The Arizona Vegetable Integrated Pest Management Update (Part 2) from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Oct. 18, 2012.

Prefar SLN herbicide in Arizona-grown lettuce

By Barry Tickes, UA Area Agriculture Agent

A special local-need registration was recently approved for the herbicide Prefar in Arizona that raises the herbicide use rate in lettuce from 4-6 quarts (acre) to 5-9 quarts.

These higher rates will help reduce some of the variability in weed control characteristic of Prefar and will also increase the possibility of crop injury.

Variable weed control from zero to 100 percent with Prefar can be frustrating and is often caused by three factors: weed spectrum, water use, and timing.

Weed spectrum: Prefar is active on a relatively small spectrum of weeds. These include most grasses, purslane, pigweed, goosefoot, and lambsquarter. There are more than 50 weed species common in lettuce in this low-desert region. Prefar has no activity on most of them. It is normally used in combination with other herbicides to broaden this spectrum.

Irrigation method and amount: These are important variables which impact the performance of Prefar. Prefar is exactly the opposite from Kerb in this regard.

Prefar strongly adheres to the soil and works best when large amounts of water are applied by sprinklers to push it down to where the weeds germinate. It does not work well when incorporated with furrow irrigation or light amounts of water.

Kerb does not strongly adhere to the soil and works best with furrow irrigation or delayed applications applied just before or after weed germination to avoid leaching below the germinating weeds.

Time of application: This is another important variable impacting Prefar performance. Prefar only works on the roots of developing seedlings. It does not move in the plant and only stops cell division at the tip of the roots that it contacts.

Prefar must be in the soil before weeds germinate. It differs significantly from Kerb in this regard. Kerb does move in the plant and will still work on small seedlings of many weed species. Early post-emergence applications can be effective.

Partial weed control can be the result of all of these variables. Higher use rates now registered will help overcome some of this. If weed infestations are heavy, however, even 90 percent control can look like a complete failure without an untreated check.

Lettuce has a good tolerance to Prefar at even higher rates than 9 quarts under ideal growing conditions. When any stress occurs, this tolerance goes down. Stress can be caused by environmental factors, pests, water, salts, and other variables. Heat and rain caused many problems earlier this season.

Unfortunately, growers have a low tolerance for crop injury. This is a major reason why few herbicides are registered for lettuce. This rate increase can be discontinued if many problems are reported.

The SLN can be found by clicking on the following link: Bensulide EPA SLN No. 120004

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Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.

Natural enemies of Bagrada bugs

By Ta-I Huang, UA post doc research associate

Bagrada bugs have been very active in desert cole crops since September. The population in the Yuma Valley slightly decreased over the last week due to relatively lower temperatures. The number of Bagrada bugs is expected to increase as the temperature keeps warm for at least another week.

Insecticides are the only and most promising weapon to knockdown Bagrada bugs.

The good news is that several egg parasitic wasps have been found to attack Bagrada bugs. Walker Jones, a USDA research entomologist, discovered one native natural enemy, Telenomus podisi, an egg parasitoid wasp known to attack other stink bug species that can successfully develop and emerge from Bagrada eggs, along with two species of egg parasitoids in the genus Trissolcus from France.

Surveys for Bagrada bug parasitoid in its native ranges, including India and South Africa, are being conducted by entomologists.

However, this is only the beginning. The forging behavior and parasitism rate on Bagrada bugs in the field remains unclear.

Other natural enemies observed to attack Bagrada bugs include spiders, assassin bugs, and earwigs. Dead Bagrada bodies on spider webs are often seen in field and greenhouse cages. Late instars or adults of assassin bugs have been observed in the field to prey on Bagrada bugs. Other predatory stink bugs may feed on Bagrada as well.

Omnivorous earwigs were found to feed on Bagrada eggs in the greenhouse colony at the Yuma Ag Center and possibly feed on early stages of Bagrada nymphs.

These natural enemies may somehow balance the pest population from outbreak but they may not provide the same protection to a crop as insecticide use, especially in the low desert.

Keep in mind that biological control works better when conservation practices are applied, including a selective insecticide, preserving natural habitats, or planting nectar-rich flowering plants neighboring to crops.

Although a biological control program for Bagrada bugs may not be achieved soon, we can start at a small scale and conserve those natural enemies.

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Contact Ta-I: huang@cals.arizona.edu.