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”
Click picture to listen to John’s update
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.
Click picture to listen to Mike's update
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.
Click picture to listen to Barry'update
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
Results of pheromone and sticky trap catches can be viewed
No corn earworm moth were captured in traps over the past 2 weeks.
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.
Captures of cabbage looper moths decreased slightly in the past two weeks, and remains seasonably low.
Adults were not captured on sticky traps over the past two weeks.
Thrips movement was very low at all trapping locations.
Numbers on traps are very low across all trap locations.
Alate (winged) aphids continue to show up on traps at low-moderate levels. However, green peach aphids are the prominent species identified on traps.
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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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