Biological Control of Common Greenhouse Pests - Part 2

Mealybugs – Pseudococcus obscurus

These scale-like wingless insects are covered with a powdery white protective wax. They feed on the sap of many greenhouse plants, amongst the most susceptible being vines, begonias, cacti, chrysanthemums, and fuchsias. They cause leaves to yellow, and unsightly sooty moulds form on the sticky honeydew that they secrete.

Prevention

Mealybugs almost always come in on plants, so examine any new ones carefully before putting them in the greenhouse.

Treatment

Cut out heavily infested shoots. Hose large sturdy plants with a strong jet of water. As a last resort, spray with insecticidal soap. Remove mealybugs from small delicate plants with a paintbrush dipped in surgical spirit or insecticidal soap.

Biological control

A predatory beetle, the ladybird Cryptolaemus montrouzieri, is marketed to commercial growers to control mealybug, and could be used in amateur greenhouses during the summer months.

Tomato moth – Lacanobia oleracea

The yellow-green or brown caterpillars of this moth eat holes in leaves and fruits of tomatoes, and may attack other greenhouse plants as well as tomatoes, although damage is rarely severe. The caterpillars pupate in cocoons on walls, woodwork, or plant debris.

Prevention

Clean out the greenhouse thoroughly in autumn to destroy overwintering pupae.

Treatment

Seek out and destroy caterpillars if damage is seen.

Biological control

Very bad attacks could be controlled by spraying with a formulation of the bacterium Bacillus thuringiensis.

Red spider mites – Tetranychus urticae

The first signs of this pest are a light mottling of affected leaves. Look underneath them with a magnifying glass and you should see the tiny mites which, despite their name, are usually greenish in colour; only the overwintering females are bright red. Many greenhouse plants are susceptible to attack including cucumbers, aubergines, fuchsias, and strawberries. If pest numbers build up, then the leaves become more discoloured and the plants become covered in a fine white webbing.

The mites breed from spring to autumn, particularly encouraged by hot dry. Conditions. From early autumn onwards, the females find sites to hibernate, eg. in cracks and crevices, under dead leaves, and in potting mixtures.

Prevention

Provide good growing conditions, as vigorous plants are less likely to be attacked. Spray susceptible plants with water when it is hot and dry, and do not let them dry out. Clean out the greenhouse thoroughly in autumn.

Treatment

Insecticides are not usually very effective, but as a last resort try derris.

Biological control

The slightly larger mites Phytoseiulus persimilis eat red spider mites in all their stages. These predatory mites multiply rapidly at temperatures of 20-30°C, and when all the red spider mites have been consumed Phytoseiulus will rapidly die out.

Sciarid flies – Lycoriella

These small black flies, often called mushroom flies or fungus gnats, are attracted by the smell of decaying organic matter. They hover around plants in pots and trays, and lay their eggs in the compost. Many species are harmless, but the larvae of some feed on young roots. Seedlings and cuttings can be killed and older plants weakened by severe attacks. Look out in the compost for white maggots 4-6mm long with shiny black heads.

Prevention

Remove any dead plant matter from the greenhouse before it has a chance to rot. Use well-drained seed and potting composts, and do not overwater plants as those in waterlogged conditions are particularly susceptible to attack. Covering the surface of the compost in a pot with horticultural grit will help deter the flies from laying their eggs.

Treatment

Repot affected plants in fresh compost, destroying any larvae that you see.

Biological control

The same or similar control to that used against vine weevil larvae is also effective against sciarid larvae.

Scale insects, soft scale – Coccus hesperidum

The species of these insects usually found in greenhouses are covered with roughly hemispherical brownish waxy scales. They live mainly on the under-sides of leaves alongside the veins, and on plant stems. They attack many ornamental plants, particularly those with shiny leaves such as bays and camellias, and also peaches and vines. Like aphids, they feed on sap, weakening the plants, and excrete sticky honeydew upon which sooty moulds grow. The adult scales rarely move, but lay eggs under the scales from which young ‘crawlers hatch. These move over the plants before settling to feed and forming their own protective scales.

Prevention

Scale insects are generally brought into the greenhouse on plants, so examine any new ones carefully.

Treatment

Remove scales from plants with relatively robust leaves by wiping them off, using a cloth or soft toothbrush dipped in soapy water. Crawlers can be sprayed with insecticidal soap — if you can catch them. They are most likely to hatch during late spring and early summer, although they could be about during any warm spell. You need a magnifying glass to see them.

Biological control

No biological control widely available to amateur gardeners as yet.

Vine weevil – Otiorhynchus sulcatus

The legless white larvae of this weevil feed on the roots, corms, and tubers of many plants. This checks their growth and often causes them to wilt suddenly. Cyclamen, primulas, and begonias are particularly susceptible but other pot plants are often affected. The adult weevils hide at ground level during the day; they cannot fly, but at night crawl up on to plants and eat holes in the leaves. They lay eggs in the potting compost, mostly in spring and then again in mid-summer. The larvae hatch a few weeks later.

Prevention

Remove any debris that could harbour adult weevils. Inspect plant roots when repotting and destroy any larvae you see. Plants in pots on greenhouse benches can be protected with a band of non-hardening glue, sold for catching crawling pests.

Treatment

Examine the roots of wilting plants immediately; destroy larvae and repot into fresh potting compost.

Biological control

Nematodes (Steinernema sp. or Heterorhabditis sp.) which parasitize vine weevil larvae can be watered on to pots and soil. These microscopic creatures enter the body of the pest and multiply within it, killing it within a few days. Further nematodes are thus released which will move around in the soil or compost provided that it is kept moist. The control is best applies either in mid to late spring or in late summer, which are the periods when most weevil larvae are active. It is most effective at temperatures above 20°C.

Woodlice Oniscus, Porcellio, and Armadillidium

These familiar garden creatures usually feed on decaying organic matter, but in greenhouses they may eat seedlings at soil level and chew the leaves, fruit, and roots of older plants. They hide and breed in moist cool places.

Prevention

Keep the greenhouse clear of any type of debris which could provide the woodlice with refuge. Sticky bands as used for vine weevil could prevent woodlice reaching plants in pots.

Treatment

Kill woodlice found under bricks, stones, and debris with boiling water. Leave one or two such hiding places as traps for survivors and keep checking them. You can also buy traps for woodlice, which are baited with a pheromone (a synthesized sex attractant).

Biological control

No biological control available as yet.