DEALING WITH LEAF MINER INFESTATION

Press, Volume CV, Issue 31041, 22 April 1966, Page 8

DEALING WITH LEAF MINER INFESTATION

If you are a grower of cinerarias, or have a specimen of Kaka - beak (Clianthus) somewhere in the garden, or even a thriving patch of fat hen, you may have noticed a peculiar tracery on the leaves. It may be evident on one or two leaves only on the plants

concerned. The cause of the tracery, or mines as they are called, is the larva or maggot of a leaf miner, a fly or a moth, a different one being responsible for the damage on each of the plants mentioned so far. The leaf miner maggots are very small and the adults are even smaller. It is doubtful whether a random inspection of likely hosts would reveal their presence to anyone who has not an extremely sharp eye as the maggots, when ready to pupate and therefore are at their maximum size, are much less than one quarter of an inch long. There are two distinct types of miners, the true leaf miners or linear miner, the larvae of which, as the name implies, tunnel progressively while eating through the leaf tissues. The other type of miner is known as the blotch miner and its offspring are of rather untidy habit, eating in all directions. There are also a few leaf miners who produce both mines and blotches. Egg Laying Miner flies differ in their egg-laying habits from those of the moths; the former lays individual eggs in punctures made underneath the leaf from which the emerging maggots rapidly burrow into the tissue; the latter lays the eggs on the leaf surface and the young bore in from that point. Once the maggots attain full size after travelling between the upper and lower leaf surfaces, they become sluggish and pupate, either within the mine or in its vicinity, emerg-

ing about two months later as fully equipped adult flies or moths. Other Victims The cineraria leaf miner is perhaps the most commonly found miner in New Zealand. It is a linear miner, the adult form of which is a rather sluggish, tiny grey black and yellow fly and it has a fairly wide host range. Apart from cinerarias, the maggots may be found on dahlias, peas, chrysanthemums and other plants, including such weeds as dandelion and ragwort. Referred to either as cineraria, or ragwort leaf miner, it pupates within the mine.

The species found on fat hen also attacks silver beet and spinach, and duckweed. It is known as the beet fly and is indigenous. A linear miner, it leaves the host plants when fully grown and pupates in the soil. Oak trees, and beech, both native and introduced, liquid amber and horse chestnut are among the plants which are possible hosts to the oak blotch miner, although I have only seen it on the evergreen oak, Quecus ilex. The yellow eggs are laid separately on the underside of the leaf and the larva produces an oval Notch which may extend over the

entire leaf surface by the time it is ready to pupate within it. Earlier reference was made to the clianthus. a plant with quite a number of common references, among them Red Kowhai, Kaka's Beak and Parrot's Bill. The generic name has a Greek derivative meaning. reclining flower, in allusion to the received standard. This plant plays unwitting host to the clianthus fly which lays its eggs in “pockets” on the under side of the leaves. Subsequently the emerging grubs make linear mines through the upper layers of the leaves, later pupating on vegetation closer to the ground. There is one other miner which may sometimes become noticeable in the garden and that is the Azalea leaf miner, from Japan. Predominantly prevalent on both azaleas and rhododendrons overseas, it frequents azaleas in New Zealand. It has a more variable approach to life. After the single eggs have hatched on the underside of the leaf and a linear mine has been formed, the maggot leaves it before half grown and procedes to roll the leaf surface edge, or top, over the under side. Here, in the cell formed it lives and feeds. Sometimes it may form two or three cells before pupating, either in the cell or on an irregularity on the leaf. Successful control is not easy although there are a number of products most suitable for the task. They include dipterex, BHC, paratheon and dimecron. The requirements for applications are either to use a substance which will penetrate the leaf such as paratheon, or one which will kill the Insect as it leaves the plants such as BHC. The advent of granular insecticides, which are scattered on the soil, watered in, and absorbed by the plant as it grows, provides an immunity of lasting effect. For the purpose of preventing leaf miner incursions there is available one such material known as bidrin which could be used to advantage.