Maataloustieteellinen A ikakauskirja Vol. 63: 411—414, 1991 Effect of intercropping carrots and onions on damage caused by the carrot fly, Psila rosae (F.) (Dipt., Psilidae) ANNA-LIISA VARIS Department of Agricultural and Forest Zoology, University of Helsinki, SF-00710 Helsinki, Finland Abstract. In small-scale field experiments carried out in southern Finland in seven succes- sive years, rows of carrots were grown a) between carrot rows, b) between a carrot and an onion row, and c) between onion rows. The damage caused by the carrot fly, Psila rosae (F.) was slightly less in carrot rows adjacent to onion rows than in those adjacent to carrot rows. The infestation by the carrot fly was generally very low. Index words: intercropping, Psila rosae, carrots, onions Introduction It is often claimed, especially in papers giv- ing advice for horticulturists, that mixed crop- ping of carrots and onions can benefit carrots by reducing damage by the carrot fly (e.g. Philbrick and Gregg 1973). Heidema (1923) reported observations made by carrot grow- ers that carrot flies can be controlled by grow- ing onions among carrots. In the studies of Whitcomb (1938) and van Poeteren (1939), some protection against the carrot fly was to be seen in experiments in which onions were grown between carrot rows, while Pether- bridge et al. (1942) found the damage in plots with alternate rows of onions and car- rots very similar to that in plots with carrots alone. Uvah and Coaker (1984) obtained a distinct reduction in the damage caused by the carrot fly by different row-intercropping ar- rangements of carrots and onions. In recent times the increased efforts to use non-chemical methods in pest control have led to re-investigation of old practices. This pa- per describes the effect of row-intercropping carrots and onions on attacks by the carrot fly, Psila rosae (F.), in south Finland. Materials and methods Field experiments were carried out in 1976 —1982 at the experimental farm of the University of Helsinki, and in 1981 also at the experimental farm of Hankkija Plant Breed- ing Station in Tuusula near Helsinki (experi- 411 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND ment 1981b). Carrots (cv. Feonia Fina Hun- derup) and set onions (cv. Liva or Koma Stutt- garter Riesen) were grown in randomized blocks with five replicates. Each plot con- tained one test row 2 m long at a distance of 0.35 m from the adjacent rows. The onions were planted 10 cm apart, and the carrots thinned to 4 cm apart in the row. The test rows of carrots were situated a) between carrot rows, b) between a carrot and an onion row, and c) between onion rows. Guard rows of carrots were sown at each end of the experi- ment. The carrots were sown and the onions planted always on the same day in May, how- ever, the date was not necessarily the same in different years. The damage caused by the carrot fly was assessed immediately after the harvest in the beginning of September, by grading the roots according to the scale o—s (fully healthy totally destroyed). G2-test with log-linear transformation was used for percentage data and the analysis of variance was used for mean damage. The analysis were also made with the pooled values of treatments b) and c). Results and discussion The carrot fly populations were very sparse except in the experiment 1981a. In the other experiments, the mean percentages of damaged plants in the treatments varied from 0 to 11.5, the annual variation being significant (G2 = 430.8, df = 7, PcO.Ol). In twelve cases out of sixteen there were fewer damaged plants in rows adjacent to onion than in those growing between carrot rows (Table 1) and the differ- ence between these treatments was significant (G2 = 5.19, df = 1, P<0.05) whereas the difference between single treatments without pooling (G2 = 5.27, df = 2, P<0.10>0.05) was only indicative. The mean degree of damage was very low (Table 1) varying between 0 and 0.13 (scale o—s). However, it showed the same trend; there was only one case out of sixteen in which the degree of damage was higher in carrots ad- jacent to onions than in those growing be- tween carrot rows. There were no visual differences in the vigourness of the carrot and onion stands be- tween different treatments and competition caused by row-intercropping was thus not ap- parent. In the experiments of Uvah and Coaker (1984), the frequency of damaged roots was considerably higher, and the effect of intercropping was also better. In the only case of the present study in which the dam- Table 1. Damage caused by the carrot fly, Psila rosae (F.), in carrots grown between a) carrot rows, b) carrot and onion rows, c) onion rows. Year Number of carrots Percentage Mean damage examined of damaged carrots (scale o—s)0—5) b+c b+c abc abc2 abc2 1976 358 280 296 5.7 2.0 2.2 2.1 0.06 0.03 0.02 0.02 1977 284 280 262 2.2 1.6 1.9 1.8 0.02 0.02 0.02 0.02 1978 311 244 229 0.3 0 0.4 0.2 0 0 0 0 1979 190 190 169 7.0 4.0 5.9 5.0 0.07 0.05 0.07 0.06 1980 156 114 178 0.8 2.5 0 1.3 0.03 0.02 0 0.01 1981 a 255 246 297 23.5 24.5 23.7 24.1 0.29 0.28 0.32 0.30 1981 b 229 243 257 6.8 4.9 4.9 4.9 0.08 0.05 0.05 0.05 1982 179 185 178 11.5 5.3 7.4 6.4 0.13 0.07 0.12 0.10 Mean 7.2 5.6 5.8 5.7 0.08 0.06 0.08 0.07 F F Years 16.18*** 14,38*** Treatments 1.57 1.68 Years x treatments 0.42 0.28 412 age caused by the carrot fly was considerable, there was no reduction in damage in the car- rots adjacent to onions. As the reduction of infestation by the car- rot fly may depend on other features of the crop, such as plant density, age of the onions, and the pattern of mixed cropping (Uvah and Coaker 1984), the method could proba- bly be improved to some extent. However, even at its best, this method does not seem to be feasible for more extensive plantings. In small domestic gardens, it could be of some help, and could perhaps be integrated with other methods of control, but even there pro- tection of carrots against the carrot fly may be more easily achieved by covering the plots with sheets (see e.g. Haseli and Konrad 1988), which would also give protection against other carrot pests. Acknowledgement. The author is grateful to Mrs Silja Makela for technical assistance and to Dr Juha Helenius for statistical assistance. References Haseli, A. & Konrad, P. 1988. Schadlingsbefall- Regulierung mit Netzen an Gemiise. Pfl.schutzber. 49: I—B. Heidema, J. 1923. Voorkoming van de schade der made van de wortelvlieg aan gele en roode penen. Tijdschr. Pl.ziekten 29: 81. Petherbridoe, F.R., Wright, D.W. & Davies, P.G. 1942. Investigations on the biology and control of the carrot fly (Psila rosae F.) Ann. Appl. Biol. 29: 380 —392. Philbrick, H. & Greco, R.B. 1973. Companion plants & how to use them. Devin-Adair Company, Old Greenwich, Connecticut. van Poeteren, N. 1939. Verslag over werkzaamheden van den Plantenziektenkundigen Dienst in het jaar 1938. Versl. PI. Ziekt. Dienst Wageningen no. 93. 92 p. Uvah, 1. 1.1. & Coaker, T.H. 1984. Effect of mixed crop- ping on some insect pests of carrots and onions. Ent. Exp. & Appl. 36: 159—167. Whitcomb, W.D. 1938. The carrot rust fly. Mass. Agric. Exp. Stat. Bull. no. 352. 36 p. Ms received March 22, 1991 SELOSTUS Porkkanan ja sipulin sekaviljelyn vaikutus porkkanakarpasen aiheuttamaan vioitukseen Anna-Liisa Varis Helsingin yliopislo. Maalalous- ja metstieloinlieteen laitos, 00710 Helsinki Porkkanan ja sipulin sekaviljelyn on usein varsinkin neuvonnallisissa kirjoituksissa sanottu vahentavan pork- kanakarpasen vioitusta. Asiaa selvittavia tutkimuksia on vahan ja niissa saadut tulokset vaihtelevat. Joidenkin mu- kaan mitaan tehoa ei ole havaittu, toisissa vioitusta on saatu vahenemaan. Seitsemana perattaisena vuotena Viikissa jayhtena vuo- tena Tuusulassa jarjestetyissakenttakokeissa tutkittavat porkkanarivit sijoitettiin a) porkkanarivien valiin, b) porkkana- ja sipulirivin valiin ja c) sipulirivien valiin. Porkkanat kylvettiin jasipulit istutettiin vuosittain samana paivana, ja porkkanakarpasen vioitus arvioitiin korjuun 413 yhteydessa. Vioitus oli yleensa hyvin lievaa. Porkkana- karpasen aiheuttama vioitus oli hieman vahaisempaa si- puliin rajoittuvissa riveissa kuin porkkanarivien valisis- sa riveissa. Eri tutkimuksissa saatujen tulosten erilaisuus voi aina- kin osittain johtua erilaisista kasvustotekijoista ja koe- jarjestelyista, kuten kasvuston tiheydesta, sipulien kehi- tysasteesta, kummankin kasvilajin suhteellisesta osuudesta seka kasvilajien keskinaisesta sijoituksesta kentalle. Nain ollen on ilmeista, etta menetelmaa voitaisiin jossain maa- rin kehittaa edelleen. Parhaimmillaankaan siita ei kuiten- kaan olisi suurten porkkanaviljelysten menetelmaksi. Ko- tipuutarhoissa sekaviljely voisi olla avuksi, ja sita voitai- siin yhdistella muihin torjuntamenetelmiin, mutta pienilta alueilta porkkanakarpanen lienee helpommin torjuttavissa kayttamallakateharsoa, joka samalla suojaa kasveja myos muilta tuholaisilta. 414