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Leaf miners are often found on chrysanthemums and related plants. Normally, these leaf miners are native insects, belonging to the species known as the chrysanthemum leaf miner (Phytomyza syngenesiae (Hardy)), which is a common pest widely distributed throughout the British Isles. However, alien leaf miners are sometimes introduced with imported cuttings and these species are described on pages 271–273.

SUMMARY Pyrazophos was more effective than dioxanthion against chrysanthemum leaf miner (Phytomyza syngenesiae Hardy) when applied as a mist onto bed-grown plants irrespective of the stage of development of the pest. When pyrazophos was applied at the feeding mark stage no eggs developed but when it was applied at the young mine, mature mine or pupal stages 3–5%, 3-1% or 45-5% reached adulthood. Pyrazophos applied every 17-5 days kept damage levels acceptably low. The potential of pyrazophos within an integrated control programme for chrysanthemum is discussed.

Eighteen cultivars of indoor chrysanthemums were tested in the laboratory for susceptibility to adult female Phytomyza syngenesiae (Hardy). Host susceptibility was assessed by the density of feeding punctures, the density of eggs laid, and the amount of leaf damaged by the mining larvae. Eight of the cultivars were also tested in the field for their susceptibility to leaf damage. Cv. Mary Poppins, a ‘Mini-mum’, was the most susceptible, for it had most feeding punctures and most eggs per unit area, the largest percentage of mined leaves and a large percentage of damaged leaf area. Cv. Thumbelina, also a ‘Mini-mum’, was the least susceptible to leaf damage, for it had the smallest percentage of damaged leaf area, although a large number of the leaves were mined. Feeding puncture/egg ratios were not found to be a consistently reliable indicator of susceptibility.

A mixed population of Phytomyza syngenesiae (Hardy) (formerly P. atricomis Meigen) from material originally collected from tansy ragwort, Senecio jacobaea L., at Fort Bragg and Albany, California (132 airline miles apart) increased remarkably on S. jacobaea plants in laboratory cages and in a greenhouse. The huge populations, which often appeared to arise from isolated individual insects, were not the result of parthenogenesis (44 mated females laid viable eggs; 40 virgin females laid only nonviable eggs). The parasitic Hymenoptera of this leaf miner do not reproduce in cages or greenhouses, and the life cycle of the leaf miner is short (about 28 days) with 12 hours of 24 and 12 hours of 12.75, and it can be completed and sustained in small cages. Virginity did not affect the fecundity of the females, but the preoviposition period of virgins was longer than for mated females (average 4±2 days vs. 2±0.5 days).