08 December 2008

The effectiveness of the gorse seed weevil and gorse pod moth


Ulex europaeus
Originally uploaded by Mollivan Jon
Gorse (Ulex europaeus) is a prickly shrub that is the number one weed in New Zealand. Due to the favourable climate in New Zealand, in a short time gorse was producing a lot of seeds to store in the seed banks.

To combat this problem, the gorse seed weevil (Exapion ulicis) was introduced into New Zealand in the 1930s. However, due to the favourable climate in New Zealand gorse was having two reproductive periods per year and the gorse seed weevil was only active during spring.

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Recent Lincoln University Masters student, Craig Sixtus, who has been fascinated by this particular weed for a long period, as part of his study for his Masters degree, he studied the viability of gorse seed that had been attacked by the gorse seed weevil. In this study he found that the gorse seed still had a high percentage of viable seed even though the gorse seed weevil had been having a picnic. He tested the viability of gorse seed that had had some of it eaten by the gorse seed weevil (New Zealand Plant Protection Volume 2003).

To combat this, the gorse pod moth (Cydia succedana) was introduced into New Zealand in the early 1990s. This moth has two reproductions per year, to coincide with the reproduction of the gorse. Craig had trials throughout the South Island, New Zealand, from Golden Bay to Mackenzie Basin. He studied the effectiveness of both of the seed eaters.

The Golden Bay sites had active gorse pod moth for most of the year, whereas the sites further south only had a short period when they were active. The gorse seed weevil was more active at southern sites. The highest percentage (45%) of pods damaged by gorse pod moth was at East Takaka, Golden Bay. Lake Ohau, Mackenzie Basin had a short period where gorse was reproducing, mature pods being produced during December with a few left to open in January. The gorse seed weevil was most active here, damaging 76% of pods. At the southern sites there was a shorter period when the monthly average maximum temperature was above 10ºC, therefore there was less insect activity. However, there were still ample pods available. These results were published in the Plant Protection Quarterly Volume 21.


The gorse pod moth larvae eat all of the seed in a pod before they enter another. Previous studies have found that the larvae will enter 2-3 pods before they pupate. Craig found that the pods in Golden Bay had more seeds per pod than the pods from further south (8 v 4.5). It is assumed that as the Golden Bay pods have more seeds, the larvae would enter less pods.

Now Craig is about to begin his study for his PhD where he will study the phenology of the gorse pod moth and the impact that it has on the seed production of gorse throughout New Zealand, including sites in the North Island. He will also see if this seed muncher feeds on other legumes when gorse seed isn’t available.

03 December 2008

Gorse seed production and viability

Gorse (Ulex europaeus) produces many seeds per season which can be viable for a long period, especially if the seed is buried underground. In the more temperate climate areas gorse has two reproductive periods per season. Craig Sixtus, who was studying gorse for his master's degree, investigated gorse seed viability to see if viability was influenced by altitude or climatic conditions. The sites were different altitude and climatic areas. The sites were Bainham, Onekaka, East Takaka, Hinewai Reserve, Trotters Gorge and Lake Ohau.
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Sites were inspected monthly and if there were seeds in the seed trays, they were removed and taken back to the laboratory. Samples were also taken from the pods on the bush when they were mature. When the seeds were tested for viability, scarification with concentrated sulphuric acid was used, which Craig had previously tested for (New Zealand Plant Protection, 2003).

The results showed that gorse produced mature seed pods almost continuously in the Golden Bay region (Bainham, Onekaka, and East Takaka). In the southern regions seed was only produced in the spring. The seed fall for each month varied from 0–256 seed/m2.

The germination of all fallen seed was similar, regardless of soil type, climatic conditions or site altitude. However, there were month-to-month differences. Seed from pods sampled during summer show that, and Golden Bay, gorse seed viability (60%) was higher than seed produced further south (30% at Lake Ohau). These results were published in the Agronomy New Zealand Volume 34.

Now Craig is about to begin his study for his PhD where he will study the phenology of the gorse pod moth and the impact that it has on the seed production of gorse throughout New Zealand. He will be testing the moth at various sites, including several North Island ones, as far north as Northland.