Ag Alert March 17, 2021

Trials provide guide for controlling blue alfalfa aphid ByBob Johnson

emies,” the farmadvisors said. “Some, like lady beetles, syrphid flies and parasitoid wasps, target aphids. Others, like damsel bugsandminutepiratebugs, aregeneralist predators and feed on a variety of prey.” The UCCE specialists said aphids can be very susceptible to predators, with few defenses, “and serve as abuffet for natural enemies.” They said killing most, but not all, of the aphids while killing its natural enemies can lead to aphid resurgence or secondary pest outbreaks. Monitoring pest populations is com- mon practice to determine whether an insecticide application is necessary and to evaluate how effective the material is. But in the case of the blue alfalfa aphid, advisors recommend that the ratio of beneficials to pests after the application should also be evaluated. “A high natural-enemy-to-aphid ratio indicates that natural enemies are helping keep aphids in check,”Grettenberger said. “Alowratioofnatural enemies toaphids in- dicates that theinsecticideisdoingallof the workandnatural enemynumbersare low.” Aphids can be difficult to control be- cause they reproduce asexually, bear live young and can increase very quickly. Blue alfalfa aphids can be particularly

damaging toalfalfabecause they inject tox- ins intoplantswhenfeeding, causingstunt- ing, reducing yields and sometimes killing plants.Thesetoxinscanremainintheplants afterharvestandpersist intothenextcutting or even the subsequent twocuttings. Thekey to telling thedifferencebetween the blue alfalfa aphids and other aphids is markings on the antennae. “The blue alfalfa aphid is a large, blue- green aphidwith long legs, antennae, cor- nicles andcauda,” according to theUCPest Management Guidelines. “It is similar in appearancetothepeaaphidbutcanbedis- tinguishedbyexaminingtheantennae.The antennaeofthepeaaphidhavenarrow,dark bands on each segment, whereas those of the blue alfalfa aphid gradually darken to brownas younear the tipof theantennae.” The trial in the intermountain region foundsignificantdifferences inthe import- ant beneficial-to-pest ratio, depending on the type of insecticide used. When broad-spectrum organophos- phate or pyrethroidmaterials were used, Grettenberger said, the lady beetle pop- ulation was wiped out within a few days. Butwhenmore-selective insecticideswere applied, the ratio of lady beetles to aphids was as high as 1 to 25 after twoweeks. The choice of insecticides had a similar impact on parasitoid wasps, which were flattened by the organophosphates and pyrethroids but reached ratios as high as one parasitoid for every six aphids after twoweeks whenmore selectivematerials were applied. The work of these beneficial insects in keeping theaphidpopulationsunder con- trol weeks after the insecticides knocked down thepest populationpaidoff inhigh- er yields at thenext cutting, the trial found. In the trial, the plots where softermate- rials reduced the blue aphid populations

salmon survival rates were exceptionally low. The survival rate was still more than four times as many as the survivors from a control group of young salmon raised in more conventional ways. “Our hopewas todemonstrate rice field- rearedfishmightsurvivetheir journeyout to theoceanbetterthanthosethatareraisedin hatcheries,”Buttnersaid. “Wehypothesized this possible outcome based on previous studiesshowingthatsalmongrowextremely fast in rice fields because of the abundance of their natural food sources in these fields. I’mhappy to say that our 2020 graduating classof young salmondidnot let usdown.” Winter-flooded rice fields are naturally a rich sourceof nutrients for baby salmon, he said, and the research now focuses on learning how the fields canmost easily be modified toallowthe fishaccess to the riv- er to begin their journey to the sea. Information about the salmon proj- ect is available at calricewaterbirds.org/ salmon-program. ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.) The secondmonitoring after the appli- cation would measure both the knock- downof thebluealfalfaaphidand the ratio of the beneficials to the pests. Another implication of the trial is that the priority for new aphicides should be onmaterials that arebotheffectiveagainst thepest and relatively easyon ladybeetles and parasitoidwasps. “Treatment options in California are currently limited, butwehopefullywill see more selectivematerials enter themarket in the future, such as afidopyropen and sulfoxaflor,”Grettenberger said. “There isa strongneed foraphid-specificalternatives, whicharemuchsofteronnatural enemies, to prevent aphid pest resurgence and sec- ondary pest outbreaks.” ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.) and kept significant numbers of lady bee- tlesandparasitoidwaspsaroundaveraged 1. 97 to 2.1 tons in the first cut. But where organophosphates or pyre- throids knocked down the blue aphids but also wiped out the beneficials, the yields dropped to just 1.19 to 1.58 tons, which was lower than in plots where nothing was applied. The take-homemessage,Grettenberger said, is that the prudent strategy is to em- ploynot onebut twomonitoringprotocols whenmanaging blue alfalfa aphids. “Watch for natural enemies, and treat when thresholds are reached or when you consistently see aphids in alfalfa crowns and the plant is growing slower than ex- pected,”headvised. “Takenatural enemies intoaccountwhendecidingif/whentotreat and when choosing a material. Continue monitoring post-treatment, especially if usingbroad-spectrummaterialssuchasan organophosphate or pyrethroid.”

Few insect pests demand a soft touch that keeps beneficials alive and working more than the blue alfalfa aphid. The application of organophosphate or pyrethroid materials controlled this pest just fine after three days in a trial in the in- termountain region near the Oregon bor- der. But two weeks later, there were even more blue alfalfa aphids than in nearby plots where nothingwas sprayed. “Accounting for natural enemies can help take advantage of their services and avoid potential pest resurgence and secondary pest outbreaks,” said Ian Grettenberger, University of California Cooperative Extension entomology spe- cialist. “Choosing a selective insecticide that targets aphids withminimal effect on natural enemies helps extend control and reduce the need for additional insecticide sprays. After the insecticide loses effica- cy, preservednatural enemies are there to suppress anyaphids that remain, abio-re- sidual effect.” Grettenberger has worked to develop a programfor thisaphidincollaborationwith UCCEfarmadvisorsandspecialistsRachael Long, DanPutnamandRobWilson. “Inalfalfa, aphidshavemanynatural en-

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consider fallowing some of their ground. “Wealsohaveanuplandbirdandnative pollinator habitat program,” Buttner said. Asmany as 230 species have been identi- fied as using California rice fields as habi- tat, most of thembirds. Thestate’s rice fieldsprovidebirdhabitat equal to 300,000 acres of wetlands, which would cost nearly $2.8 billionup front. TheRiceCommissionisworking incon- certonthesalmonprojectwithresearchers fromtheUniversityofCalifornia,Davis,and Point Blue, a group of scientists who lend their expertise to private and public land managersengaged inconservationefforts. Buttner said baby salmon raised in nu- trient-rich, winter-flooded rice fieldswere tagged and released into the Sacramento River in the early spring of 2020, to learn how many would successfully make the trip to the Pacific. Although only one in a hundred sur- vived the elements and the predators to complete their journey, the result came after a low-water, drought winter when

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8 Ag Alert March 17, 2021