The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Feb. 6, 2013.

Western flower thrips populations are building

By John Palumbo, UA research scientist and Extension specialist

Western flower thrips populations are increasing on produce crops throughout the Yuma County, Ariz. area. Numbers have slowly increased in the Yuma Valley and will likely increase as the days grow longer.

With the heavy rain last week in the Yuma area (.75 to 1.2 inches), one might anticipate reductions in numbers typical in wet springs. Heavy rainfall can dislodge or even drown adult thrips on plants, plus suffocate larvae in the soil if it remains wet for prolonged periods.

The recent rainfall did not appear to slow down thrips population development in lettuce at the Yuma Agricultural Center as we would have expected. Populations were relatively light prior to the rain, but immediately following the storm, samples showed a significant increase in larvae.

Around Feb. 5, the population density increased almost five fold. The rainfall was not sufficient enough to impact these populations in the field.

Based on historical data, if temperatures remain moderate and rainfall is light, we can expect thrips numbers to reach high levels by the end of February.

Another factor which pest control advisers should be concerned about this time of the year is thrips “bio-concentration” which occurs each year in late February and March as lettuce acreage declines. This could be especially important this season since there appears to be less lettuce at this time of the year relative to years past.

Each time a lettuce field is harvested and disked, adult thrips populations disperse from these areas into the next available lettuce field. This is generally found with our seasonally warm temperatures which are suitable for thrips development.

As the number of lettuce acres is reduced near the end of the season, this creates a bottleneck effect which concentrates high numbers of thrips adults on remaining fields under production. This can often make chemical control very difficult, particularly in March, as thrips adults may continually re-infest fields following spray applications.

Furthermore, by mid-March when most lettuce production is finished, these populations can pose a threat to seedling cotton.

The key to preventing thrips from significantly scarring leafy vegetable plants is to prevent immature populations from establishing.

For more information on the identification, biology, ecology, and management of thrips on desert produce, check out this link: Insect Management on Desert Produce Crops.

Click this link to listen to John.

“Remember, when in doubt - scout.”

Contact Palumbo: (928) 782-3836 or

Weather impacts plant disease development

By Mike Matheron, UA Extension Plant Pathologist

In the last update, I commented on how the dry weather pattern experienced in the Desert Southwest has greatly reduced the progression of diseases, including downy mildew which thrives when relative humidity and free moisture are abundant.

The weather can change rapidly in the desert. With the wet conditions experienced last week, one should look for increased activity for diseases including downy mildew, Sclerotinia drop, and others.

In the first half of January, crops were subjected to several nights of temperatures at or below freezing in many areas. The results of physical damage may become more apparent as crops progress to maturity.

Crops which sustained freeze damage should be monitored carefully in the weeks ahead for the appearance of disease symptoms caused by opportunistic microorganisms. These can colonize freeze-damaged tissue and cause the additional deterioration of plants.

Click this link to listen to Mike's Update

Contact Matheron: (928) 726-6856 or

Pre-emergent herbicides

By Barry Tickes, UA area agriculture agent

Using pre-emergent herbicides correctly to kill weeds before or soon after emergence is often more complicated and difficult than post-emergent herbicide use.

Most pre-emergent herbicides do not kill weed seeds. The products only work on weed seeds which have germinated. The herbicides must be in the right place at the right time and at high enough concentrations or failure or partial control can result.

Weed seeds are killed by some products including fumigants chloropicrin, metam sodium and Telone, and by soil solarization and flooding.

Most pre-emergent herbicides only work once absorbed by the roots and shoots of germinated seeds.

Herbicides including Goal and Chateau are absorbed at the soil surface as the shoot emerges. Others are absorbed only at the root tips as weeds grow in the treated soil.

The location of the herbicide is critical. Roots or shoots already well established before herbicide contact often survive. Roots which have grown past the herbicide and still have vigor can often recover.

Some herbicides, including Kerb (pronamide), are absorbed and translocated throughout the plant. Others, including trifluralin (Treflan) and bensulide (Prefar), only work at the root tips.

Click this link to listen to Barry's Update.

Contact Tickes: (928) 580-9902 or

A question for the Yuma IPM team

By Marco Peña, UA research specialist

The UA IPM team received recently a greenhouse tomato sample with what appeared to be a disease symptom similar to a mold. The leaves were drying and the stems and fruit appeared bronzed and generally unhealthy.

When examining under a stereomicroscope, we noticed thousands of conical-shaped creatures, smaller than thrips larvae with four legs at the anterior part of the body and hairs at the tapered posterior end.

Mike Matheron discarded any possible disease. The sample was sent to John Palumbo who identified the pest as the tomato russet mite, Aculops lycopersicii.

To watch a short video on this pest, click on this link: “Tomato Russet Mite.”