IPM strategies for desert seed alfalfa production

Lygus, Lygus hesperus, is the peskiest of all pests that continually bothers seed alfalfa growers and pest control advisers (PCAs) in low desert production.

“If insecticide applications are timed properly, seed alfalfa growers can make a crop despite the lygus bug,” said Eric Natwick, University of California Cooperative Extension (UCCE) entomologist and director, UCCE Imperial County, Holtville, Calif.

“We’ve always been chasing the lygus bug with new insecticides to keep ahead of the game.”

Natwick shared various integrated pest management strategies for desert seed alfalfa production with growers and PCAs during the 3rd annual Southwest Agricultural Summit in Yuma, Ariz., in March.

Properly-controlled insecticide treatments help minimize control costs and are important to successful lygus bug control. Natwick says it’s important to understand the biology of lygus.

The lygus bug has three stages of development: egg, nymph, and adult. There are five nymphal “instars” (periods between molts). The lygus life cycle is complete after 28 days during the summer.

Lygus is best controlled with early insecticide applications, Natwick says. While adults are often repelled from seed production fields following an application, mature lygus quickly fly back once the repellency has subsided. Nymphs that hatch a few days following an insecticide application often survive so an additional insecticide treatment may be required.

“My recommendation is to spray when most eggs have hatched and there are some third instar nymphs,” Natwick said. “Lygus in the fourth and fifth instars are very difficult to control; especially the fifth instar nymphs.”

There is no strict timetable for lygus control, but Natwick suggests three action thresholds (spray periods). The first is early alfalfa bloom and prior to bee pollination when four to six bugs are collected with a single net sweep.

“This is a good time to use a pyrethroid or a strong organophosphate material including Carzol that’s registered for lygus bug control,” Natwick said. Carzol has a 24c emergency-use exemption in seed alfalfa with specific crop restrictions including the non-use of the hay or straw for livestock feed.

Natwick says several new, softer insecticides including Beleaf are demonstrating excellent lygus control. Studies indicate the insecticides cause no harm to pollinating honey bees and leaf cutter bees.

The second threshold is bloom to seed set when eight to 10 bugs are gathered per sweep. Seed set to maturation is the third threshold when 10 to 12 bugs are collected. Action levels vary with actual field conditions.

Over the last three years Natwick has compared the efficacy of older and newer pesticides on lygus and the stink bug in research trials at the University of California Desert Research and Extension Center in Holtville.

Experiments included four replicates of the CUF 101 variety on plots measuring 50 feet by 13.3 feet with four beds per plot. Natwick tested about a dozen formulations of insecticides.

The conclusions indicate that all insecticide treatments provided some efficacy against lygus. Treatments containing a pyrethroid insecticide consistently resulted in lygus resurgence over the three-year period from mid- to late season.

“I think there’s something going on with pyrethroids,” Natwick said. “I don’t know if the pyrethroids are knocking out a particular parasite of lygus bug, or it could be hormolygosis (increased egg production) or insecticide resistance. We just don’t know what caused the resurgence.”

Plots treated with NAI-2302 (Tolfenpyrad), Beleaf, Lorsban, and a pyrethroid had significantly less stink bug-damaged seed compared to the untreated control plots.

The trials were partially funded by the California Alfalfa Seed Production Research Board.

For a full report on Natwick’s insecticide-seed alfalfa trials, go online to http://ceimperial.ucdavis.edu/, click on ‘Ag Briefs Newsletter’ and then click on ‘April, 2009.’

• Alfalfa seed chalcid

Another target of desert-grown seed alfalfa is the alfalfa seed chalcid insect, Bruchophagus roddi Gussakovsky. Adult seed chalcids enter the field during bloom and oviposit (lay) eggs in the soft, immature green seed within the alfalfa pod.

Larvae hatch in four days, feed on the seed internally, mature in about 10 days, and then pupate. Pupae mature in 12 days and the adults exit the seed. The adults feed on nectar and then mate. Females insert their eggs into developing seeds three to four days later. Chalcids overwinter as larvae in the seeds.

Natwick says chalcid-damaged seed should be detected and assessed from hand-threshed seed pods collected at random during harvest and at the seed-cleaning facility.

Insecticidal control of seed chalcid is impractical since the chalcid wasps are continually entering the field and depositing eggs into seeds in the pods. The eggs and developing larvae are protected from insecticides.

Management techniques include destroying volunteer alfalfa on field borders, fence rows, and ditch banks, Natwick says. Infected seed or chaff should be destroyed. Managing the field for uniform seed set and plant clip back is recommended; the earlier the better.

“Clip back is a community project,” Natwick said. “It’s ideal to get all seed alfalfa growers in the same area to clip back at the same time to go to seed, preferably in March or April. Clip back later in the season increases chalcid-related problems.”

• Web-spinning spider mites

Four web-spinning spider mites are found in desert seed alfalfa fields: twospotted, carmine, strawberry, and desert mites. The insects are typically found on lower alfalfa leaves in seed and hay fields throughout the season.

Spider mites insert needle-like mouthparts into the leaves to eat plant sap resulting in chlorotic spot stippling on leaves. Severe feeding damage causes leaves to turn brown and then dehydrate leading to defoliation.

Damage usually begins in the lower plant canopy and then moves upward as the mite’s appetite grows for new leaves. Severe feeding damage reduces yield and can retard re-growth.

For good spider mite control Natwick suggests using Brigade 2EC, Carzol, Neem oil or sulfur mixed with the first insecticide application for lygus. Do not apply oil and sulfur in the same field, he says.

Natural predators assist in spider mite control including western flower thrips, minute pirate bugs, bigeyed bugs, and predaceous mites.

• Stink bug

Growers and pest control advisers should also stay on the lookout for stink bug, Chlorochroa sayi Stal and Euschistus conspersus Uhler. Adults overwinter in and around alfalfa fields and other crops and then emerge in the spring. The stink bug feeds on green pods and seed causing significant damage. Lygus-control insecticides are usually successful for stink bug control.

• Aphid

The spotted alfalfa aphid favors hot weather and can be a problem when susceptible seed alfalfa varieties are grown for seed. Pea and blue alfalfa aphids are primarily a problem when cool weather follows clip back. Cowpea aphids are not a problem in desert seed alfalfa production, Natwick says.

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TAGS: Alfalfa
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