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University of Arizona
March 2nd 2014 Vegetable IPM Updates
Insect Management
Weed Science
Insect Management:

Thrips Populations Are Building on Spring Produce

Thrips populations are slowly building in the Yuma Valley, and will likely continue to increase as the days grow longer and warmer. I expected that the rain we experienced in early January (0.75-1.0”) would reduce population numbers because heavy rainfall can dislodge or even drown adult thrips on plants, and can suffocate larvae in the soil. However, the recent rainfall did not appear to slow down thrips population development on untreated lettuce at the Yuma Ag Center. In fact, thrips larval numbers have been 2- times greater on untreated romaine as compared to this time last season (see graph below). It appears the rainfall had a negligible impact on these populations in the field; nor has the cooler weather we’ve experienced so far this winter. Based on historical data, if temperatures remain moderate and rainfall is light we can expect thrips numbers to reach very high levels by the end of the month. Note: the key to preventing thrips from significantly scarring leafy vegetable plants is to prevent immature populations from becoming established. The cryptic or thigmotactic bevahoir of thrips often makes them difficult to find on lettuce plants. Research has shown us that if you can see a few adults and larvae on the plant, it likely means that as many as 8-10 fold more thrips are actually on the plant (hiding near the base of the plant between midribs). This behavior also means that spray coverage is important, particularly with contact insecticides like Lannate, Torac and pyrethroids. For more information on the identification, biology, ecology and management of thrips on desert produce please visit Western Flower Thrips Management on Desert Produce.

Did you know?: “Like the words sheep, deer and fish, the word thrips is used for both the singular and plural forms, so there may be many thrips on a lettuce plant or a single thrips”.


Remember, When in Doubt . . . . . “SCOUT”

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Lettuce Powdery Mildew

February is a month when powdery mildew can make its initial appearance in maturing lettuce plantings. The disease, caused by the fungus Golovinomyces cichoracearum, is first observed as very small spots of white fungal growth on both upper and lower leaf surfaces of the oldest leaves. From these initial infection sites, the fungus continues to grow on the leaf surface and release vast quantities of spores which are carried in the air, and upon landing on lettuce leaves initiate additional infections under favorable temperature and moisture conditions. The most favorable temperature range for spore germination is 65 to 77 °F. Relative humidity at or above 85% is required for infection, growth and sporulation by the pathogen; however, free moisture will actually kill powdery mildew spores. Low light intensity also favors powdery mildew development. These requirements are often all met for several hours daily, especially on lettuce leaves near or at the soil surface in a maturing lettuce planting. As little as 4 days are needed from infection to production of a new crop of pathogen spores. Depending on environmental conditions and the particular susceptibility of the lettuce variety being grown, preventative applications of a fungicide may be needed to prevent economic loss to the crop. The oldest leaves containing the first powdery mildew infection sites will not be harvested; however, these leaves serve as nurseries for production and release of spores, which can infect the marketable portion of the lettuce plant. In recent field trials conducted at the Yuma Agricultural Center, fungicides that provided excellent control of powdery mildew on lettuce included Fontelis (penthiopyrad), Merivon (fluxapyroxad+pyraclostrobin), Microthiol Disperss (wettable sulfur), Procure (triflumizole), Quintec (quinoxyfen), and Rally (myclobutanil). Initiating fungicide treatments before or at the very latest at the very first sign of infection on the oldest leaves will result in the best levels of disease control.

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Weed Science:

Summer Annual Weed Emergence

It is not too early to consider applying herbicides for summer annual grasses. Preemergence herbicides are available and effective for us in many summer annual crops. They are only effective however when applied before weeds emerge and it is better to be a month early than a minute late. The graph above is based on a trial we did in Yuma several years to determine when summer annual grasses emerge. Weeds seed germination varies by species and will be affected by temperature, soil type, depth in the soil and other factors. This chart is a general guideline. Summer weeds began to germinate in March, reached a peak in June but continued to germinate through October. Pre-emergence herbicides should be applied, when possible, starting in late February and may need to be reapplied in June or July. The herbicide label should be checked to see what is allowed.


Question to IPM team: What’s this weed in my alfalfa field?
Corn Spurry (Spergula arvensis), January 29, 2016 Yuma, AZ.

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Area wide Insect Trapping Network:

February 3, 2016

Our area-wide trapping network is up and running. The project is designed to measure the activity and movement of adult populations of a number of key pests. The project is being funded by the Arizona Iceberg Lettuce Research Council, and will hopefully provide an indication of when pest activity (e.g., corn earworm moth flights) is increasing based on pheromone/sticky trap captures. The data is not intended to indicate field infestations, as trap data is largely a reflection of adult movement. If nothing else, the data may make PCAs aware of increased pest activity in some areas and encourage intensified scouting in susceptible produce fields. The pests being monitored include: corn earworm, tobacco budworm, beet armyworm, cabbage looper using pheromone traps; aphids, thrips and whiteflies using yellow sticky traps. A total of 15 trapping locations have been established. Traps will be checked weekly and data will be made available in the bi-weekly Vegetable IPM updates. If a PCA or grower is interested in weekly counts, those can be made available by contacting us.
Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: No corn earworm moth were captured in traps over the past 2 weeks.

Beet armyworm: Captures of beet armyworm moths were very low over the past few weeks, and are currently at their lowest point of activity for the season thus far.

Cabbage looper: Captures of cabbage looper moths decreased slightly in the past two weeks, and remains seasonably low.

Whitefly: Adults were not captured on sticky traps over the past two weeks.

Thrips: Thrips movement was very low at all trapping locations.

Leafminers: Numbers on traps are very low across all trap locations.

Aphids: Alate (winged) aphids continue to show up on traps at low-moderate levels. However, green peach aphids are the prominent species identified on traps.

To contact John Palumbo go to:


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The Vegetable IPM Updates Archive page provides links to updates from previous weeks.

The Vegetable IPM Video Archive page contains a collection of educational videos from current research work in vegetable crops by University of Arizona Researchers.


For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.

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