Several species of leafminers are found in the UK on a wide range of crops but all produce the characteristic 'mines' within the leaf, in which the larvae feed. Adult feeding produces small discrete white spots (1 mm diameter) on the leaves, usually on the upper leaf surface. 

The larval stages inside the leaf can be controlled using one or both of two parasitic wasps. Some leaf miner species are notifiable insect pests, particularly on ornamental crops. Correct identification by PHSI (Plant Health and Seeds Inspectors) for any suspect insects is essential.

Type: Parasitic wasp. 

How it works: The wasp lays its eggs next to the leafminer larvae, damage ceases immediately as the larvae are paralysed. The wasp parasite larvae then feed on the leafminer larvae. Adult females kill and feed directly on small leafminer host larvae that are too small to support a developing parasite larva. 

Species controlled: Liriomyza (Tomato Leafminer) and Phytomyza (Chrysanthemum Leafminer). 

When to use: Ideal for use from mid-February in heated crops and April in unheated crops. The establishment will only occur if the wasp is released when sufficient numbers of reasonably sized larvae are present within the crop.

Rate of use: 1 wasp per 1 m2 every week for 3 to 4 weeks.

Type: Microscopic nematodes. 

How it works: A special formulation for foliar application by high volume spraying. The nematodes swim through a film of water and enter the mine to attack and kill the larva within. 

Species controlled: Leafminer and Western Flower Thrips. 

When to use: At any time of the year, ideal at temperatures above 16°C. Do not use under strong lights or when the sprayed crop is likely to dry within 3 to 4 hours. A series of 4 or 5 weekly sprays is best to control a thrips population. Nemasys can be used alongside Conserve and several other pesticides for a wider range of activity. 

Rates of use (for foliar application):
  • Preventative: 50 million per 400 m2 or 250 million per 2000 m2
  • Standard control: 50 million per 200 m2 or 250 million per 1000 m2

Leafminer damageAdult Didlyphus
(Photo: Holt Studi