Q What is pear midge?

A It's a tiny fly (Contarina pyrivora), the larvae of which damage immature pears. This pest is widespread, damaging and especially severe in gardens where it occupies lone trees.

The adult is only 2.5-4mm long and darkly coloured. It is on the wing in April and May, during the peak of the pear-blossom season. It lays between 10 and 30 elongated eggs in each open flower. These, too, are extremely small. After four to six days, the eggs hatch and the grubs emerge. Since more than one fly will lay eggs in each flower, there can be up to 100 grubs in each developing fruitlet. These feed inside the fruit as it develops, forming a black cavity filled with seething maggots. The pest is usually first noticed when the fruits are damaged and fall off.

Caption: Fruitlets affected by pear midge turn black and fall off in June

Q How do I recognise pear midge?

A Pear midge only affects pears. The damaged fruitlets turn black from the base and fall in June when still immature. If you cut one open, you’ll find that the centre is hollowed out and you may see small, yellowish-white grubs inside. They take about six weeks to become mature. When fully grown they are 4-5mm long. Then they force their way out of the fruitlet and head for the soil. Most fruitlets drop before this happens. They are surprisingly mobile for such tiny grubs and can flick themselves along by flexing their bodies. Once in the soil, they burrow to a depth of about 5-7.5cm and spin silken cocoons. This is where they spend the winter, turning into adults the following spring. There is only one generation per year.

Q Could I mistake it for anything else?

A Not really; the presence of the maggots is a give-away. However, it's worth noting that frost damage can also cause fruits to blacken, and natural thinning of a heavy crop can result in fruits falling.

Q What damage does pear midge do?

A At first, no damage is visible. In fact, the infested fruitlets grow faster than unaffected ones. However, the fruits become rounder than usual or deformed. When they are about 20mm in diameter the fruits split, wither and rot, before falling from the tree. The uninfested fruits are also damaged. The speed of growth of the infested fruits fools the tree into diverting resources to these fruits at the expense of the normal ones. The result is that normal fruits fall, too, from starvation. Very heavy crop loss will follow.

Q How do I control this pest?

A There is nothing you can do once an outbreak has occurred, but you can reduce the extent of the problem next year. Start by picking up all the blackened fruitlets you can find and put them in the bin, not on the compost heap. Then spread polythene, fleece or old sheets under the tree, held down with bricks or pegs. This will collect other fallen fruit and help to prevent the midge grubs burrowing into the ground to complete their life cycle, so should mean far fewer midges next year.

The old method of control was to cultivate the ground up to 7.5cm deep beneath the trees during late summer and winter. This is when the grubs and pupae are vulnerable to damage and can be exposed for birds to eat. Such cultivation is hard work and may reduce crops by damaging roots. An alternative is to cover the soil beneath the trees with black polythene. This should help to prevent the flies escaping to attack the fruit. Cover the ground in late winter, before the pears flower.

Q Is there a spray I can use?

A You could try spraying with a contact insecticide approved for fruit trees. Do this at white-bud stage – the point where the petals begin to show. You will probably need to do this two years running, though routine treatments are not usually needed.

Q Are some varieties of pear more resistant than others?

A Yes. The tiny adult midges lay eggs in the buds in mid-spring, so very early or very late varieties may escape damage. The flowers of very early varieties such as 'Packham's Triumph' can be damaged by frost however. Good later-flowering varieties include 'Concorde' and 'Onward'.

Q Is there any biological control?

A Yes, Keep chickens. Free-range poultry gobble up the larvae.