Maataloustieteellinen Aikakauskirja Vol. 62: 349—355, 1990 Conventional and organic cropping systems at Suitia VI: Insect populations in barley JUHA HELENIUS Department of Agricultural and Forest Zoology, University of Helsinki, SF-00710 Helsinki, Finland Abslract. In 1988, insects were sampled from tillers and by pitfall trapping in a long-term field experiment consisting of plots of organic and conventional barley. The organic crops suffered from lack of nitrogen and from draught stress, and growth and development were retarded. The stand characteristics were suggested to be the major determinants of the four- fold higher densities of Rhopalosiphumpadi(L.) (Horn., Aphididae) and the 50 % lower den- sities of Frankliniella tenuicornis Uzel (Thys., Thripidae) in the conventional than in the or- ganic barley. The 75 % lower catch rate of Bembidion spp. (Col., Carabidae) in the conven- tional barley was attributed to lower activity resulting from the more closed canopy and more dense crop. However, the effect of an aphicide applied to the conventional crop was confounded with the true habitat effects. The densities of Coccinella seplempunclata L. (Col., Coccinel- lidae) were almost three times higher in the conventional than in the organic barley, which in turn was attributed to the higher prey (aphid) densities. Index words: Hordeum vulgare, Rhopalosiphum padi, Coccinellidae, Carabidae, Staphylinidae, Araneae, Frank- liniella tenuicornis Introduction One of the major problems encountered in developing organic agriculture is how to avoid excessive pest damage to the crop plants, as pesticides are not allowed. The effects of crop- ping systems on insect populations have been dealt with in several experimental studies (e.g. Dritschilo & Wanner 1980, Wichtrup et al. 1985, Hokkanen & Holopainen 1986, Let- schert 1986, Burn 1987, El-Titi & Landes 1988; see also Potts & Vickerman 1974), many of which have concentrated on epigeal faunas, especially Carabidae (Col.). In this study, the focus was on populations of the main cereal pest, Rhopalosiphum padi (L.) (e.g. Rautapää 1976) and its predators in barley (Hordeum vulgare (L.)) in conventional and organic experimental systems. The object was to describe rather than explain the possi- ble differences in aphid infestation and num- bers of their epigeal polyphagous predators 349 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=Mq7ueCnGNmPjJkeC.VhH76sMflCdplTj9njCc1g.8HIVZCinealB7FoL19T4WtkiX-C0-lr9O23LUIFNwGdfsHUu8gBZ9IhUwViXN_vX0PcfcGDj48sll34XRlmyCyOw-p6q2YNjT6hcpSk5dmawIOo28bybYf_ncSCpzUhagNFo2K5mFYDTvjr9hDcyp-X_juYtug-Gb2qU (e.g. Helenius 1990) between the systems. Sampling was confined to just one year, 1988, of the experiment, started already in 1981 (see Hannukkala et al. 1990, in this issue). Material and methods Details of the experimental design are given by Hannukkala et al. (1990), in this issue. The entomological sampling was done in the rotation a in 1988, in which barley was grown in all the plots. The cultivar was Pokko Hja. The sampling was confined to a 2 mx6 m area in the centre of each of the experimental plots. The treatments (cropping systems) nested with the cropping pattern (organic or conventional) are given in Table 1. Spraying with dimethoate, at a rate of 268 g/ha, was made in the conventionally grown plots on 3rd June, when the crop was at the two-leaf stage. There were no controls for the effects of spraying, which thus became confounded by other effects of the cropping pattern. The populations of R. padi were monitored, from colonization to collapse, by sampling tillers of barley between Ist June and 29th June. Incidence counts (e.g. Ekbom 1987) were made on Ist June and 7th or Bth June, and score estimation (see Lowe 1984) was used on 15th June, 22nd Juneand 29th June. The sample size was 20 tillers per plot, except on 29 June, when the collapse of the popula- tions could be confirmed visually. The total number of aphids was counted in situ. At the peak density, on 22nd June, the sampled tillers were pulled up gently, and the aphids located on the stem base (c.f. Wiktelius 1987) below the soil surface were included. In order to monitor the activity abundance (c.f. Halsall & Wratten 1988) of epigeal predators, three pitfall traps, with a diameter of 120 mm, were placed in a row, at 2 m in- tervals in each plot. The medium was water with detergent added. The traps were emptied once a week from 25th May to 29th June. The numbers of carabids (Col., Carabidae), staphylinids (Col., Staphylinidae), spiders (Araneae), adults of Coccinella septempunc- tata L. and coccinellid larvae (Col., Coccinel- lidae), and the total numbers of aphids in the catch were counted. On 29th June two quadrates of 0.092 m 2 per plot were sampled for absolute densities of coccinellid adults, larvae and pupae. On 11th July, 20 panicles per plot were sampled to determine the infestation level of Frank- liniella tenuicornis (Uzel) (Thysanoptera, Thripidae). The adults and larvae of the thrips were flotated from the panicles by submerging the samples in water (ca. 20°C) containing de- tergent for one day, after which the dead animals were decanted and counted. The dry matter weight of the panicles was determined. Standard analysis of variance procedures were used for testing statistical significance of treatment effects. The model was described by Hannukkala et al. (1990). The notation S.E. Table 1. Treatments representing the different systems in the conventional and organic cropping. The treatments were nested within the cropping patterns. There were three blocks. For details, see Hannukkala et al. (1990), Crop in 1988 Preceding crop in 1987 Conventional: 1. Monoculture barley barley 2. Cereal rotation » » 3. Diverse field crop rotation » turnip rape 4. Ley rotation » oats Organic: 5. Composted green manure barley + clover undersown oats + faba bean mixed crop 6. Green manure » » 7. Composted slurry barley » 8. Slurry » » 350 is used for standard errors of means, and these are given as mean ±S.E. Results and discussion R. padi in barley The conventional barley was colonized quicker by R. padi, i.e. the proportion of in- fested tillers grew faster, and the populations reached peak densities four times higher in this as compared to the organic barley (Table 2). The reasons for the obvious failure of the aphicide in controlling the aphids are not known. There were no significant differences in aphid densities between the systems in con- ventional cropping, or between the systems in organic cropping. The mild infestation in organic cropping is evidently related to the fact that the growth of barley was very poor. The above-ground biomass of barley at about the time (4th July) of peak aphid density was only 18.9 % of that in the conventional farming (Korva & Varis 1990, in this issue). The canopies were not closed, and the growth stage was 31 (Ist node visible; Tottman et al. 1979), while the cano- pies of the conventionally grown barley crops were closed, and the growth stage was 53 (one- fourth of the ear emerged). Nitrogen deficiency, which was especially pronounced early in the season during the aphid period and led to draught stress due to insufficient root growth, was obviously one of the major reasons for the poor performance of the organic barley (Korva & Varis 1990). The nitrogen content and metabolism in host plants plays an important role in the growth and development of herbivorous insects and their populations (Mattson 1980), and it is realistic to assume that this nitrogen/host plant/herbivore relationship also contributed to the results presented here. By the end of June, practically all aphids had disappeared from the crops; small colo- nies were encountered occasionally, most of which were in the organic barley. The rapid development and degeneration of the aphid populations was caused by exceptionally hot weather conditions combined with exception- ally high rates of colonization by alate migrants. The numbers of aphids in pitfall traps were high (Table 3), confirming the finding of Sopp et al. (1987) and Wiktelius (1989) that cereal aphids are active on the soil surface. The 27-fold greater catch in the conventional as opposed to the organic farming during the collapse period is best explained by the larger peak populations on stems combined with the Table 2. Incidence (proportion of infested tillers) and mean density (numbers/tiller) of Rhopalosiphumpadi and mean density of Frankliniella tenuicornis (numbers/panicle) in organic and conventional barley. (±S.E.) Table 3. Mean catch of Rhopalosiphum padi in pitfall traps (numbers/trap/day) in organic and conventional barley. (±S.E.) Date Organic Conventional p R. padi incidence 1/6 0.1+0.03 0.4 ± 0.03 0.04 7—B/6 0.210.05 0.4 1 0.05 0.2 22/6 1 1 mean 22/6 19.6 + 2.38 88.42115.98 0.03 F. tenuicornis mean 11/7 28.712.23 16.451 1.26 o^o2 Trapping period Organic Conventional p 15—22 June 15.0± 1.89 269.3 ± 19.06