Monday, June 10, 2013

Central Valley Alfalfa a Haven for Good and Bad Bugs

We talk about bugs a lot, especially those pests that can cause heavy damage to crops. It is basic economics: Damaged crops equal loss income.

One place where you can find lots of bugs is an alfalfa field. These fields are home to more than 1,000 species of bugs, according to University of California ag researchers.
Carlos uses his sweep net to catch bugs in a local alfalfa field.

In fact, alfalfa fields are dubbed the “insectary of the Central Valley” because they host many predators and parasites that travel to nearby fields. At the same time, alfalfa plays an important role in biological controls of pests in diversity of crops, including alfalfa. Of course, out of the hundreds of bugs that call alfalfa fields home, there are only a handful of bugs that can do serious damage to alfalfa. These pests are worrisome at different times of the season.
Right now, we’re at the tail end of the threat from alfalfa weevils. Our field scout Carlos Silva found some potential problems in a couple alfalfa fields last week. His sweep net snagged 18 weevils per sweep – near the threshold for treatment. However, these growers are likely to avoid spraying because the alfalfa is ready for the third cutting of the season. Harvesting would take away the food source for alfalfa weevils.

This week, Carlos will be keeping an eye out for beet armyworms, which are usually active from June to September. Here is a UC list that highlights the seasonal period for other major pests:
  •           Leafhooper – now until August.
  •           Alfalfa caterpillars – now until August
  •           Cowpea aphid – July to August
  •           Spotted alfalfa aphid – July to September

We can’t stress enough the importance of weekly monitoring because pest populations can explode with little notice. Monitoring is key for an effective integrated pest management program.

To get started, you simply need a sweep net. Here’s a quick field scout guide on sweep net sampling: Pick three or four different locations of the cotton field to perform the sweeps. Picture a big “X” overlaid on top of your field and take your pest samples at each leg of the X. This ensures your locations are equally spaces out to give you a good overview or sample from your entire acreage. You also can follow a “V” pattern. 

Net sampling should be done weekly. Also be sure to rotate the sample locations to ensure you cover all areas of the field.

Lygus is starting to show
up in a few cotton fields.
- UC IPM photo
The technique is pretty straight forward. As you walk down a row, you move in a sweeping motion back and forth or right to left (left to right if you’re a lefty). Image the sweep net as a tennis racket. One sweep is a forehand move and the second sweep is a smooth backhand motion – just like last Saturday’s French Open champ Serena Williams.

The same sweep net monitoring technique applies to cotton. Carlos is seeing some early concerns with lygus in cotton. One grower has already treated for the pest.

During one field visit last week, Carlos found four lygus after 50 sweeps. UC guidelines put the treatment threshold during June at two lygus bugs per 50 sweeps. Growers can learn more about lygus in cotton from UC IPM online.

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