Impacts of temperature, on the oviposition and mortality of adults, and altitude, on larvae density, on the broom twig miner (Leucoptera spartifoliella) : A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Science with Honours at Lincoln University

Abstract

The broom twig miner, Leucoptera spartifoliella (Hilbner) (Lepidoptera: Lyonetiidae) is a biological control agent of Scotch broom, Cytisus scoparius L. (Link) (Fabaceae). The larval stage of the life-cycle causes damage to the Scotch broom. Successful establishment of agents is critical to the success of biological control programmes. Temperature and altitude can affect establishment and hence impact of the programmes. A laboratory study was conducted at Landcare Research, Lincoln, New Zealand, to determine the effect of six temperatures (4, 13, 18, 23, 27 and 30°C) on the oviposition of the adult moth. Cages, containing five pairs of insects and a 40 cm branch of Scotch broom, supported in a flask with water, were placed in temperature controlled rooms for four days. Numbers of tips on the Scotch broom branch, numbers of eggs laid and mortality of the insects were recorded. Maximum oviposition occurred at 27°C, with 17 times as many eggs oviposited, compared with 4 °C. Mortality increased with increasing temperature, with 10% of the insects dying at 4°C compared with 90% at 30°C. A field study was conducted near Hanmer Springs, New Zealand, to determine the effect of altitude on the abundance of mines produced by larvae. Scotch broom branches were sampled from bushes on sites at 100 m altitudinal intervals (from 350 m to 950 m) from four transects. The altitude and aspect of the site, and height and stem diameter of the bush were recorded. The length of the branches and the tips were measured and the number of mines on the branch and tips were counted, then a density of mines (mines cm-I) was calculated. As altitude increased, there was a significant decrease in the density of mines, decreasing from 0.5 mines cm⁻¹ to less than 0.1 mines cm⁻¹.The density of mines was also significantly affected by the number of tips on the branch of Scotch broom, but this depended on the aspect of the site. There was no effect of the number of tips if the site was southerly facing, but if the site was northerly or east/west, the density of mines decreased as the number of tips on the branch increased. From this study, it was concluded that the.broom twig miner is likely to have increased chances of establishing if released at sites with temperatures ranging from 18 to 27°C as oviposition is maximised within this temperature range. It was also concluded that lower altitudes would increase the chances of the agent establishing as the density of larvae decreased with increasing altitude. Further studies are needed to determine whether oviposition of the broom twig miner is limited by available laying space and the reason for the response of mine densities to the interaction between the number of tips and the aspect of the site.