The following is a possible action project that students and teachers could investigate, if feasible. The gorse spider mite is a common biological control agent used for gorse and is relatively easy to rear. You may have found it out in the field during the 'explore' section.

How to rear gorse spider mites

Spider mites live in communal webs on gorse bushes.

The adults are brick red in colour, females are smaller than the size of a pinhead and the males are even smaller. New adults feed for two weeks before they begin to breed. The warmer the temperature the faster gorse spider mites can breed. The entire life cycle, from egg to adult, takes around 6 weeks at 15°C, but can take only 3 weeks at 23°C!

Eggs are brownish and laid in clusters tucked in close to the gorse stems. Spider mites have six juvenile stages. Juveniles are smaller and more orange in colour than adults.

Materials needed for your spider mite colony

• Rearing Cage
  
• Florescent lamp
  
• Timer for lamp
  
• Gorse sprigs *
  
• Trays for adults and nursery
  
• Oasis foam (from the florist)
  
• Margarine containers
  
• Adults spider mite colony

* Fresh gorse sprigs 20-30cm long will need to be harvested by students daily/weekly. Alternatively gorse seedlings could be collected and potted up 8 weeks before the spider mite rearing is to begin.

Setting up the spider mite breeding colony

Step 1:

Place the rearing cage in position in your classroom, away from direct sunlight.

Step 2:

Set up the fluorescent lamp in the corner of the cage and plug into a power point using the timer.

Step 3:

Set the timer for 16 hour days and 8 hour nights. i.e. Set it to come on at 6am and go off at 10pm.

Step 4:

Using a sharp pair of secateurs, take cuttings/sprigs from mature plants, approximately 20 - 30 cm long. The mites prefer the harder, older growth that is at least a few months old. It is a good idea to wear thick gloves ( e.g. welding gloves) to protect your hands from the spines of the plant.

Step 5:

Place the nursery tray and adult tray inside the cage. On the adult tray put a pottle containing the gorse sprig with the supplied mites.

Harvesting gorse sprigs

Fresh gorse sprigs will need to be collected regularly to replenish the mites food supply. Locate a healthy infestation of gorse. As the sprigs are collected, place them into a plastic bag. Seal the bag until the food is ready to be used. When transporting the bag containing the food material, ensure that it is not left in direct sunlight, as this will cause the material to deteriorate and wilt. The mites will not feed on poor quality food.

A healthy food supply is required to maintain a healthy colony of spider mites.

On Mondays, collect enough cuttings/sprigs to supply the spider mites with food for one week. Store extra sprigs in a cool environment, preferably in a fridge, in a sealed plastic bag for up to a week. Discard any unused cuttings on Fridays and collect fresh sprigs again the following week.

Adding new sprigs to the gorse colony

The gorse mites will feed and lay eggs all the way up the stem of gorse. When they reach the top it will be necessary to add a fresh sprig of gorse. Gorse spider mites are attracted to the light and will move together as a colony towards the tip of the sprig as they feed. The mites will not descend the sprig to feed once they have made their way to the top. They will remain in large numbers, spreading out to the ends of the webbing, congregating together on the gorse spines. Once the mites begin clustering at the top of the shoots, they must be transferred to new food material so that they do not starve. If fresh gorse is not supplied the mites may run off the gorse looking for food.

NB. How fast the mites move up the cuttings and cluster at the ends of shoots will depend on the temperature and how fresh and healthy their food supply is.

Step 1:

Place some Oasis foam cubes into a container and fill with water until the oasis is completely wet, tip out any excess water. Ensure that the foam is kept wet at all times.

Step 2:

Take a freshly collected sprig of gorse, and remove the lower spines by pulling them downwards so that there is 4-5 cm of clear stem to hold. Insert the stem of the fresh gorse sprig into the Oasis foam. Only insert the stem about half way.

Step 3:

Place this fresh cutting in the container alongside the original sprig. Arrange the original sprig so that it sits on an angle, its tip in contact with the base of the fresh cutting. This will allow the mites to transfer themselves onto the new cutting. It may be necessary to stand the fresh sprig on top of another container to ensure that the mites transfer to the bottom of the new sprig.

Step 4:

Once the mites have moved across to the new cutting, do not discard the original sprig. It will contain tiny eggs and juvenile mites left behind by the advancing breeding colony. The original cutting should now be placed on the tray labelled 'Nursery'.

There should now be two trays in your rearing cage, one with the advancing 'Adult' breeding colony and another set up as a 'Nursery' for the eggs and juvenile mites.

When the adult mites again begin clustering at the top of the cutting in your breeding colony, repeat the above steps. Remember to keep the old sprigs and place them in the 'Nursery'. The juveniles may also move up to the top of the sprig when there is no food left. When this happens, use the same method to transfer them to a new sprig as was done with the adult colony.

Releasing your gorse spider mite population

Release your population before the holidays.

Step 1:

Find a patch of gorse in your local area and contact the owner to find out what their plans are and whether a release into this gorse would be suitable.

Step 2:

Place all the gorse shoots into a paper bag and carry to the site.

Step 3:

Pull green shoots of the gorse bushes apart so that the shoots with the spider mites on can be place securely amongst them.

Step 4:

Release Information Sheet

This sheet is designed to be downloaded, printed out and filled in. For instructions about what to fill in see below

For further information about biocontrol, contact: Lynley Hayes:  HayesL@landcareresearch.co.nz
For comments on the Landcare Research website, contact: Chrys Horn: HornC@landcareresearch.co.nz

Download -Gorse spider mite study doc