Maataloustieteellinen Aikakauskirja Vol. 62: 21—31, 1990 Plant size, nutrient composition and biomass productivity of oats and faba bean in intercropping, and the effect of controlling Rhopalosiphum padi (Horn., Aphididae) on these properties JUHA HELENIUS Department of Agricultural and Forest Zoology, University of Helsinki, SF-00710 Helsinki, Finland Abstract. Effects of mixed intercropping on plant size, content of mineral nutrients and biomass yields were examined in three field experiments in Southern Finland in 1983—1985. The stand types were monocrops and replacement series of mixtures with 2/3 and 1/3 or 1/3 and 2/3 of oats (Avena saliva) and faba bean (Vida faba), respectively. In one of the experi- ments control of R. padi, by means of deltamethrin sprayings, was an additional experimental factor having two levels. The height of stems or the above ground biomass of oats either were not affected or were increased by crop diversification. Bean plants remained smaller in the mixtures than in the monocrop. In plant size, there was a significant interaction between stand type and the effect of aphicide spraying: Oat benefitted most from being grown in the mixture containing most bean, and there was an indication (not statistically significant) that in these mixtures bean had proportionately higher weight loss. This result was interpreted as giving some support to the hypothesis of interspecific compensation between oats and bean against aphid damage to oats. In oats, the content of N, P, K, Ca, and Mg all decreased from the stage of inflorescence emergence to the stage of the onset of milk development. Mixed cropping increased the con- tent in oats of all these nutrients except Ca. At the same time, contents of P and K in bean were decreased. The changes in growth form and composition in oats induced by intercrop- ping are discussed from the point of view of host plant relationship and damage function of the aphid pest. In terms of relative yield total (RYT), there was no overyielding in the dry matter, and in one case only was there overyielding in the nitrogen. During the period of populationgrowth of R. padi, the daily maximum temperatures within the canopy were higher in the mixtures than in the monocrop of oats. Index words: Avenasaliva, Vidafaba, mixed intercropping, plant size. N, P, K, Ca, Mg, biomass, insect pest manage- ment 21 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=HBVkrWiS3pWRVrSu.6YuJt8HTD9ZIwjVTyeg6fA._QA5z-W1SFRhylGrkBH9dcF-qIa4gRqHlvKVcd5t8iruxZQgkJqtrTJNU_RLK34g3pZD_9kJ9C-c3nyswuiOD_g9zkFTxzfxdOr9qr-0Cwn0hkh1b9Ba-fw5tNOxUKazV6u9lh9TWNBi7xee0HXB9gHndvEtbKVcSCKo Introduction Intercropping has gained interest in research primarily because of the two potential advan- tages it has to offer: overyielding, i.e. im- proved utilization of growth resources by the crop, and improved reliability from season to season. Intercropping cereals with legumes is a common practice all over the world. In Fin- land, the new cultivars of faba bean (Vida fabaLinnaeus) dobetter when grown togeth- er with oats (A vena salivaLinnaeus) as mixed intercrops than when grown as monocrops (Hovinen 1982, 1984). In mixtures with faba bean, oats suffers higher infestation by cereal aphids, especial- ly Rhopalosiphum padi (Linnaeus) (Horn., Aphididae) (Helenius 1989). This phenome- non made it possible to study mixtures of oats and faba bean from thepoint of view of pest management: Why did diversification of the crop lead to higher (and not lower, see Risen et al. 1983) pest infestation in oats? The ef- fects of mixed cropping on the colonization process of the aphid and on natural enemies have been reported earlier (Helenius 1989, 1990 a, b). This paper describes the changes in the growth form and nutrient content of the com- ponent crops in the mixtures as compared to the monocrops; these changes induced by crop diversificationmay affect the host plant rela- tionships and performance of the pest insects and may even alter the damage function. The temperatures within canopies in monocrops and mixtures were compared as a preliminary attempt to judge the extent to which an abiotic factor may contribute to the increased aphid numbers in the mixtures. Also, the hypothe- sis of interspecific yield compensation against pest damage (Trenbath 1976, Perrin 1977) was considered in this study. The results con- cerning grain yields and their formation, and the results concerning pest incidence in these systems, have been presented earlier (Hele- nius and Ronni 1989). The results for oats in Experiment II of this study have been presented by Ronni (1987). Material and methods Experimental designs Monocrops of oats cv. Nasta (Experiment I) or Puhti (Experiments II and III) and field beans cv. Mikko were compared with mixed intercrops of these two plants in three field experiments, carried out during 1983—1985 in Helsinki. The cv. Puhti ripens normally in 103 days and grows 98 cm high, cv. Nasta re- quires 4 days less and is on an average 9 cm shorter at maturity, and cv. Mikko requires 108 days and the mean height is 75 cm with a vide variation (Hovinen 1982). The experi- ments consisted of four differentcrop types: (1) Monocrop of oats (notation OOO) with a normal seed density of 500 germinating seeds per m 2. (2) A mixture of 2/3 oats and 1/3 beans (OOB) following the replacement principle (de Wit 1960, see also Willey 1979): the mono- culture sowing density of field beans was 100 germinating seeds per m 2, and the 008 mix- ture was established by drilling 2/3 X 500 ger- minating seeds of oats and 1/3xloo ger- minating seeds of beans per m 2. (3) A mixture of 1/3 oats and 2/3 beans (OBB). The 088 mixture was established by following the same replacement principle used for the 008 mixture. (4) Monocrop of beans (BBB) with a nor- mal seed density of 100 germinating seeds per m 2. The mixtures were established by drilling the seed mixture. The rates of nitrogen fer- tilization were 80 kg/ha for 000, 40 kg/ha for 008 and 088, and 0 kg/ha for 888. The routine procedure for weed control was employed, applying dinoseb (Experiment I) or bentatzone (Experiments II and III) at the recommended rates. In Experiment 111 control of R. padi, by means of sprayings, was an additional ex- perimental factor having two levels. All the eight treatment combinations, i.e. four stand types times two spraying levels (unsprayed or sprayed), were randomized within each repli- 22 cate block. During the sprayings 6.25 g/ha deltamethrinwas applied, for the first time on 20 June at oats G.S. (growth stage) 21 and for the second time on 3—4 July at oats G.S. 30, the same time as the first bean flowers opened. The experimental design used in Experiment I (1983) was similar to that of Experiment II (1984), but the 008 mixture was absent and there were only two replications (blocks). In the other cases there were three replications. The experiments have been explained in de- tail in the previous papers in which they were referred to as follows: Experiment I (1983) II (1984) 111 (1985) no. 6 no. 7 no. 5Helenius 1989 Exp. I Exp. IIHelenius & Ronni 1989 Sampling and analyses Sampling from the plots was made at G.S. 70, at the onset of the milk development stage of oats (Tottman and Broad 1987) and pod development of bean (Stulpnagel 1984), on 26, 23 and 29 July in Experiments I, II and 111, respectively. In Experiment I the sample consisted of four, and in Experiment II of two randomly chosen 0.25 m 2 circles per plot. Within the circles, all oats and beans were re- moved by cutting at the soil surface level. In Experiment 111, no area based sampling was done, but eight oat and bean plants were ran- domly sampled within the inner rows of each plot. The height and the dry weight (above ground biomass) of stems were measured from the samples. The height was measured by stretching the shoot to the tip of the up- permost leaf or flower, whichever reached fur- thest. The data for dry weight in oats became inconsistent, as in Experiment I all stems, in- cluding adventitious stems, had been weighed, while in Experiment II whole plants, main stems and tillers combined, had been weighed. Only in Experiment 111 were the main stems weighed separately from the adventitious stems. In addition to the sampling at the ‘harvesta- ble stage’ G.S. 70, in Experiment II sampling for nutrient content of oats was made on 25 June at G.S. 37 (flag leaf just visible) and for height of main stem and nutrient content of oats at G.S. 55 (inflorescence emergence) and height of bean at G.S. 65 (full-blossom) on 9 July. In Experiment 111 sampling for height of main stem was made on 2—3 July at G.S. 31 of oats (first node detectable) and at G.S. 65 of bean, and for height, weight and nitro- gen content of main stems of oats on 18 July at G.S. 55. The sampling methods were the same as at G.S. 70. For oats, total nitrogen content was ana- lysed in all the experiments, phosphorus and potassium in experiments I and 11, and calci- um and magnesium in Experiment II only. For bean, no analyses were performed in Experi- ment I, nitrogen was analysed in experiments II and 111, and phosphorus, potassium, calci- um and magnesium in Experiment II only. The nutrients were analysed, following the standard procedures of Association of Offi- cial Analytical Chemists (Williams 1984), by Viljavuuspalvelu Co. (Helsinki). Comparisons of the mixtures with the monocrops were made in terms of relative yield, using the concept of Relative Yield To- tal ( = RYT: de Wit and van den Bergh 1965). In Experiment 111, daily minimum and maximum temperatures at ca. 2 cm above soil surface level within the canopy were measured by minimum-maximum thermometers. One thermometer was placed in the middle of each plot. The measurements were made during population growth of R. padi, and only the insecticide treated plots were used. Standard analysis of variance tests were utilized to test for statistical significance. The following abbreviations are used: S.E. for standard error of mean (with the mean given as ±S.E.), G.S. for growth stage and D.M. for dry matter. 23 Results Plant size In Experiment 111, mixed cropping signifi- cantly increased the above ground biomass of oats. At G.S. 70 the main stems in the monocrop and 008 mixture weighed the same, but were 25.2 % heavier in the 088 mixture. At the same time, the adventitious stems weighed 0.29 ±0.07, 0.38 ±O.lO and 1.02 ±0.22 g/plant in the monocrop, 008 mixture and 088 mixture, respectively (n = 6, /7C0.01). Aphicide spraying had no signifi- cant effect on the weight of the adventitious stems. In the weight of the main stems there was a significant (p< 0.001) interaction between growth stage, stand type and spraying. In the monocrop the biomass growth from G.S. 55 to G.S. 70 was 17.9 % and did not depend on the sprayings. In the 008 mixture the esti- mated growth in the unsprayed plots was negligible, while in the sprayed plots the growth was 24.1 %. In this mixture, the gain in weight by sprayings was not yet detectable at G.S. 55, but at G.S. 70 it averaged 33.3 %. In the 088 mixture the growth was, on an average, 16.6 % in the unsprayed plots but 27.1 % in the sprayed plots, and the increase obtained by the sprayings was 11.8% already at G.S. 55, and 22.0 % at G.S. 70 (Fig. 1). In Experiment 111, there was also a sig- nificant (p< 0.001) three way interaction in shoot height between the growth stage, stand type and spraying with insecticide. In the monocrop, the height increased 11.9 % from G.S. 55 to G.S. 70, and the increase did not depend on spraying. However, in the mixtures the growth in height during this period was, on an average, only 5.0 % if not sprayed but 20.1 % if sprayed with insecticide. This re- sulted in oats shoots 21.8 % taller in the sprayed mixtures as compared to those in the unsprayed mixtures (Fig. 2). In Experiments I and 11, there were no sig- nificant effects of mixed cropping on the size of oats. In Experiment 11, the height of the oats stems increased 39.7 % from G.S. 55 to G.S. 70 (p<0.001). The leaf/stem ratio in oats was not sig- nificantly affected by mixed cropping, but the insecticide treatment in Experiment 111 reduced the ratio by 22 % from 0.23 ±O.Ol to 0.18±0.01 (n = 9, /7C0.01). The stems of bean tended to remain lighter in the mixtures (Fig. 3), significantly so in Fig. I. Above ground biomass of main stems of oats when not sprayed (Control) or sprayed (Aphi- cide) against cereal aphids, at growth stages (G.S.) 55 and 70 in the monocrop, 008 mixture (1/3 faba beans) and 088 mixture (2/3 beans) (Experiment III). Fig. 2. Height of the main stems of oats when not sprayed (Control) or sprayed (Aphicide) against cereal aphids, at growth stages (G.S.) 31, 55 and 70 in the monocrop, 008 mixture and 088 mixture (Experiment III). 24 Experiment 11, where the reduction from the monocrop to the 088 mixture and 008 mixture was 9.4 % and 36.1 °7o, respectively (p<0.01). In Experiment I, the differences were not significant and in Experiment 111, these were statistically only indicative (p=0.06). In the plots treated with insecticide, as com- pared to the untreated plots, the mean weight of stems was 8.2 % higher in the monocrop, but 22.1 % and 34.6 % lower in the 088 mix- ture and 008 mixture, respectively: howev- er, the effect of spraying (p= 0.22) or inter- action spraying x stand type (p = 0.36) were not statistically significant. The reduction in stem weight of bean was accompanied by reduction in height. In Ex- periment 11, the reduction in height was 1.1% and 15.6 % in the 088 mixture and 008 mix- ture, respectively, as compared to monocrop {p < 0.001). In this case the growth in height from G.S. 65 to G.S. 70 was 50.4 % (p< 0.001), equally in all the stand types. In Experiment 111, there was a significant interaction in stem height between growth stage and stand type (/jc0.05); the mean heights at G.S. 65 were 31.6±0.8, 37.8±1.6 and 41.811.6 cm, but the increase from that stage was largest in the monocrop and smallest in the 008 mix- ture, resulting in mean heights of 95.313.7, 89.313.7 and 85.814.6 cm at G.S. 70 in the monocrop, 088 mixture and 008 mixture, respectively (n = 6). Nutrient content The results on the effects of stand type on nutrient content of the plants varied from one experiment to another. In Experiment I, there were no differences between the monocrop and the mixture in the Table 1. Content of mineral nutrients, g/kg D.M., in the above ground tissues of oats, at growth stages 37, 51 and 70 in the monocrop (OOO), 008 miture (1/3 faba beans) and 088 mixture(2/3 beans) (Experiment II). Mean±S.E., n = 3. Nitrogen Phosphorus Potassium Calcium Magnesium G.S. 37 000 30.3±0.8 4.2±0.1 32.210.5 6.1 ±0.2 1.28±0.05 008 29.911.6 4.5±0.2 32.1±0.5 6.1 ±0.4 1.35±0.05 088 34.9±0.6 5.110.2 35.9±1.3 6.5±0.0 1.52±0.04 G.S. 51 000 17.2± 1.7 3.710.2 25.211.4 3.910.5 0.9810.07 008 20.010.6 4.210.2 25.811.6 4.410.2 1.0810.06 088 25.211.4 5.210.2' 31.911.8 5.210.2 1.2710.03 G.S. 70 000 14.010.3 3.310.1 20.410.6 3.210.2 0.9410.05 008 15.810.7 4.010.1 23.210.9 3.710.1 1.0810.02 088 18.610.9 4.510.3 27.011.1 3.910.3 1.0710.02 level of p: G.S. 0.001 0.01 0.001 0.001 0.001 Crop 0.05 0.01 0.05 n.s. 0.05 Interaction n.s. n.s. n.s. n.s. n.s. 25 Fig. 3. Above ground biomass of bean at growth stage 70 in the monocrop, 088 mixture and 008 mix- ture. In Experiment 111, the effect of aphicide spraying is shown. nitrogen, phosphorus or potassium contents in oats. The mean contents at G.S. 70 were 11.0±0.3 N, 2.2±0.1 P and 24.0±0.7 K in g/kg dry matter (n = 12). Only nitrogen was analysed in Experiment 111, and again, there were no differences between stand types: In oats, the mean nitrogen content at G.S. 55 was 15.1 ±0.5 (n=18), and it was not yet in- fluenced by spraying with insecticide, but at G.S. 70 the mean content in the unsprayed plots was 14.4+ 0.6 (n = 9), and significantly lower, 12.510.5 g/kg dry matter (n = 9) in the sprayed plots (interaction between growth stage and spraying: /?<0.05). The mean con- tent in bean at G.S. 70 was 29.5+0.4 in g/kg dry matter (n= 18). In Experiment 11, mixed cropping signifi- cantly increased the nitrogen, phosphorus, potassium and magnesium contents of oats. In all these nutrients there was a steady de- crease from G.S. 37 to G.S. 70. Calcium was not affected by mixed cropping but, like the other nutrients, it also decreased over time (Table 1). The only statistically significant ef- fects on bean were the decreases in phospho- rus and potassium contents in mixed cropping (/7<0.05) (meanlS.E. in g/kg dry matter, n = 3): KP 4.1 ±O.l 26.6 ±0.3 3.9±0.1 25.0 ±0.7 3.5 ±O.l 22.2 ±0.9 bean monocrop 088 mixture 008 mixture The mean contents of the other nutrients in bean were 32.9 + 0.4 N, 7.910.2 Ca, and 1.6010.04 Mg in g/kg dry matter (n =9). Biomass and nitrogen yields At G.S. 70, the highest above ground bio- mass yields were always produced by oat monocrop and the lowest by bean monocrop. The mixtures were intermediate in this respect. Table 2. Above ground biomass yields, D.M. kg/ha, and relative yields of the component crops and the relative total yields (RYT) in the mixtures, at G.S. 70 (Experiments I and II). Mean±S.E., n =6. Oats rel. Bean rel. Total RYT Oat monocrop Exp. I 9 308 + 962 100 Exp. II 8 268 ±638 100 008 mixture Exp. II 56641523 68 1 261 ±309 24 6925±412 92 088 mixture Exp. I 2386±171 26 4 139±430 73 6526 + 436 99 Exp. II 2626±356 31 4094±521 78 6720±783 109 Bean monocrop Exp. I . . 5 6381547 100 Exp. II . . 5 284 ±636 100 Table 3. Absolute and relative nitrogen yields, kg N/ha, in the above ground biomass (c.f. Table 1). Oats rei. Bean rel. Total RYT Oat monocrop Exp. I 102.4 100 Exp. II 115.8 100 008 mixture Exp. II 89.5 77 41.5 24 131.0 101 088 mixture Exp. I 26.2 26 124.2 73 150.4 99 Exp. II 48.8 42 134.7 78 183.5 120 Bean monocrop Exp. I . . 169.1 100 Exp. II . . 173.8 100 Note: Nitrogen yields were calculated from the dry matter yields using the determined nitrogen contents except for bean in Experiment I, where a nitrogen content of 30 g/kg dry matter was assumed. 26 The RYT valuesremained close to unity in all the cases (Table 2). Using the results from the nitrogen deter- minations given above, the nitrogen yields can be calculated (for bean in Experiment I, 30.0 g N/kg dry matter is assumed): Either thebean monocrop or the 088 mixture produced the highest nitrogen yields in the harvestable green biomass. In terms of RYT, because of the relatively high nitrogen yield in oats, only the 088 mixture in Experiment II overyielded, by 20 % (Table 3). Temperature within canopy Over the time of the growth and peak den- sity of the R. padi populations, at G.S. 16—31 from 26 June to 8 July (from the onset of tillering to stem elongation of oats), the means of the daily minimum temperatures did not differ between the oat monocrop and mix- tures, but in the bean monocrop, the mini- mums were O.B°C lower than in the other stand types (p< 0.001). During the same peri- od, the means of the daily maximum temper- atures were I.O°C higher in the 088 mixture than in the oat monocrop or 008 mixture, and still I.3°C higher in the bean monocrop than in the 088 mixture (p< 0.001). Discussion In mixtures with faba bean, oats may grow taller than in the monocrops, whereas bean usually remains shorter; this is due to the dom- inance of oats in the competition (Vullioud 1969, Kortesmaa 1982, Hovinen 1983). In their study on mixtures of barley and faba bean, Martin and Snaydon (1982 b) showed that root competition alone can increase the shoot weight of the cereal component and de- crease that of bean. In the present study, a de- crease in bean weight and height, and in one case an increase in oats weight and height could be shown. Controlling the cereal aphids resulted in a relatively greater increase in the growth of oats in the mixtures than in the monocrop: In this case, the aphid loads at the peak population densities were 18 % and 28 % higher in the unsprayed 008 mixture and 088 mixture than in the monocrop (Helenius 1989, He- lenius and Ronni 1989). The trends in the heights of the stems of bean in the sprayed plots, although not statistically significant, supported the hypothesis of compensatory dynamics in the growth of the component crops in the mixtures. An indication of a simi- lar interaction between stand type and the ef- fect of sprayings on grain yield has been reported earlier, from the same experiment (Helenius and Ronni 1989). More substantial evidence of this kind would be needed to en- courage research on developing risk adverse intercropping systems, especially in order to respond to the need to overcome plant pro- tection problems in low input sustainable agriculture. The case also serves to illustrate the point that control thresholds for pesticide use intended for monocrops do not necessarily apply to intercrops. Mixed cropping had varying effects on the mineral nutrient contents in theabove ground tissues of oats and bean. In two experiments out of the three, there was no change in the nitrogen content of oats. In one case, nitro- gen and all the other minerals that were ana- lysed, except calcium, were increased in oats by mixed cropping. The differences between the monocrop and the mixtures were consis- tent over the three growth stages that were sampled, in spite of the steady decrease in the contents in the course of growth and develop- ment. Along with the increase in oats, phos- phorus and potassium were decreased in bean. Martin and Snaydon (1982 a, b) also found varying effects of intercropping on the nitro- gen, phosphorus and potassium contents of barley and faba bean: Most consistent was the increase in nitrogen in barley, and it seemed as if barley was a stronger competitor for P and K. By using 15N labelling, Patra et al. (1986) showed that up to 28 % of the N uptake by the cereal (maize) can be obtained by trans- fer of biologically fixed N by the legume (cow- pea) in intercropping. The most likely expla- 27 nation for theresults presented here would be a relieved intraspecific competition for nitro- gen in oats and dominance of oat in the in- terspecific root competition for the other nutrients, such as phosphorus and potassium, in the mixtures with faba bean (c.f. Martin and Snaydon 1982 a). There was no evidence for transgressive yielding in harvestable biomass in the mix- tures. However, the relative yields of bean in the 088 mixtures tended to be higher than ex- pected on the basis of the 60 ®7o sowing ratio. Martin and Snaydon (1982 a) reported over- yielding in alternate row mixtures of barley and faba bean, but not in the within row mix- tures. There is some evidence for superiority of alternate row systems also with respect to the intercrops of oats with faba bean (unpub- lished results). Concerning nitrogen yields, on both absolute and relative bases, the best al- ternatives were either the bean monocrops or the 088 mixtures. In one case, the latter over- yielded by 20 %, here, the relatively higher up- take of nitrogen by oats contributed most. Obviously, 088 mixture could provide an al- ternative for making silage as green fodder (Ingalls et al. 1979, Kiviniemi 1982). Changes induced by intercropping in growth form or nutritive quality of a host plant can affect pest numbers. In this study, the focus was on the aphid pest specific to the component crop, namely oats, which was the dominating of the two. As expected, com- petition between the component species had a greater effect on the growth form of the dominated beans. The most marked response in the growth form of oats was the increase in tillernumber, which could be attributed to the reduced plant density rather than to inter- specific interaction (Helenius and Ronni 1989). No changes in leaf/stem ratio could be proved, but a marked increase in shoot weight was shown in Experiment 111, where the methods to estimate this were more ap- propriate than in the other two experiments. The difference between the monocrop and the 088 mixture was evident already at in- florescence emergence, two weeks after the peaking of the aphid populations. Theo- retically, even a small difference between monocrops and mixtures in size of stems during the latter parts of the aphid popula- tion development (exponential growth and peak) could contribute to the difference in numbers per stem of cereal aphids (see Hele- nius 1989). Larger stems could delay the ef- fects of intraspecific competition by provid- ing more space, thus delaying crowding in the aphid colonies, and by tolerating more sap feeding, thus delaying deterioration of the plant as a host to the aphids. Plant mineral content, especially that of nitrogen (e.g. Mattson 1980), often relates to its quality as a host to an insect herbivore. Content of free amino acids in tissues of a cereal plant has been shown to affect behav- iour and reproduction of R. padi (Havliökova 1987, Weibull 1987). There was no consis- tent pattern in the effect of mixed cropping on the nitrogen content of oats, and no con- clusions can be drawn concerning the possi- ble contributionof host plant nutritional qual- ity to the difference in cereal aphid infesta- tion between the monocrops and mixtures. Controlling the aphids decreased the nitrogen content in oats, obviously because the increase in dry matter growth was relatively more than the increase in nitrogen uptake. The measurements of temperatures within the canopies indicated I°C higher daily max- imums in the 088 mixture than in the mono- crop of oats. This difference was measured justabove soil surface level, where before the canopy closure the maximums are higher than at upper layers due to heating of the soil (Cordukes and Robertson 1963). Tempera- ture affects development time, survival and fecundity of cereal aphids (e.g. Dean 1974), but such a small difference as found here be- tween the crop types in maximum tempera- tures only is not likely to be critical to the growth rate of aphid populations. However, it is obvious that microclimatic differences be- tween monocrops and intercrops in some cases may become a major cause for differences in 28 pest infestations (e.g. Kyamanywa and Ampo- fo 1988). An agronomically important emergent property of a multispecies crop community is the possibility of compensatory dynamics in growth of component crops. This is a feature which should be taken into account whenaim- ing to improve reliability in arable field crop- ping. As indicated in this study, the inter- specific dynamics of component crops in in- tercropping could be utilized in crop protec- tion by planning communities where compen- sation against pest damage to one of the com- ponent crops is operative. Crop diversification is likely to lead to changes in component crops, in their growth form and nutritional quality, and to other changes in the crop com- munity that are important to the population dynamics of pest insects, thereby affecting both the numbers and the damage function of the pests. These effects together with the in- terspecific dynamics of the crop species deter- mine the level of resistance against pest dam- age in an intercropping system. Acknowledgements. I would like to thank Päivi Ron- ni, M.Sc. (Agr. & For.), for help in compiling the data, Prof. Anna-Liisa Varis for comments on the manuscript and Sevastiana Ruusamo, M.A., for its linguistic revi- sion. The study was funded by the National Research Council for Agriculture and Forestry, and by the Tiura Foundation. References Cordukes, W.E. & Robertson, G.W. 1963. Note on the temperature distribution within an oat crop. Cana- dian J. Plant Sci. 43: 235—239. Dean, G.J. 1974. 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Koejäsenet olivat kauran ja pavun puhdaskasvustot ja korvaussarjoina järjestetyt sekakasvustot, joissa oli jo- ko 2/3 kauraa + 1/3 papua tai 1/3 kauraa + 2/3 pa- pua puhdaskasvustojen kylvömääristä. Yhdessä kokees- sa (1985) toisena faktoriaalisena koetekijänä oli tuomi- kirvan (Rhopalosiphum padi) torjunta deltametriini- ruiskutuksin. Tässä kokeessa mitattiin myös tuomikirva- populaatioiden kasvun aikaiset päivittäiset lämpötila- minimit ja -maksimit kasvustojen sisältä. Sekaviljely jokoei vaikuttanut kauran versojen kokoon tai lisäsi sitä. Papu jäi seoksissa yleensä lyhyemmäksi ja kevyemmäksi kuin puhdaskasvustoissa. Tuomikirvan torjuntaruiskutuksen jakasvustotyypin välillä oli merkit- sevä yhdysvaikutus, siten että puhtaassa kasvustossa tor- junta lisäsi kauran pääversojen pituutta ja massaa vähi- ten ja papuvaltaisessa seoksessa eniten; keskiarvojen pe- rusteella vaikutus papuun oli juuri päinvastainen, mutta ei tilastollisesti merkitsevä. Tulosten tulkittiin kuitenkin tukevan hypoteesia lajienvälisestä satokompensaatiosta sekakasvustossa, tilanteessa jossa tuholaisvioitus kohdis- tuu komponenttilajeista vain yhteen. Samansuuntainen tulos on aikaisemmin raportoitu saman kokeen siemen- sadoista (Helenius ja Ronni 1989). Yhdessä kokeista kauran maanpäällisten osien typpi-, fosfori-, kalium-, ja magnesium-pitoisuudet (kuiva- aineessa) olivat merkitsevästi suurempia seoksissa kuin puhdaskasvustossa, suurimpia papuvaltaisessa seokses- sa. Kaikkien näiden ravinteiden sekä kalsiumin pitoisuudet laskivat merkitsevästi kauran röyhylletulosta maitotuleen- tumisvaiheen alkuun, mutta kasvustotyyppien välinen ero ei riippunut kehitysasteesta. Samassa kokeessa pavun fosfori- ja kalium-pitoisuudet olivat merkitsevästi pienem- piä seos- kuin puhdaskasvustoissa. Luontevin selitys täl- le tulokselle on se, että vallinneissa kasvuoloissa kaura hyötyi pavun typensidonnasta ja oli papua voimakkaampi juuristokilpailussa fosforista ja kaliumista. 30 Suhteellisten kokonaissatojen vertailu osoitti, että kuiva-aineen tuotannossaseokset eivät olleet puhdaskas- vustoja edullisempia. Kuiva-ainesadot mitattiin kauran maitotuleentumisvaiheen alussa, jolloin papu oli palko- jen täyttymisvaiheessa. Kokeessa, jossa kauran typpipi- toisuus nousi sekaviljelyn ansiosta, typpisato muodostui 20 % suuremmaksi kuin puhdaskasvustojentyppisatojen perusteella olisi voitu odottaa. Puhdas kaura tuotti siis suurimmat kuiva-ainesadot japuhdas papu tai papuval- tainen seos suurimmat typpisadot. Tuomikirvapopulaatioiden kasvun aikaan, kauran ver- soutumisen alkamisesta korren pidentymiseen, päivit- täiset maksimilämpötilat noin 2 cm:n korkeudella maan pinnasta olivat papuvaltaisessa seoksessa keskimäärin I.o°C:tta korkeammat kuin puhtaassa kaurakasvustos- sa. Vastaavaa eroa minimilämpötiloissa ei havaittu. Tulokset osoittivat että sekaviljelyyn siirtyminen voi aiheuttaa kasvien koossa jakoostumuksessa sekä kasvus- ton mikroilmastossa muutoksia, jotka osaltaan selittävät sekaviljelyn vaikutusta tuhohyönteisten runsauteen ja vahingollisuuteen. Sekaviljelyn aiheuttamat muutokset tuholaisten runsaudessa ja vahingollisuudessa, sekä lajien- välisen satokompensaationmahdollisuus edellyttävät tor- junnan kynnysarvojen tarkistamista aina sekaviljelyyn siirryttäessä. 31