Butterflies May Signal Future Alfalfa Problems

Alfalfa caterpillars, the larvae of butterfly eggs, can cause reductions in yield and quality Butterflies-Alfalfa

Author: Mike Rankin, Hay & Forage Grower

Butterflies in alfalfa fields may be free and make for a good photo opportunity, but they also can indicate future alfalfa worm-feeding issues. That’s the warning coming out of Central California where alfalfa fields are awash with yellow and white butterflies this summer.

“Some alfalfa fields appear more yellow and white than green with outbreaks of alfalfa caterpillar butterflies in the Sacramento and San Joaquin Valleys,” notes Rachael Long, an extension farm advisor in the region. “In certain cases, the populations have been massive,” she recently reported in the University of California extension’s Alfalfa & Forage News.

It’s not that the butterflies do any direct damage, but the eggs they lay soon develop into larvae known as alfalfa caterpillars. The yellow butterflies, sometimes referred to as sulfur butterflies, are cyclical, occurring in large numbers every few years. According to Long, contributing to high populations are factors such as slow and uneven alfalfa growth, a lack of predator insects such as the parasitoid wasp, and hot, dry weather.

The alfalfa caterpillar worm is green with a white stripe along each side. “They consume entire leaves and strip a plant, causing significant reductions in yield and quality if numbers are high enough,” Long notes. In severe cases, plants can be completely stripped of the high-value leaves.

Sulfur butterflies and the associated alfalfa caterpillar are not confined to California. The species can be found throughout the United States. According to the “Compendium of Alfalfa Diseases and Pests” (Third Edition), this pest does the most damage in the southwestern U.S., usually on irrigated fields. In southern regions, up to seven generations can occur, whereas in northern locations there may be as few as two.

The economic threshold for controlling alfalfa caterpillars is 10 healthy, nonparisitized caterpillars per sweep of the net. California specialists recommend scouting for the pest in conjunction with armyworms. They have produced a video that helps growers and consultants identify the worms, their natural parasite enemies, and how to differentiate a healthy worm from one that is parasitized.

If treatment thresholds are reached and the field is not close to harvest maturity, there are several chemical control options available for spraying alfalfa. Products containing the bacteria Bacillus thuringiensis (Bt) are also an option. Long reports that a number of fields in Central California have already required chemical applications to control their worm outbreak.

Full article shared from Hay & Forage Grower website.

Limiting Spider Mite Damage in Vineyards

Pacific spider mite prefers hotter, dryer part of the season California-Vineyard

Author: Greg Northcutt, Western Farm Press

As summer temperatures start to soar and the number of Willamette spider mites in vineyards begin dropping, the population of Pacific spiders can take off.

Typically, the early-season Willamette spider mite begins appearing when the leaf buds open. It can be a problem in coastal areas and Sierra foothill. However, it seldom is a pest in the San Joaquin Valley, especially in Thompson Seedless vines.

The larger Pacific spider mite, on the other hand, is the main pest mite species in the San Joaquin Valley and may also be the primary pest mite in certain coastal grape-growing areas. It prefers the hotter, dryer part of the season

Of these two pest mites, the Pacific poses a bigger threat if populations develop, notes Glenn McGourty, University of California Cooperative Extension viticulture farm advisor for Lake and Mendocino Counties.

Whereas the Willamette mite stays closer to the cordon where it affects older leaves, the Pacific mite is found on the tips of shoots where newer leaves play a bigger role in producing sugar later in the season. As a result, unlike the Willamette mite, the Pacific mite can affect ripening of the grapes.

Dusty conditions, like where vines are close to unpaved roads, favor populations of both species, McGourty says. So do sandy, gravelly sites where drier soils may stress the vines.

“In our area last year, the Pacific mite pressure was a little higher than usual in some upland vineyards where the drought was full blown, and growers were unable to irrigate as much as normal,” he says.

However, dust from roads isn’t the only type of dust that encourages a buildup of mites. Sulfur dust kills beneficials, such as predatory mites, that feed on both Willamette and Pacific mites. That’s why he doesn’t advise relying solely on sulfur dust to control powdery mildew.

“We like to apply sulfur dust post-bloom until the weather gets hot,” McGourty says. “Here in Lake and Mendocino Counties that’s from late May until mid-July. It would be sooner in the Central Valley. When temperatures get above 95 degrees, sulfur is likely to burn foliage.”

Because mites live underneath leaves, which shield them from sprays, they can be difficult to control with sprays. He recommends growers who use miticides to apply them before the canopy gets big, which may hinder penetration of the spray through the foliage.

However, he reminds growers that, depending on the material and mite population levels, a miticide treatment may not be effective against the various stages of mites present in the population. So an additional treatment may be required. In time, that can lead to development of resistance to the material. It is important to understand optimum timing—usually earlier rather than later. Looking for predatory mites is also important, since they may already be present in many vineyards.

McGourty favors an integrated biocontrol program using natural enemies to limit numbers of both Willamette and Pacific mites. These beneficials include six-spotted thrips as well as several commercially available predatory mites, like Galendromus occidentalis.

Some growers have achieved good success with Galendromus occidentalis, he notes, adding “These predatory mites, released at the rate of 5,000 per acre and preferably in the spring, come on bean plants that are placed in the canopy,” he says. “These predators scatter quickly, hunting for the pest spider mites, and they reproduce pretty effectively.

More information on controlling spider mites is available at http://ipm.ucanr.edu

Full article shared from Western Farm Press website.