Some aspects of the biology of the ragworth flea beetle Longitarsus Jacobaeae (Waterhouse) relating to its role as a biological control agent in New Zealand

Abstract

An Italian biotype of L. jacobaeae (Waterhouse) was introduced in to New Zealand in 1981 as a biological control agent for ragwort Senecio jacobaea, a problem weed in dairy farms. This thesis investigated various aspects of the biology of L. jacobaeae and its interactions with its host plant that could be helpful in furthering the aims of the biological control programme.

Laboratory experiments showed that the black spot stage and larval movement in the egg are two distinct characters which can be useful for determining the imminent hatching of eggs. The finding of Cullen (1981) on storing ability was confirmed. Eggs of L. jacobaeae can be stored at 4 °C and used for incubation to obtain healthy larvae even after 16 weeks. Experiments under controlled temperature conditions showed that the eggs have the ability of to overwinter in the field and then hatch with the return to favourable conditions. Mean incubation periods at 15 °C and 18 °C were 25 days and 18 days respectively.

Analysis of head capsule widths of laboratory hatched and field collected L. jacobaeae larvae confirmed that this species possesses three larval instars and supported the description of Newton (1933). Larval development at different constant temperatures showed that at lower temperatures tested (10 °C; 12 °C), larvae fed and developed slowly in the root crowns whereas at higher temperatures tested (18 °C; 20 °C), larvae fed heavily on lateral roots and developed at a higher rate. According to the temperature conditions of different areas of New Zealand, eggs which hatch before the winter may produce adults in December, but eggs that do not hatch before the winter may overwinter in the field and with warm spring and summer temperatures hatch and produce adults in January.

Laboratory experiments using controlled day lengths showed that short days (8 hours light + 16 hours dark) initiated heavier feeding, reproductive maturation, mating and oviposition of L. jacobaeae, whereas long days (16 hours light + 8 hours dark) maintains them in diapause. Therefore, there will not be a time lag in egg production between early emerged adults (adults that emerge during a long day period) and late emerged adults (adults that emerge during short days). Measurements of flight muscles of reproductive and non- reproductive females showed that there was a complete resorption of flight muscles as the insect produced eggs. It is argued that whether the insect use flying for dispersal or insect use jumping for dispersal.

Experiment on oviposition of L. jacobaeae with relation to different temperature conditions showed that the range 15-20 °C were the most suitable temperatures for oviposition. Insect may oviposit for a longer time at a higher rate in North Island of New Zealand than in South Island of New Zealand.