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Desert vegetable research yields impact of wind, temperature, and rain on aphids

Growing vegetables is not exactly a piece of cake, especially considering the damage aphids inflict on lettuce, broccoli, and the like. Yet veggie growers in the desert-growing areas of the Imperial and Yuma valleys in California and Arizona can have their cake and eat it too, if they follow guidelines designed to improve aphid management in vegetables.

“The best economic management of aphids in desert-grown vegetables includes early detection, accurate aphid identification, using the best insecticides for the right aphids, thorough insecticide spray coverage, and spraying when at least 10 percent of the plants become infested,” according to John Palumbo.

The University of Arizona research entomologist and Extension specialist based at the Yuma Agricultural Center, Yuma, Ariz., has conducted numerous research trials on factors that impact aphid development and movement in desert-based vegetable production.

Palumbo's research has also placed aphid control-insecticide products under the field microscope. Findings show that several new products, Beleaf and Movento, work the same as or better than conventional products with fewer spray applications.

Palumbo spoke during the 18th annual Western Farm Press-sponsored Desert Vegetable Crops Workshop in Holtville, Calif., in November. Commercial sponsors of the event included BASF, Booth Machinery Inc., Bayer CropScience, Dow AgroSciences, Syngenta, Tessenderlo Kerley Inc., FMC Corporation, Valent U.S.A., and Advan.

Factors that influence aphid abundance and control include wind, rainfall, and temperatures, Palumbo said.

In terms of foliar sprays and control, wind can ground aerial applications of insecticides, plus impact drift and product deposition. Yet what impact does wind have on aphid colonization and abundance?

“Ever wonder what happens in the Yuma and Imperial valleys to green peach aphids during the summer - you always see them in the spring in desert produce, but they are few and far between in June, July, and August.”

Palumbo concurs with 1960s research conducted by University of California, Riverside entomologists. The findings conclude that green peach aphids and other aphid species almost achieve extinct status in the desert in the summer due to high temperatures above their physical tolerance and a lack of suitable plant hosts.

To prove the point, the UC entomologists established a sticky trap line in California in the mountains above Riverside and San Bernardino, through the Coachella Valley, and then through passes down to the Imperial area. Aphid findings were zero in the summer, yet in September and October aphids began appearing in the traps.

The conclusion — aphids start to migrate out of the coastal mountains in early fall as hosts become unsuitable, and move to the desert areas to recolonize on suitable hosts like lettuce, sugar beets, and weeds. Aphid populations build quickly since parasites were left behind and more favorable wind and weather conditions exist.

Palumbo's own research at the Yuma Agricultural Center has confirmed that aphid buildup occurs this time of year. During the winter of 2006-2007, wind was common in the Yuma Valley. When the wind blew particularly in October and November, and then stopped, lettuce and broccoli were covered with winged aphids.

Palumbo has also tracked aphid infestations in various temperature conditions. Results indicate that in years with moderate winters peaks in aphid abundance were typically observed throughout the course of the growing season.

“Conventional wisdom would tell you that insect mortality would occur with most insects under extreme temperatures or during sub-zero periods. In the study, the six to eight days with nighttime temperatures below freezing in January 2007 in Yuma caused aphid populations to slow but not crash,” Palumbo noted. When temperatures warmed up in late January and early February, aphid populations began to grow more rapidly.

Aphids are cool-season pests, Palumbo concluded. Their lethal mortality rates are much lower than whiteflies and most worm species.

Rainfall can also influence aphid control by delaying foliar application or washing products from the plant. Moisture also impacts aphid population numbers. In a Palumbo study conducted from 1999 to 2006, annual vegetable plantings included five plantings per season — one in October, two in November, and two in December. Many aphids were counted in untreated plots throughout the growing season. While the numbers varied from year to year, aphid infestations were lower when little or no rainfall occurred during the season.

“Rainfall definitely has an influence on aphid populations. We don't necessarily see heavy aphid populations when there's a lot of rainfall. Aphids typically don't cause problems during dry winters,” Palumbo said.

A lot of it has to do with the desert. Fall and winter rainfall creates an abundance of native and weedy hosts. This provides a reservoir of aphids that can continually migrate into desert produce. In dry years, the desert flora or the aphid numbers don't exist.

Palumbo's research also delved into the effectiveness of aphid insecticides. While soil-applied imidacloprid is the industry standard for aphid control, Palumbo's research studied action thresholds for the timing of foliar spray applications, not in terms of counting aphids on plants, but looking for the presence or absence of aphids.

In particular, he noted the percentage of plants with colonizing aphids. All aphid types were present, not just green peach, foxglove, and potato aphids. He determined that a threshold of 10 percent aphid infested plants was the best time for initiating treatments.

In insecticide trials, Palumbo utilized conventional insecticides, products with more reduced risks, and combinations of both for aphid control.

“If you look at a matrix of the products available for aphid control in lettuce and look across the growing season whether soil or foliar, ultimately what we found was that using a combination of insecticides was the most effective method of controlling the aphid complex,” Palumbo said.

“Products like the Neonicotinoids are not the most effective against foxglove and lettuce aphids, and some older chemistries are not very effective against green peach aphids. Combinations worked the best, particularly when more than one or two species were present,” he said.

The 2003 to 2007 studies conducted at the Yuma Ag Center yielded positive findings for Beleaf (FMC Corporation) and Movento (Bayer CropScience).

“Beleaf is a real interesting compound that fits nicely with other products available to pest control advisors and growers,” Palumbo said.

Beleaf (flonicamid) is a new chemistry that prevents aphids from feeding — it's basically a feeding blocker. Aphids basically starve to death because they can't eat. Beleaf is very effective on aphids in produce and also has a good fit for lygus in other crops. Other pros include a clean toxicity profile and a zero days to harvest application rating. Knockdown can be expected in four to seven days depending on temperature, and a residual length of control from 14 to 21 days depending on rate.

Palumbo called Beleaf a good alternative in the mix for aphid management.

“Using three sprays at 10 to 15 day intervals, Beleaf equaled other products in green peach aphid control, and rated a little better than the Neonicotinoids for foxglove aphid control. Beleaf also rated better than average on lettuce aphids.

“With lettuce aphid, we learned Beleaf is not a rescue product — you just can't come over the top and clean up something that's enclosed within a head or within a romaine heart. That goes with all the available products,” Palumbo said. “You can slow them down and reduce the numbers somewhat, but you can't clean them up once the aphid is established inside the head. Beleaf is a nice product that will fit well in aphid management programs.”

A product expected to gain a label for use in 2008 is Movento (spirotetramat) by Bayer CropScience. Movento is a new chemistry with essentially the same mode of action as Oberon (spiromesifen) used in cotton and melons for whitefly control.

Movento's foliar systemic action moves the product up and down the plant much like soil-applied insecticides. It also has a reduced risk toxicity profile and the residual period is 21 days on average. Movento should be a foliar alternative to the popular Neonicotinoids.

In six trials with aphids present at harvest, Movento controlled green peach aphids about equal to the industry standard yet accomplished the mission with fewer sprays. That's an indication of the product's residual control, Palumbo said.

For foxglove aphids, Movento worked a “little to significantly better” than industry standard products with fewer sprays. For lettuce aphids that are difficult to control depending on the time of the season, Movento worked “significantly better” than the industry standards, much better than the untreated, and with fewer sprays. Since lettuce aphids are a problem pest, more Movento applications were required.

Palumbo was impressed by Movento's systemic action. In tests where more than 200 aphids had colonized inside Romaine hearts just prior to harvest, Movento applications were successful in eliminating the aphids from the entire plant.

“Ten days after the first application of Movento the aphids were reduced to very low numbers. By the second application (seven days before harvest) the aphids were essentially zeroed out. Movento has real potential as a clean up or rescue-type product in lettuce,” Palumbo said.

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