Cone, Wyatt W.

Wright, Larry C.

Undetermined

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Section IVBiological & Cultural ControlTESTING AN ALTERNATIVE METHOD FOR CONTROLLING HOP APHIDS:SPRAYING THE APHID'S WINTER HOSTS, PLUM TREESL.C. Wright and W.W. ConeIrrigated Agriculture Research & Extension CenterWashi ngton State Un iversityProsser, WA 99350-9687(509) 786-2226FAX (509) 786-4635Ornamental and fruit plum trees are the only known over-wintering hosts for the hopaphid. Hop is the sole summer host and only wingless females are produced on hopduring the summer. In surveys done in 1990 and 1991 in the lower Yakima Valley,we found large numbers of hop aphids infesting red-leafed ornamental varieties ofplum trees, while fruit varieties had much fewer aphids. We concluded that themajority of the hop aphids infesting hops came from the ornamental plums that areused in landscaping. In 1993 and 1994, we did an experiment to test the hypothesisthat reducing the number of hop aphids on ornamental plums would reduce thenumber of aphids infesting hops.1993 The area selected for the study is the part of the Yakima Valley around Harrah,which is south of the city of Yakima. The valley was divided into a northern part ofabout 60 square miles and a southern part of about 77 square miles. One hundredforty six plum trees were located in the northern part and 144 trees were sprayedwith diazinon and oil on 31 March and 2 April. The southern area was not sprayedand was used as the control. Forty five of the trees were still infested on 7, 11 and12 May and 43 of them were sprayed with bifenthrin on 2 June. Hop aphids weresampled on plums to determine if the number of aphids in the two areas differed. Tenshoots were examined per tree in the unsprayed area on 15, 16 and 19 July and on21 and 23 July in the sprayed area. The trees in the sprayed area averaged 4.1aphids per shoot with 10 of 63 trees infested. The trees in the unsprayed areaaveraged 6.1 aphids per shoot and 25 of 85 trees were infested.Hop aphids were sampled in hop yards in the two areas to determine if spraying theplums had an effect on the number of aphids on hops. Sampling was done weeklyfrom 9 June to 30 July for a total of eight weekly samples. Nine to 12 yards weresampled each week in each area and the aphids were counted on 50 leaves in eachyard. Over the whole sampling season, the hops sampled in the sprayed areaaveraged 2.19 aphids per leaf, while the hops in the unsprayed area averaged 2.142728per leaf. The difference was not statistically significant (t = 0.2587, P = 0.796,9348 df). The two most likely explanations for the failure of the plum sprays toreduce the number of aphids on hops were: 1. The application of oil and diazinonwas not very effective. 2. The sprayed area was too small; hop aphids probablyinvaded the sprayed zone from the surrounding unsprayed areas.1994 This year we expanded the sprayed area to about 130 square miles in thevalley around Harrah, and used the hop growing area around Moxee, which is east ofYakima, as the unsprayed control. A total of 235 plum trees were located in theHarrah area and 224 trees were sprayed from April 13 to 22 with one application ofTalstar (bifenthrin) using 3.2 oz per 10 gallons of water. To see how well the sprayworked, hop aphids on 36 trees on a transect of the Moxee area were counted onMay 18 and 20, and 49 trees randomly selected in the Harrah area were sampled onMay 20 and 23. Ten shoots were examined per tree. The trees in the Moxee areaaveraged 21.3 aphids per shoot, while the Harrah plums averaged 0.78 aphids pershoot. In a second sampling, 31 plum trees were examined on July 7 in the Moxeearea and 36 trees were sampled on July 7 and 8 in the Harrah area. We found anaverage of 0.17 aphids per shoot in the Moxee area and no aphids were found on theHarrah trees. One application of Talstar controlled the hop aphids very well.Weekly hop yard sampling started June 13 and continued for eight weeks, concludingthe week of August 1. Fifty leaves were sampled per hop yard and 10 yards weresampled in each area per week. The number of aphids on hops in the sprayed areawas not significantly different from the number in the unsprayed area over the wholesampling season. The Harrah hops had an average of 2.80 aphids per leaf and theMoxee area had 2.56 per leaf (f = 1.335, P = 0.182, 7,998 df). Taken one weekat a time, the Harrah area had significantly more aphids than the Moxee area in threeweeks, the Moxee area had significantly more aphids in two weeks, and there wasno difference in three weeks.Despite good control of the aphids on the plums and the large size of the sprayedarea, the aphids were still able to infest hops in relatively high numbers, probably byflying into the area from the surrounding unsprayed areas such as the cities ofYakima, Wapato, Toppenish and White Swan. Based on our knowledge of the hopaphid, the aphid depends on plums to complete its life cycle. Therefore, reducing thenumber of over-wintering aphids by spraying or removing plum trees should reducethe number of aphids infesting hops. However, to be successful, the controlmeasures would have to be done over a very large area, probably the whole YakimaValley.

Testing an Alternative Method for Controlling Hop Aphids: Spraying the Aphid's Winter Hosts, Plum Trees

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Testing an Alternative Method for Controlling Hop Aphids: Spraying the Aphid's Winter Hosts, Plum Trees

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