CABBAGE APHID, BREVICORYNE BRASSICAE (L.),' CONTROL IN BRUSSELS SPROUTS IN RELATION TO CROP DEVELOPMENT Frank G. Zalm and Carlyn Pickel'!PM Implementatin Grup University f Califrnia Davis, CA 95616 Abstract: Brevicryne brassicae (L.) infestatins in brussels spruts heads were characterized seasnally in relatin t sprut develpment frm Santa Cruz C., Califrnia field plantings during 1984 and 1985. Plants received t 9 insecticide sprays applied at 2 week intervals in 1984. Plants received t 5 insecticide sprays applied at 3 week intervals in 1985. Number f Breuicryne brassicae cntaminated spruts at harvest were significantly greater (P < D.OS) n plants sprayed, 1 r 2 times than n plants sprayed 3 r mre times in 1984. A significant (P <.1) relatinship between prprtin infested spruts and number f insecticide applicatins was bserved in bth years. The perid f initial sprut infestatin appears t cincide with lateral bud develpment which ccurs rapidly after apical stalk remval (tpping). A significant difference (P <.5) in sprut plant weight between treatments was fund in 1984, with lwer weight fr plants receiving mre sprays. Key Wrds: Cabbage aphid, Breuicryne brassicae, brussels spruts, damage, develpment, distributin. J. Agric. Ent<>ml. 5(3): 16H67 (July 1988) Apprximately 22 ha f brussels spruts Brassicae leracea gemmi{era are grwn in Califrnia, with the majrity being lcated n castal plateaus alng the Pacific Ocean frm Mnterey Bay t Half Mn Bay suth f San Francisc. Aphids are cnsidered the mst serius pests f brussels spruts in this area (picket et al. 1983) because f quality standards impsed n lcal grwers by packers wh permit at mst 1% f the spruts at harvest t be infested with this insect fr market. The cabbage aphiq. Breuicryne brassicae (L.), is particularly imprtant because f the difficulty in cntrlling this species r remving it frm the prduce nce it has entered individual spruts. Fear f aphid cntaminatin and t a lesser degree yield lss has resulted in a situatin where insecticides are rutinely applied fr aphid cntrl frm early seasn t harvest. Wilsn, et a!. (1983) develped a sampling prgram fr bth B. brassicae and the green peach aphid, Myzus persicae (Sulzer), which incrprated prvisinal cntrl actin threshlds based n thse develped fr brccli by Trumble, et a1. (1982). Pickel and Bigelw (1983) demnstrated that the prgram culd reduce insecticide treatments as much as 5% when cmpared t cnventinal preventative spray prgrams. In Califrnia, brussels spruts are transplanted int fields in late spring. The stalks are allwed t grw until late summer when the apical prtin f the stalk is remved (tpping), frcing the lateral bud ndes alng the stalk t mature int harvestable spruts. The specific bjective f this research fcused n characterizing, HOMOPTERA: Aphididae. Accepted rr publicatin 2 June 1988. 2 Cperative Extensin; 1432 Freedm Blvd., Watsnville. CA 9576. 161
162 J. Agric. Entml. Vl. 5, N. 3 (1988) the infestatin f the develping spruts by B. brassicae spatially n the plant and seasnally in relatin t sprut develpment in rder t determine the ptential fr delaying r mitting early seasn aphid treatments. METHODS Small Pl/s Brussels spruts were transplanted nt twelve 3m lng beds at the Sheriffs Rehabilitatin Center in Watsnville, Santa Cruz C., CA n 31 May, 1984. The plants were tpped n 7 September, and harvested n 2 Nvember. Apprximately five field cllected B. brassicae were transferred t each plant after the plant became established t insure unifrm infestatin. Plant grwth was mnitred weekly by cunting the number f ndes present n each f 1 plants randmly selected frm the field. Plants frm each treatment were examined fr the presence f aphids t insure that all test plants were infested befre initial treatments were applied. Treatments cnsisted f t 9 applicatins f xydemetn. methyl applied t ten successive plants in a rw with fur replicatins f each treatment placed at randm in the field. The insecticide was applied with a backpack sprayer at a rate f.92 kg (A.I.)/hect8I'e. Tw untreated plants were left between each plt as a buffer t prevent insecticide drift. Plants receiving 9 applicatins were first treated 33 days after transplanting (3 July) with the remainder f the sprays being applied n tw week intervals until tw weeks befre harvest (19 Octber). Plants treated fewer times received xydemetnmethyl at tw week intervals beginning successively ne spray perid later. At harvest, leaves were remved frm each plant and the remaining stalk f spruts cut and weighed. Ten plants were sampled frm each treatment replicate. Every 4th sprut was remved frm each plant s that a ttal f 2 spruts pel' plant culd be examined fr the presence f B. brassicae. Plant weight and prprtin f aphid infested spruts were cmpared between treatments by multiway analysis f variance and Duncan's (1951) multiple range test Binmial data were transfrmed using an arc sine cnversin prir t analysis. Large plts ]n 1985, the results f the 1984 study were tested n a 2 ha cmmercial field transplanted n 4 June at Wilder Ranch State Park nrth f Santa Cmz C., CA. The plants were tpped n 16 September, and the field hatvested n 28 Octber. The entire field was divided int 6 blcks f 8 rw buffers n each end. The rws were ca. 13m lng. Treatments cnsisted f t 5 applicatins f insecticides applied with a tractr driven grund rig and 9 rw wide bm. The 8 rw buffers were sprayed at the grwers' discretin apprximately e\'ery 2 t 3 weeks. Plts receiving 1 t 4 treatments were sprayed with a mix f xydemetn-methyl at a rate f.92 kg (A.l.)/hectare and mevinphs at a rate f.18 kg (A.I.)/hectare n each date. The plt receiving 5 treatments was side dressed with disulftn at a rate f.23 kg (AJ.)/hectare fr the first treatment and spl'6yed with the mixture f xydemetn-methyl and mevinphs fl' the subsequent fur treatments. Plants receiving five applicatins were first treated n l5 July. Additinal sprays were applied n 7 August. 29 August, 2 September, and 1 Octber. Plants receiving fewer treabnents were first sprayed n thse dates, but successively later during the seasn. BreuicT)rne brassicae ppulatins were mnitred weekly in untreated
ZALOM and PICKEL: Cabbage Aphid Cntrl in Brussels Spruts 163 plts using the binmial sampling plan develped by \Vilsn et al. (1983). In this methd, each plant is examined fr presence f aphids, but ttal number f aphids are nt cunted. It is pssible t estimate ppulatin density because f the crrelatin between prprtin f plants infested and mean number f aphids per plant. At harvest, 2 plants were remved at randm frm the middle rws f each treatment. The leaves were remved frm each plant and the remaining stalk f spruts weighed. Twenty spruts were remved at randm frm the full length f each stalk, cut pen, and examined fr the presence f B. brassicae t determine prprtin infestatin and within-plant distributin f aphids. RESULTS Aphid infestatin A significant relatinship (P <.1) was bserved between the mean prprtin f cabbage aphid infested spruts at harvest and the number f insecticide applicatins in bth 1984 (Y = 75.865-1 168,) and 1985 (Y = 46.739-1 115l ) (Fig. 1). Breuicryne brassicae infestatins n plants receiving 3 r mre applicatins in L984 were nt significantly different (P >.5; n = 4) frm ne anther while thse receiving, I, r 2 sprays were significantly different (P <.5; n = 4) frm thse receiving mre than 2 sprays and frm ne anther. The same. trend ccured in 1985 n the cmmercial plts. In bth years, this crrespnds t the perid after tpping in terms f sprut frmatin (Fig. 2). In 1984, plants were artificially infested t insure unifrm infestatin befre initial treatment. The prprtin f aphid infested plants remained abve.5 in the untreated plants the remainder f the seasn. In 1985, the prprtin f B. brassicae infested plants in untreated plts remained abve.5 fr mst f the seasn (Fig. 3). Brussels spruts weight There was a significant difference (F = 2.2; P <.5; OF = 9) in the '...eight f sprut plants treated with different numbers f insecticide applicatins in 1984 (Table 1). In general, plants receiving mre sprays tended t weigh less that plants with fewer sprays. Plants sprayed 4 t 9 times weighed n average 18% less than plants sprayed t 3 times. N relatinship between plant weight and number f insecticide applicatins was bserved in 1985. Within-plant distributin A greater prprtin f B. brassicae infested spruts tended t be fund meal' the apex f the plant than the base at hblvest (Fig. 4). Analysis f variance, hwever, shwed n significant differences (P >.5) by sprut lcatin. It is ur bselvatin that spruts nearer the apex tend t grw larger than thse lwer n the plant, and that leaf infestatins appear t be greater n the mre apical leaves thrughut the seasn.
164... '" ::> 1..9 C en.8..,..., - CIl.7 '" CIl.6 \.E: '\. \ c:.5 :;:;.4 \ C-.3 a..2 en.1 1 1>< J. Agric. Entml. Vl. 5, N. 3 (1988) K-. \ -...:. 2 3 BRUSSELS SPROUTS INFESTED WITH Brevicryne brassicae 1984 -._._. 1985 -. -...-._ 1.._...J._._. 4 5 6 7 8 9 Number f Applicatins Fig. L Mean (± SD) prprtin f cabbage aphid infested spruts at harvest in 1984 and 1985 fr plants receiving different numbers f insecticide applicatins.... c:..!!! a. CIl C... '" ::> C en z en 1 1>< 1 9 8 7 6 5 4 3 2 1 BRUSSELS SPROUTS H 1984 1\ HARVEST 1985 TOP f HARVEST 1984 JUNE I JULY AUGUST SEPTEMBER! OCTOBER I JUNE r JULY AUGUST SEPT.! OCT. I 1985 1 2 3 4 5 6 7 8 9 1111213 Degree-Days> 1 C Frm Transplanting Fig. 2. Develpment f brussels spruts plants in relatin t degree-days greater than loc (Baskerville and Emin 1969) nnd calendar time in 1984 and 1985.
ZALOM and PICKEL: Cabbage Aphid Cntrl in Brussels Spruts 165 1. Brussels Spruts Infested with.9 Brevicryne brassicae 1985 rn -c.8 <ll a...7 " Qḻ.6 rn Ql... c.5 c.4 - c..3 a...2.1. July August September Octber Fig. 3. Prprtin f Breuicryne brassicae infested plants in untreated plts during 1985. Table 1. Weight (grams) f brussels spruts plants treated with different numbers f insecticide sprays in 1984. # f Sprays X ± SD 21. ± 429.3 AB' 1 199.6 ± 52.4 ABC 2 1959.5 ± 23.8 ABC 3 2186.3 ± 52.2 A 4 1614.8 ± 416.4 BC 5 1873.3 ± 221.2 ABC 6 1592.1 ± 341.8 C 7 1891.4 ± 653.5 ABC 8 1651.1 ± 541.9 Be 9 1596.6 ± 65.8 C Means fllwed by the same let.ter are nt significantly diuerenl (P -.5; Duncan's 119511 multiple range test).
166 J. Agric. Entml. Vl. 5, N.3 (l9bs) -'" (jj c: :; -. TOP I 2 3 4 5 6 7 B 9 Cf) 1 II c 12 (; 13 u 14 --' '" > 15 '" /7 <r 16 IB 19 2,,,, BOTTOM.2.4.6 O.B 1. x: ± SD Prprtin f Brevicryne brssice Infested Spruls Fig. 4. Prprtin f Breuicryne brassicae infested spruts at harvest in relatin t sprut lcatin n the planl DISCUSSION The results f ur study suggest that insecticide applicatins applied after tpping are necessary t reduce B. brassicae infestatins in spruts at harvest, and tht at least ne applicatin prir t tpping may als be necessary t reduce aphid abundance t manageable levels. The imprtance f keeping aphid ppulatins lw after tpping is likely due t sprut frmatin. Individual spruts enlarge as the lateral buds arc frced t develp after tpping. We assume that Pl;OI' t tpping, spruts are t cmpact t permit aphid entry. Once aphids are within the sprut heads, it appears that xydemetn-methyl, and presumably mst insecticides with cntact actin, will nt sufficiently reduce infestatins t meet quality standards. Church and Strickland (1954) wrking in England stated that apprximately 95% f B. brassicae infesting a brussels spruts plant in late summer were n the leaves. Hafez (l961) in the Netherlands cnfirmed that aphids ccur primarily n
ZALOM and PICKEL: Cabbage Aphid Cntrl in Brussels Spruts 167 leaves earlier in the seasn, but nted that in autumn a cnsiderable fractin f the ppulatin were fund n the spruts. This is cnsistent with ur bservatin that when nt excluded by treatment in the perid f sprut head maturatin, B. brassicae d clnize the spruts. In 1984, a reductin in plant weight appears t crrespnd t a greater number f xydemetn-methyl applicatins. Sances et al. (1981) has shwn similar weight reductins in ther crps fllwing repeated insecticide treatment. They speculate that excessive insecticide use disrupts nrmal physilgical prcesses f the plant The phenmenn was nt bserved in 1985. Hwever, the maximum number f insecticide applicatins were fewer with n plant receiving mre than 4 xydernetnmethyl treatments. CONCLUSION It is necesary t initiate cntrl actins if B. brassicae are present when the spruts becme hstable after tpping r when sprut heads begin t develp and mature. It may be necessary t initiate cntrl actins prir t tpping t reduce aphid infestatins t manageable levels fr the perid f sprut enlargement. Multiple insecticide sprays applied prir t this time reduce the abundance f aphids, but d nt by themselves appear t significantly imprve yield r quality. ACKNOWLEDGMENT This study WllS funded in part by the Califrnia State Parks Department, the Califrnia General Services Administratin, the Assciatin f Mnterey Bay Area Gvernments, and the Santu Cruz Cunty Farm Bureau. REFERENCES C1TED Baskcf\lille, G. L., and P. Emin. JDG9. Rapid estimntin f heat accumulatin frm maximum and minimum temperatures. Eclgy. 5: 514-517. Church, B.M., llnd A. H. Strickland. 1954. Sampling cabbage aphid ppulal:ins n brussels spruls. Plant Path. 3: 76 8. Duncan, D. B. 1951. A significance test fr differences between ranked treatments in an analysis f variance. Va. J. Sci. 2: 171-189. Hafez, M. 1961. Seasnal fluctuatins f ppulatin density f the cabbage aphid, BreuicryJle brssicac (L.), in the Netherlands and lhe rle f its parasite, Aphidius (Diaeretiella) rapa (Curtis). T. PlantenziekL 67: 445-547. Pickel, C., and R. Bigelw. 1983. [nregrated pest management prject fr brussels spruts and strawberries. Assc. Mnterey Bay Area Gvernments Rept. 13 pp. Pickel, C., R. C. Munt. F. G. 2alm, and L. T. Wilsn. 1983. Mnitring aphids n brussels spruts. Calif. Agric. 37 (5&6): 24-25. Sances, F. V., N. C. Tscan, M. W. Jhnsn, and L. F. La Pre. 1981. Pesticides may reduce lettuce yield. Calif. Agric. 35 (11&12): 4-5. Trumble, J. T., H. Nnkakihara, and W. Carsn. 1982. Mnitring aphid infestatins n brccli. Calif. Agric. 36 (11&12): 15 16. Wilsn, L. '1'., C. Pickel, R. C. Munt, and F. G. Zalm. 1983. Presence-absence sequential sampling fr cabbage aphid and green peach aphid (Hmptera: Aphididae) n brussels spruts. J. Ecn. Entml. 76(3): 476-479.