate for maintaining non-stressed plants.  Thus, fields that are requiring an irrigation in shorter intervals than usual can possibly be an early sign of increasing salinity and it is good to follow-up with a good set of soil samples and a sample of the irrigation water to check.
Early identification of problems in the field with increasing Na concentrations are commonly noticed with increasing tendencies of the soils in the field to form surface crusts very easily, which reduces water infiltration into the soil surface.  This is often recognized at the time planting and stand establishment with germination problems resulting from increasing degrees of soil crusting.

Viruses vectored by insects 
 
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses. 
 
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses. 
Insect transmit viruses in 4 distinct modes: 
 

1. Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high. 

Viruses transmitted in non-persistent manner:
 Papaya ringspot virus 
Zucchini yellow mosaic virus 
Watermelon mosaic virus

1. Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category.  Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies. 

1. Persistent circulative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper. 

Example: Beet curly top virus transmission by beet leafhopper 
 

1. Persistent propagative:  Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips. 

 

Save the Date : 2024 Plant Pathology Workshop 
When: August 29^th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF) 
Where: Yuma Ag Center, 6425 W 8^th Street 
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks! 

Earlier this year, we completed fabrication of a prototype commercial scale steam applicator for injecting steam into the soil prior to planting. The concept behind soil steaming is similar to soil solarization - heat the soil to levels sufficient to kill soilborne pathogens and weed seeds (typically 140 °F for >20 minutes). The self-propelled machine is principally comprised of a 100 BHP steam generator mounted on tracks and a steam applicator sled (Fig. 1). Steam is applied via shank injection as the machine travels through the field. After cooling (< ½ a day), the crop is planted into the disinfested soil.

The device has been demonstrated in several on-farm, field-scale (>1-acre plots) tests in Salinas, CA this summer. Although the trials are still in progress, preliminary results indicate that the machine is performing well and similar to our previous steam applicator prototypes. In those trials, we found that soil steaming provided excellent weed control (>90%), suppressed problematic soilborne diseases (Fusarium wilt of lettuce >50%, lettuce drop >70%), reduced Pythium spp. counts in soil assays (>93%) and increased crop yields (>24%).

For this upcoming season, we are seeking collaborators to conduct similar field-scale on-farm demonstrations in Yuma, AZ. The primary objectives would be to assess the viability of soil steaming at the field-scale level and obtain grower feedback on the device’s commercial potential. The machine can be adjusted to work with most bed configurations including narrow (40”, 42”) and wide (80”, 84”) beds, and is suitable for use in conventional or organic crops (soil steaming is organically compliant). To date, the device has been successfully tested in iceberg lettuce, romaine lettuce, baby leaf spinach and carrot crops.
If you are interested in an on-farm demo of soil steaming, please let me know. We have resources to conduct 3-4 on-farm demos, so space is limited. I’d be happy to work with you.

Fig. 1. Self-propelled steam applicator principally comprising a a) 100 BHP steam
generator mounted on tracks and a b) steam applicator sled that applies steam via
c) shank injection as beds are formed.

We are planning some Broccoli herbicide trials at the Yuma Agricultural Center. The idea is to have some demonstration plots for you to come and visit.  Will evaluate new and old products with different rates and incorporation methods hoping to obtain additional data on phytotoxicity and weed control. The goal is to have the demo plots available to PCAs and growers at the Yuma Agricultural Center as well as document the observations and make them available to all people in the industry.
Do you have an idea of a treatment or combination that could possibly perform great for our weed complex? If you like to participate creating this protocol please send us your ideas and suggestions. As an example we could play with older products such as Treflan, Prefar, Prowl, Devrinol, Goal Tender, Clethodim etc. Some suggest different levels of sprinkler incorporation (8, 12, 24, 36 hours) for Devrinol. Others apply Treflan to flat ground then disk and put beds, come back with Goal, or Clethodim.
 If you like to include a treatment please send it to marcop@ag.arizona.edu

A species' persistence requires the ability to adapt to the biotic and/or abiotic conditions present within its environment. This adaptation may occur through natural selection, which is the process by which organisms that are better adapted to their environment tend to survive and have better fitness. Natural selection is believed to be the motor of evolution. Evolution has occurred within all groups of organisms, including plant and animal populations. 

Plants have evolved resistance in many circumstances, including resistance to pests and pathogens attacks. In response to insect herbivory attacks, plants have been using chemical defenses to resist. Consequently, to maximize their fitness, plant-feeding insects have co-evolved with plants to overcome plant defenses utilizing an array of strategies. Insects have evolved the ability to detoxify plant chemicals used for defenses and use the compounds as cues that favor the detection of the plant host. Insects have also evolved an adjusted sensory system allowing host cues detection and a nervous system that is able to integrate inputs from sensory neurons. The enhancement in the sensory and nervous systems allows the detection and avoidance of toxic plants as well as the excretion, sequestration, and degradation of plant toxins. Additionally, herbivory insects utilize target-site mutation, cuticular, humoral, and cellular defenses against plant chemical defenses. Moreover, insects have evolved to resist predation, parasitism, and pathogen attacks by means of a series of mechanisms, including cuticular adjustment, adaptive behavior, and chemical defenses. 

With the intensive use of pesticides to manage agricultural pests, insect pests have evolved resistance to an array of insecticides using a variety of mechanisms. This type of evolution has been described as field-evolved resistance, which is a “genetically based decrease in susceptibility of a population to a toxin caused by exposure to the toxin in the field”. This is due to strong selection pressure that favors rapid evolution of resistance. For example, the widespread adoption of Bt crops in the U.S. has led to field-evolved resistance of corn earworm, also known as cotton bollworm, against Bt toxins. In some regions, Texas, for example, cotton bollworms being exposed to the Bt toxins in both corn and cotton throughout the year have been subjected to a high selection pressure, causing the pest to become quickly resistant to Bt toxins. 

The use of beneficial arthropods to manage insects can favor a decrease in insecticide use and consequently reduce the selection pressure caused by pesticides. Although the evolution of resistance to predators and parasitoids tends to be prohibited by some factors (special and temporal refuges from enemies’ attacks, reciprocal evolution by control agents, and contrasting selection pressure from enemy species), the evolution of resistance to biological control agents has been reported for several insect pests including Argentine stem weevil, greater wax moth, and fruit fly. This is likely due to reduced plant and natural enemy diversity caused by intensive large-scale agriculture. 

Several factors may play a role in the development of resistance. Large-scale homogenous agricultural systems do not allow enough refuges to sustain the susceptible strains, which would then mate with the resistant strains to dilute the resistance genes and maintain the susceptibility of the pest populations. Additionally, low biodiversity within the natural enemy population may favor the selection pressure. Coevolutionary arms races may play a significant role in that this may favor one participant in mutation and recombination rates. 

Mechanisms of resistance: 

Physiological resistance: Insects use physiological processes to become resistant to enemies and insecticides. In a pesticide use context, physiological resistance is defined as the capacity of an insect population to survive after being exposed to a concentration of insecticide that is known to be able to kill the totality of the population completely. However, the physiological process can also favor resistance against non-pesticide control methods. For instance, the fruit fly (Drosophila melanogaster) uses encapsulation to protect itself from koinobiont endoparasitoids. The encapsulation is a cellular immune response that follows three major stages, including the recognition of the parasitoid eggs as foreign, increasing the amount of circulating hemocytes that are produced by the lymph glands, and the lysis of the crystal cells allowing the release of prophenoloxidase which results in the melanization of the capsule surface. 

Another way insects become resistant is through mutation in the target site of the toxicant. This physiological process can lead to resistance in insects against both plant defenses (toxic compounds released by the plant to protect itself from herbivory) and insecticides. This mutation can lead to target site insensitivity, meaning that even though the insect is being exposed to the toxic molecule, there will be no or reduced binding of the molecule to the target site, making the molecule ineffective. This mechanism of resistance is very common in many insecticide-resistant insect pests. Insects can also become resistant to toxic compounds from plants and to insecticides by evolving the ability to undergo detoxification of certain toxicants after exposure. This ability is also conferred by a series of mutations allowing the resistant insect to increase their enzyme production, which consequently increases their enzymatic activity and causes a rapid degradation of the toxicant into a nontoxic compound. This mechanism is also known as metabolic resistance. 

Behavioral resistance: Many insect species have become resistant to certain host plants that use defense compounds to prevent herbivory through their plant selection and feeding behavior. For this behavior to occur, they must evolve the ability to detect toxic plants, which can be determined genetically or through a series of learning processes. Some other insects evolved the ability to deactivate or suppress the toxin produced by the plant hosts. For instance, the cotton bollworm uses its saliva, which is a gluco-oxidase, to cause a reduction in the level of nicotine produced in tobacco leaves. Other insects, when they feed on toxic plants, can excrete a significantly large amount of the accumulated toxic compound. Some insects even sequester the toxic compounds and use them for their own defense against predators and pathogens. Some insects that are hosts for parasitoids use a very effective behavioral resistant strategy by avoiding parasite contact or detection by choosing to niche away from the parasitoids or by choosing to locate themselves near a deterrent. Using this behavior, these insects are not directly resistant to the attackers but use what is present in their environment as tools to resist parasitism. Some other insects use alternative strategies, such as cryptic coloration or masquerade, to prevent their detection by predators and/or parasitoids. In this situation, they disguise themselves as something dangerous or unwanted to avoid being prayed on or parasitized. 

Cuticular resistance: Insects depend heavily upon cuticular defenses to resist pathogens, parasitism, predation, and insecticides. For instance, to resist insecticide penetration, they develop a barrier in the outer layer of the cuticle either by changing the composition of the cuticle or by thickening it. This causes the toxicant to be penetrated slowly, consequently slowing the absorption of the contaminants to the insect bodies, where actions will take place. 

Although the development of resistance is mostly beneficial for insects, there are some fitness costs associated with that. Physiological resistance, behavioral resistance, and cuticular resistance require the use of a large amount of energy; some energy that would have been allocated for growth, development, and reproduction is likely to be reduced, which would consequently reduce the fitness of the insect. Thus, fitness cost may cause an evolutionary constraint, which may reduce the rate or even prevent the evolution of resistance from occurring. Given that, an increase in resource availability is likely to favor the rate at which evolution occurs within a population. 

In conclusion, resistance in insects can occur in a diversity of forms, and several factors may cause resistance to occur within insect populations. Additionally, while insect populations are more likely to be resistant to insecticide in large-scale agricultural systems, they can also become resistant to biological control agents, which underscores the importance of integrated pest management programs. The rate at which resistance occurs in a population closely depends on the intensity of selection pressure to which the insect populations are exposed. Thus, the more intense the selection pressure the quicker the populations will evolve resistant.  

 

References: 

1- Ali, J. G, and A. A. Agrawal. 2012. Specialist versus generalist insect herbivores and plant defense. Trends in Plant Science. 17: 293-302. 

2- Balabanidou, V., L. Grigoraki, and J. Vontas. 2018. Insect cuticle: a critical determinant of insecticide resistance. Current Opinion in Insect Science 2018, 27:68–74. 

3- Berenbaum, M.R. 1986. Target site insensitivity in insect-plant interactions. In: Brattsten, L.B., and S. Ahmad. (eds) Molecular aspects of insect-plant associations. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-1865-1_7 

4- Boots, M. 2010. The Evolution of Resistance to a Parasite Is Determined by Resources. The American Naturalist. 178: 214-220. 

5- Castagnola, A., and J. L. Jurat-Fuentes. 2016. Intestinal regeneration as an insect resistance mechanism to entomopathogenic bacteria. Current Opinion in Insect Science. 15:104–110. 

6- Chareonviriyaphap, T., M. J. Bangs, W. Suwonkerd, M. Kongmee, V. Corbel, and R. Ngoen-Klan. 2013. Review of insecticide resistance and behavioral avoidance of vectors of human diseases in Thailand. Parasites & Vectors. 6: 280. 

7- Dang, K., S. L. Doggett, G. V. Singham, and C-Y. Lee. 2017. Insecticide resistance and resistance mechanisms in bed bugs, Cimex spp. (Hemiptera: Cimicidae). Parasites & Vectors. 10:318, DOI 10.1186/s13071-017-2232-3 

8- Després, L., D. Jean-Philippe, and C. Gallet. 2007. The evolutionary ecology of insect resistance to plant chemicals. TRENDS in Ecology and Evolution. 22: 298-307. 

9- Dubovskiy, I. M., M. M. A. Whitten, O. N. Yaroslavtseva, C. Greig, V.Y. Kryukov, E. V. Grizanova, K. Mukherjee, A. Vilcinskas, V. V. Glupov, and T. M. Butt. 2013. Can insects develop resistance to insect pathogenic fungi? PLoS ONE. 8: e60248. doi:10.1371/journal.pone.0060248 

10- Fellowes, M. D. E., and H. C. J. Godfray. 1999. The evolutionary ecology of resistance to parasitoids by Drosophila. Heredity. 84:1-8. 

11- Ferré, J., and J. V. Rie. 2002. Biochemistry and genetics of insect resistance to Bacillus thuringiensis. Annual Review of Entomology. 47:501–33. 

12- Heidel-Fischer, H., and H. Vogel. 2015. Molecular mechanisms of insect adaptation to plant secondary compounds. Current Opinion in Insect Science. 8: 8–14. 

13- Mills, N. J. 2017. Rapid evolution of resistance to parasitism in biological control. PNAS. 114: 3792-3794. 

14- Ryan, M. F., and O. Byrne. 1988. Plant-insect coevolution and inhibition of Acetylcholinesterase. Journal of Chemical Ecology. 14: 1965-1975. 

15- Tabashnik, B. E. and Y. Carrière. 2010. Field-evolved resistance to Bt cotton: Helicoverpa zea in the US and pink bollworm in India. Southwest. Entomol. 35: 417–424. 

16- Tomassetto, F., J. M. Tylianakisb, M. Realed, S. Wrattene, and S. L. Goldsonet. 2017. Intensified agriculture favors evolved resistance to biological control. PNAS.114: 3885–3890.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts decreased considerably y in the past 2 weeks and about average for late summer
 
Beet armyworm:         
Trap counts stay active in some locations (Yuma Dome Valley and Wellton) particularly near alfalfa
 
Cabbage looper:         
Cabbage looper numbers remain low consistent previous years.
 
Diamondback moth:      
DBM moths were not caught in any trap since June; average for this time of the year.
 
Whitefly:                      
Adult movement decreased in past 2 weeks consistent with previous years
 
Thrips:                         
Thrips adults decreasing in most locations, and trending comparable to previous years.
 
Aphids:                       
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity has highest in Wellton and Dome Valley, and slightly above average for early August