Chrysolina Leaf Beetles were actively eating and depleting St Johns Wort plants in early November. They are black with bronze, dark-blue or purple reflections and are oval in shape.
During periods of cold wet weather , like the past few days they disappear . I could hardly find any to photograph this morning whereas they were swarming over the plants last week. Chrysolina hyperici and Chryolina quadrigemina were released in Victoria in the 1930’s . Because these two species have low mobility it may be worthwhile to assist the spread by collecting batches and moving them to areas that have no beetles. Larvae or adults can be shaken into a bucket and transported to new sites. These harvested insects should be kept cool and out of direct sunlight and released as soon as possible. This may be a solution when you plan to herbicide areas that support beetle populations that you wish to preserve but would be a time consuming activity.
The beetle larvae attack the winter growth and the adults attack the spring growth. After a few years at favourable sites the beetles reach densities which are high enough to cause complete defoliation. Heavy damage suppresses flowering and seed production. The damage produced by Chrysolina beetles can appear spectacular but the impact tends to be sporadic and inconsistent. They can provide effective control in open, unshaded situations but without follow-up pasture improvement , the weed frequently re-establishes. The beetles are not effective in timbered country as they mate only in sunlight. Biological control cannot eradicate the weed, it can only reduce the spread and density of infestations.
Use of herbicide is not advisable when high numbers of St Johns Wort beetles are present because partially defoliated plants are unlikely absorb enough herbicide to kill them. Biological control of St John’s Wort should not be considered the complete answer to the problem , but can be used in conjunction with other control measures in an integrated program. Reference DEPI Ag Notes Keith Turnbull Institute.1999.