STUDIES ON THE BIOLOGY OF DIPTEROUS STEM-BORING LARVAE IN GRASSES BY Oladotun Lateef Idowu B.Sc. (Ibadan) Thesis submitted to the University of London for the degree of Doctor of Philosophy Entomology Department Rothamsted Experimental Station April, 1975. 11 ABSTRACT The infestation of sown grasses, especially Lolium perenne var. S24, by stem-boring dipterous larvae was studied in the field from the time of germination onwards. Grass growing on the farm at Rothamsted Experimental Station, Harpenden, Herts., during 1972 and 1973 was attacked by five species of stem-boring fly. These were, in decreasing order of abundance, Oscinella vastator (Curt.), 0. frit L., Geomyza tripunctata (Fall.), O. frit ex anthoxanthum and O. frit ex arrhenatherum. Infestation built up rapidly in the field after the emergence of the adult flies in spring, larval numbers reaching a peak in late summer and autumn before falling with high winter mortality. G. tripunctata colonised grasses earlier in the season and was numerous during the first year of a sward's life, but was supplanted by Oscinella spp., especially O. vastator, as the sward matured. The factors influencing attack were examined in laboratory and field experiments. Infestation was governed by oviposition and larval survival which were dictated by host plant conditions: parasitism was negligible. Plant species, age and stem density all affected infestation. In laboratory tests with ten grass species or varieties, Poa trivialis, Agrostis tenuis, L. perenne S24 and Reveille, L. multiflorum, Avena fatua, Phleum pretense, Holcus lanatus, Festuca rubra, and Dactylis glomerata, specific or varietal differences in oviposition rates were not necessarily followed by similar differences in ultimate larval infestation. Oviposition on any plant species or variety depended upon the number of sites available, but larval success, and hence the level of infestation achieved, depended upon the length of time during which the tillers remained suitable for larval penetration. This period is determined 111 by the particular growth characteristics of the species involved.. The consequences of this are that under field conditions slow- developing and freely-tillering grasses such as Poa trivialis support higher larval populations than fast-growing less freely tillering species such as Dactylis glomerata. Also the increased rate of young tiller production of Lolium perenne in response to defoliation increased the populations of stem-boring larvae in this grass. The inclusion of a proportion of a less susceptible species, such as D. glomerata, in a perennial ryegrass sward, reduced the number of larvae in the latter, perhaps by functioning as a trap crop for eggs and young larvae. iv CONTENTS Title page .. .. .. .. .. .. .. .. .. .. Abstract .. .. .. .. .. .. .. .. .. • • ii Contents .. • • • . .. .. .. .. .. .. .. iv I. INTRODUCTION (i)Review of Previous Work .. .. .. .. .. 1 (ii)Aim of Study .. .. .. .. .. .. .. .. 5 II. METHODS, (i) Experimental Site .. .. .. .. • • .. .. 9 (ii) Field Sampling Methods .. .. .. .. .. .. 11 (a)Core Sampling .. .. .. .. .. 11 (b)Soil Washing .. .. .. .. .. .. 13 (c)Suction Sampling . .. .. .. .. 13 (d)Emergence Traps .. .. .. .. • • . 14 (iii) Laboratory Culture Methods . • • .. .. .. 15 (iv) Lab./Field Experiments .. .. .. .. .. 16 (a)Potted plants of different ages and densities 17 (b)Potted plants of different ages and varieties 17 (v) Laboratory Behaviour Studies .. • • .. 17 III. RESULTS (i) Population build—up and breeding cycles of flies in the field .. .. .. .. .. .. • . .. 19 (a)Pupation in the field .. .. .. 19 (b)Adult emergence in Spring . • .. 20 (c)Seasonal variation in the population of adultt flies in the field •• •• •• .. 21 (d)Comparative abundance of flies in young and old swards .. •• .. 23 (e)Comparative abundance of flies of different generations in the field .. 24 (f) Sex ratio of flies in the field .. .. 26 (g) Breeding cycles - number of generations of each species per year • • 27 (ii) General Behaviour of adults as influenced by the host plant • • .. .. .. .. .. 28 (a) Landing and resting behaviour .. .. 29 Effect of plant age .. .. .. 29 Effect of plant density .. .. ... • . 30 Effect of plant species .. .. .. 30 (b) Courtship and mating behaviour .. .. 31 (c) Oviposition movements and egg-laying behaviour 34 (d) Performance of oviposition movements .. 35 Effect of plant age .. .. .. .. 35 Effect of plant density .. .. .. • . 36 Effect of plant species .. .. .. .. 36 Fly behaviour - discussion . .. 36 (iii) Oviposition (a) The distribution of oviposition sites . 40 (b) Intensity of oviposition .. .. .. 45 Effect of host plant age .. .. .. 45 n ii, " density .. .. 53 n ir, species .. .. 57 variety .. .. .. 61 ,, I, ., genotype • • ... 62 It ill " growth habit ..• . 64 Effect of mixed-species sowing .. 66 (iv) Larval Establishment (i) Effect of plant age and density . .. .. 67 (ii) Effect of plant age and species • • .. 74 (iii) Effect of mixed-species sowing .. .. .. 77 vi (v) Field Infestation (i) Larval complex .. .. .. .. .. (ii) Estimation of damage .. • • • • .. .. 801 (iii)Seasonal pattern of larval infestation .. 82 (iv) Relative abundance of larvae of different species • • .. 86 (v) Comparative abundance of larvae in young and old swards . .. .. .. • • • • 0 0 87 (vi) Winter mortality of larvae .. .. .. 0 0 89 (vii)Parasitism .. 00 00 00 00 00 0 0 90 (viii)Comparative size of different generations .. 96 (ix) Year-to-year variation in larval numbers .. 97 (vi) Spatial and Temporal distribution of adult and immature stage .. .. 00 00 00 00 00 98 IV. GENERAL DISCUSSION AND CONCLUSIONS (i) The larval and adult complex . .. 104 (iia) Pupation of the overwintering larvae in spring .. 105 (iib) Sward damage.. • • • • •• .. •• 106 (iii) Adult emergence and sward invasion •• 106 (iv)Spatial and temporal distribution and its biological significance • • 107 (v)Seasonal variation in number of adults .. 108 (vi) Breeding cycles and number of generations per year 109 (vii):Adult activity patterns .. .. .. .. .. 110 (viii)Courtship and mating behaviour .. .. .. 111 (ix) Plant factors affecting adult behaviour .. .. 112 .(x) Plant factors affecting oviposition .. .. .. 113 (xi) Larval survival .. .. .. • • • • • • .. 114 (xii) Infestation of single-species swards .. .. .. 117 vii. (xiii) Larval parasitism . .. .. •. •. .. 119 (xiv) Winter mortality of Larvae • • .. .. • • 120 (xv) Infestation of mixed-species swards .. .. 120 (V) SUMMARY .. .. .. .. .. .. .. .. .. .. 122 (VI) ACKNOWLEDGEMENTS .. .. .. .. .. .. .. .. 125 (VII) REFERENCES .. • • • • • • • • • • • . •• •• .. 126 List of Tables . • .. .. .. .. .. .. .. .. .. 136 List of Figures .. .. .. .. .. .. .. .. .. .. 170 Appendix IA .. .. .. .. .. .. .. .. .. .. 194 Appendix IB .. .. .. .. .. .. .. .. .. .. 195 1 I. INTRODUCTION (i) Review of Previous Work Cyclorraphous larvae of some families of Diptera, and'in particular, the families Chloropidae and Opomyzidae are known to infest many species of grasses,living within them and feeding on the stem tissues. In the U.K., there are records of stem-borer damage to grasses as early as 1912 when Edmunds observed damage to the three grass species Lolium perenne, Avena flavescens and Arrhenatherum avenaceaum. Petherbridge (1917) recorded infestation of these grass species by dipterous larvae, and subsequent infestation of cereal crops following leys. In addition to the afore-mentioned grass species, Collin (1918), in his review of previous work on frit flies recorded the infestation of Poa annua, Poa pratense, Bromus sp., Triticum repens, Triticum cristatum, Phleum pratense, and Mulium effusum. Cunliffe (1921, 1922, and 1923) added to this list of grass hosts of the frit fly, the species Alopecurus myosuroides, Hordeum murinum, Holcus lanatus, Bromus sterilis, Dactylis glomerata and Agrostis myosuroides. Frew (1923) listed among other host plant species of frit flies the following grasses, Agrostis alba and Festuca elatius. Nye (1959) published the results of a complementary study in which he investigated the distribution of dipterous stem-boring larvae in pure and mixed swards of grasses during winter months. He found seventeen species of larvae attacking sixteen .grass species, the most susceptible of which were Agrostis tenuis, Lolium perenne, Holcus lanatus, Arrhenatherum elatius, Anthoxanthum odoratum and Dactylis glomerata, in decreasing order of magnitude. 2 Infestation of grasses appears to be widespread: Vasina (1929) in Russia observed that grasses always produced frit flies, especially the autumn generation. Zhukowiski (1937) and Vasina (1929) in Russia, Mayer (1926) in Germany, and Roos (1937) in Switzerland, regarded the grass species, Agropyron repens to be the principal host. By contrast, Aldrich (1920), and Wilbur and Sabrosky (1936) in North America, showed a wide distribution of Oscinella frit L. on grasses, especially in areas where Kentucky blue grass (Poa pratensis) was predominant. Wetzel (1967) investi- gated the occurrence and injuriousness of Dipterous larvae in grasses grown for seed in Germany, and found 14 spp. of larvae from four families associated with 16 spp. of grasses. Based on his rating technique, 0. frit appeared to be the most important pest attacking Poa pratensis and Trisetum flavescens in particular. 0. vastator attacked nine spp. of grasses and caused considerable damage to seven grass species including L. perenne, Alopecurus pratensis, and Festuca elatius pratensis. 0. frit L. ex. Arrhenatherum was confined to Arrhenatherum7--