The effects of crown weevil, Trichosirocalus horridus (Panzer) on the growth of nodding thistle, Carduus nutans L. : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science

Abstract

Rosettes of nodding thistle smother pasture and crops and reduce their availability to stock. It is a major weed of economic importance world-wide and a noxious weed in New Zealand. Cultural and mechanical control are difficult because of the ability of the plant to germinate most of the year round and, because of the persistence of the seed in the soil. Three biocontrol agents have been introduced into New Zealand to aid in the control of nodding thistle.

A field experiment was carried out to test the effect of varying numbers of Trichosirocalus horridus on the growth responses of nodding thistle, Carduus nutans. Plants of different germination times were subjected to infestation by five densities of T. horridus larvae. Characteristics of C. nutans growth habit were measured both during and at completion of the trial.

The extent of damage to nodding thistle is affected by size of the thistle and number of the larvae. It also appears that damage by crown weevil produces a stimulatory effect because the crowns of infested thistle plants were larger than non infested plants. However, overall thistle size, as indicated by the plant weight, was not increased so overall plant biomass was not affected by crown weevil damage.

All infested plants were shorter than non infested plants. Non infested plants did not produce any laterals, while the number of laterals generally increased in infested plants with increasing numbers of larvae.Plant mortality generally increased with increasing numbers of larvae, even though the differences were non significant.

Plants with larvae had significantly fewer seed heads than plants without. However, the seed heads generally increased in size with increasing larval numbers. The root:stem ratio showed no significant differences between treatments. Thistles showed a general increase in crown diameter in response to increasing infestation by crown weevil.

Rosette size did not differ significantly between treatments and the plants with the highest numbers of larvae had the largest rosette size.

Crown weevil has the potential to be a good biocontrol agent of nodding thistle if used in conjunction with other controls to initially reduce the thistle size. Used in this way, crown weevil has the potential to control the population of smaller rosettes