Biological Control of Common Greenhouse Pests



Biological control of common greenhouse pests

Biological control of common greenhouse pests Some pests in the greenhouse can be controlled by introducing their natural enemies artificially. A range of such biological controls, which may be predatory insects or mites, parasites, bacteria or fungi, are being used increasingly by commercial growers, and some have also been shown to work well in small garden greenhouses. Good examples are the parasitic wasp Encarsia formosa, which controls greenhouse whitefly, and the predatory mite Phytoseiulus persimilis, which controls red spider mite.

You can order these controls through the post, and they come in a form which is easy to distribute around the greenhouse. Provided that they have plenty of food and the right conditions, generally they do not try to ‘escape’, even if you have the doors and ventilators open. More recently introduced biological controls which could be successful in garden greenhouses include nematodes which parasitize vine weevil larvae, and midges which prey on aphids. Details of all these controls are given under the appropriate pest entries. Others, such as the control for scale insect, need higher and more constant temperatures or greater humidity than amateur greenhouses can normally supply, but more may become available in the future.


Using biological controls

ADVANTAGES

  • Biological controls are very specific, affecting only the target pest. They are harmless to humans, pets, and beneficial insects, and do not damage plants.
  • Ideally, the controls allow a balance to be established so that both control and pest exist at levels which are not damaging. Thus, after the initial introduction you do not have to take any further control measures.

CAUTIONS

  • Usually, specific conditions are needed for the controls to work well, eg. the right temperature, humidity, and light levels. Make sure that you can provide these conditions before introducing biological controls.
  • They do not work instantly and cannot control epidemics. You must introduce the natural enemy before the problem gets out of hand.
  • They can be very sensitive to insecticides — even organic ones. Check that any residues of chemicals you have used in the past will not harm them, and take great care if you use sprays against other pests in the greenhouse after you have introduced a biological control. The fumes from paraffin heaters can harm Encarsia.
  • Biological controls can be expensive. One delivery from the supplier is usually more than enough for a small greenhouse, but you cannot store them for future use.


Aphids

Many of the hundreds of aphid species, usually referred to as greenfly or blackfly, feed on plants in greenhouses as well as outdoors. They are mainly green in colour and appear first on shoot tips and the under-sides of young leaves. They feed on plant sap, distorting growth and weakening plants, and they can also carry viral diseases. Another problem is the sticky honeydew which they excrete and upon which sooty moulds grow. These look unsightly and can prevent light getting to the leaves.

Prevention

Look after the plants well: those under stress, particularly through under-watering or incorrect feeding, are more susceptible to attack. Encourage the aphids natural enemies such as hoverflies

(Episyrphus balteatus, Syrphus ribesii, and many other species), lacewings ( Chrysopa spp.) and ladybirds (Coccinella, Propylea, and other species).

Treatment

Examine plants, and even seedlings, regularly Infested leaves and shoots can be picked off large plants, or the aphids squashed by hand. Use insecticidal or soft soap if all else fails.

Biological control

The mite Aphidoletes aphidimyza can be introduced into the greenhouse to help control aphids. Its orange larvae, which can just be seen with the naked eye, feed on any aphids they find before falling to the ground to pupate. They are most active at temperatures of 15-20°C, and are best introduced in late spring or summer If aphid levels are high, then reduce them by spraying first.


Eelworms or nematodes – Nematodae

Several species of these microscopic worm-like creatures attack greenhouse plants. Two of the most common examples are given here:


1. Root knot eelworms – Meloidogyne

These enter into roots and cause swellings or galls. As a result, the plants become stunted and undernourished, and wilt easily. Plants commonly attacked (although not necessarily by the same species of eelworm) include begonias, chrysanthemums, cucumbers, tomatoes, lettuces, and French beans. The eelworms spread through roots, soil, or potting compost to other plants.

Prevention

Do not introduce suspect plants or soil into the greenhouse.

Treatment

Destroy infested plants and potting compost. Change soil in borders and gravel on staging if likely to be affected. Observe strict hygiene with tools, pots, etc. to prevent spread. For tomatoes, you can grow a rootstock resistant to the eelworm, on to which you can graft other varieties.

Biological control

No biological control available as yet.


2. Potato cyst eelworms – Globodera

These enter into the roots of tomatoes, checking growth and reducing yield. The eelworm eggs are contained in spherical brown cysts which you can see with a magnifying glass; these can remain dormant in the soil for up to ten years.

Prevention

Do not introduce suspect plants or soil. Rotate tomato crops in borders, or change the soil regularly, to prevent any cysts present in the soil from building up numbers.

Treatment

Destroy infested plants. Change border soil or grow tomatoes in pots for at least six years.

Biological control

No biological control available as yet.


Glasshouse whitefly – Trialeurodes vaporariorum

These tiny white moth-like insects live on the under-sides of leaves of many greenhouse plants, flying off when disturbed. Both the adults and the immobile white scales, which are their immature form, feed on plant sap. They secrete honeydew upon which unsightly sooty moulds form, and heavy infestations can weaken plants. Whitefly overwinter on plants and weeds in the greenhouse, their eggs surviving even if it is not heated, and they begin to multiply rapidly once temperatures get over 15°C.

Prevention

Examine newly acquired plants thoroughly to avoid bringing in whitefly. Clear weeds and old annual plants out of the greenhouse in autumn to help prevent the pest from overwintering. Treat or destroy any other infested plants.

Treatment

Yellow sticky traps will control small numbers of whitefly and give you an early indication that they are present. For larger numbers, try shaking the plants and using a portable vacuum cleaner to suck up the adults as they fly. As a last resort, spray with insecticidal soap.

Biological control

The parasitic wasp Encarsia formosa lays eggs inside the whitefly scales. These turn black as the eggs develop, and new adult wasps emerge. Introduce the control two to three weeks after you see the first whitefly, but before numbers buildup. Reduce numbers by spraying if necessary. The wasp works best at temperatures of 18-27°C, usually reached in an unheated greenhouse by late spring or early summer, and once established it should keep whitefly below damaging levels for the whole season.


Leaf miners, chrysanthemum – Phytomyza syngenesiae

The leaf miners commonly troublesome in the greenhouse are small flies which lay their eggs inside plant leaves. The larvae which hatch eat out meandering tunnels, spoiling the appearance of the plants. Chrysanthemums and lettuces are amongst the plants commonly affected.

Prevention

The larvae can overwinter on weeds such as sowthistle and groundsel, so make sure that you remove these from the greenhouse and garden if this pest is a problem.

Treatment

Inspect plants regularly and pick off affected leaves as soon as you see them.

Biological control

No biological control available to amateur gardeners as yet.


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.

29. November 2010 by Dave Pinkney
Categories: Garden Management, Greenhouse Gardening, Pests and Diseases | Tags: | Comments Off on Biological Control of Common Greenhouse Pests

Follow

Get every new post delivered to your Inbox

Join other followers: