By Penny Ossowski
If you see a shiny black fly about 3mm long sitting on a bean leaf, it is probably a bean fly. Look for pale yellow spots on bean leaves close to the stalk. This is where the bean fly punctures the leaf to lay its eggs. When the eggs hatch, the maggot/larvae tunnels to the leaf stalk, down the stalk and stem to the base of the plant. At this stage it pupates ready to emerge as a bean fly and so the life cycle continues and all this takes about 3 weeks.
Soil hilled around the base of seedlings will not stop the bean fly stinging the plant but may allow it to grow some extra roots and stop it falling over. Some protection may come from using Garlic Spray (below), Nettle Spray or Chilli and Wormwood Spray. John says “as soon as they are about 10cm high, hill the soil up around them and use garlic spray for bean fly”.
From the Queensland Planting Guide
Garlic Spray: It kills and repels aphids, thrips, caterpillars, snails and slugs. It can be used to repel fruit fly, codling moth and large outbreaks of leaf-eating beetles. Garlic spray has some anti-fungal properties and may be useful to control downy mildew, rust, curly leaf and brown rot. Useful for the control of chewing insects and aphids when sprayed directly onto the insect. It may also be used as a feeding deterrent for insects and possums.
Recipe: Chop 90 grams of garlic, cover with paraffin oil, let soak over night, strain, add 1 litre of soapy water and store in a glass jar with a sealed lid.
To Use: Dilute one part garlic to 50 parts water and spray.