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Desert too hot for lettuce aphid

The Salinas Valley was the first place the destructive new aphid was first identified. That was in the summer of 1998 and it has been increasing its destructive presence since.

It made its way to the desert valley later that year and was first identified in the Gila and Yuma valleys in the spring of 1999, according to University of Arizona research entomologist John Palumbo.

Palumbo, stationed at the Yuma Valley Agricultural Center, told more than 100 pest control advisors and producers at the 11th annual Desert Vegetable Crop Workshop in Yuma that he does not expect the lettuce aphid to pose as serious a threat to desert lettuce as it does in the cooler climes of Salinas’ summer.

The green peach aphid is the more common aphid pest in Yuma, and it has been around for a long time. However, it is even hard to trap in the desert in the summer.

Not only is it too hot for the lettuce, but also there are few primary hosts in the desert upon which it can survive the summers.

However, Palumbo said that doesn’t mean producers and PCAs will not have to contend with it because he believes it likely will continue to be a problem because it will hitchhike to the desert on transplants and vegetable harvesting equipment, which is how Palumbo believes it got to Yuma two years ago.

Marking differences

The lettuce aphid is distinguishable from the green peach aphid by its black markings on leg joints, antennae and abdomen. It also has a more reddish color than the pale or translucent green peach aphid, he said. "In Salinas some people call it the red aphid because of its color," said Palumbo.

The lettuce aphid prefers to feed on the lettuce’s plants growing point while the green peach aphid prefers older leaves.

The lettuce aphid has a short life cycle and disperses readily. Often its infestation pattern in a field is patchy, therefore, it requires far more thorough scouting to detect. The growing point is the primary scouting site.

And, failure to detect can cause major problems since it feeds deep in the head where major damage can occur. Palumbo has found as many as 700 aphids within a single head lettuce plant.

Its optimum growth temperature is 65 to 70 degrees — a warmer range that the 55 to 60 degrees for the green peach aphid. Therefore, Palumbo said if the lettuce becomes a problem, it likely would be late in the desert lettuce season.

This late-season potential infestation likely would run headlong into a buildup of predators. As a late-season pest, that would put it at the end of the desert lettuce season and therefore not a major pest.

Threshold at one

While Palumbo does not believe the lettuce aphid will develop into as serious a problem as it is in Salinas, he said the threshold for treatment in his book is one.

"If you do find it, you probably ought to nip it in the bud" because of the damage it can cause, he said.

Palumbo recommends the same topical pesticides as are used in Salinas: Metasystox-R, Orthene, Provado and Endosulfan. If these products are selected, they must be used before the lettuce head closes.

The widely used systemic Admire can effectively control aphids for 70 to 90 days at the 16- to 20-ounce rate. However, that control level may not be sufficient under heavy lettuce aphid populations. "If you get 90 percent control of a heavy population, that 10 percent not controlled may represent thousands of aphids," warned Palumbo.

He believes the use of Admire in the areawide plant bug and whitefly suppression program around Yuma also may be why the lettuce aphid has not established itself in the desert. Use of pyrethroids and Lannate for thrips control may also be a contributing factor.

There are highly resistant varieties of butter and head lettuce Palumbo tested. "I would have not believed the resistance level had I not seen it," he said. Where susceptible varieties had as many as 900 aphids per plant, resistant varieties had none.

"The resistant varieties currently available are not agronomically suited for our spring lettuce deal, but they are working hard in the Salinas area to get them going there," he said.

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