Monday, July 27, 2015

Controlling Aphids Can Be a Sticky Issue for Growers

This time of year cotton fields start turning the corner from a sea of pink, yellow and white flowers to an ocean of lush green bolls. 
Here is a close-up photo of aphids. -- UC IPM Photo

Lurking in the background, though, are pests that can damage the crop and inflict economic pain. Field scout Carlos Silva reports cotton aphids are taking center stage on the pest front right now in a number of fields, particularly in the Firebaugh region of Fresno County.

About of third of the cotton plant leaves he has inspected are infested with aphids. “Growers are starting to think about how they may need to get rid of the pest,” Carlos says.

Aphids can be found throughout the season and cause different kinds of problems during the growth stage of the cotton plant. During the spring, heavy aphid populations can cause leaves to crinkle and stunt seedling growth.
Aphids are found on the back of a cotton plant leaf.

Carlos says growers need to be vigilant with their monitoring programs. UC IPM says it’s important for growers to be aware of both the number of aphids and the color of the pests. Small yellow aphids develop slowly from nymphs to adults and don’t produce many offspring. As a result, their populations don’t increase quickly. On the other hand, darker green and black aphids reproduce rapidly and their populations can explode.

Natural enemies such as parasitic wasps, minute pirate bugs, bigeyed bugs and green lacewings can help control aphids. “There are a lot of green lacewings in the fields,” Carlos says.

Honeydew residue creates a sheen on a cotton plant leaf.
However natural enemies won’t be enough to tackle a heavy aphid infestation. But chemical treatment isn’t a simple solution. Here’s what UC IPM says:
“Chemical management of cotton aphid can be extremely erratic and unpredictable. Part of the problem is that cotton aphid has developed resistance to many chemical classes, including organochlorine, organophosphate, carbamate, and pyrethroid insecticides. In addition, these broad-spectrum pesticides kill the natural enemies of the cotton aphid. Another resistance concern is with the neo-nicotinoid insecticides. Repeated applications of any neonicotinoids can result in resistance to all neo-nicotinoids.”

Parasitic wasps can help control aphids.  -- UC IPM photo
“To manage resistance, follow the basic principles of IPM: (1) spray only when pests reach economic thresholds; (2) start with the most selective pesticides and avoid pyrethroids early in the season in order to preserve natural enemies; (3) save the broad-spectrum pesticides for mid- to late-season aphid outbreaks; and (4) rotate insecticides that have a different mode of action group number if you have to spray more than once.” Go online to UC IPM’s cotton aphid management site to see a table that summaries insecticide resistance guidelines.

Overall, cotton development is progressing well. Growers are irrigating their fields for the third time this season. The plants are at about 12 to 13 fruiting branches.

In alfalfa, growers are preparing for another harvest. So far, it appears many growers have enough water available for another cutting.
Carlos reminds growers to continue leaving strips of uncut alfalfa to create a habitat for lygus bugs and keep them migrating to nearby cotton fields. “There still is a lot of lygus in the alfalfa fields,” he notes.

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