Pest Problems Explained – WHITEFLY

Whitefly Description

Adult whiteflies as their name suggests are white in colour. They resemble tiny moths and are about 3mm long. When they are not flying (which they do profusely when disturbed) they can usually be found on the undersides of leaves.

Young whitefly look like clear scales and can also be found on the undersides of leaves.

NB. Aphid cast skins are sometimes mistaken for whitefly! However, whitefly will fly away when disturbed, these cast skins will fall off or not move. Just shake the plant to find out which you have!

Whitefly Damage

The whitefly eggs are laid in a horseshoe shape on the undersides of leaves. Once hatched the young nymphs move a short distance so that they are equally spaced and then moult into legless scales that spend the rest of their development feeding by sucking sap. These scales excrete a sticky “honeydew” which falls onto the leaves and fruit below.

Sooty moulds often develop on this “honeydew” and plants under attack appear to lack vigour.

Sooty moulds can reduce the amount of light reaching the leaves which can lead to leaf yellowing and plant death.

Whitefly Life Cycle

Whitefly go through several growing stages between egg and adult. The whole cycle takes about 30 days at 20ºC (68ºF). The length of the adults life depends on the plant on which it was raised.

Freshly hatched adults from the lower leaves of plants fly up to the younger leaves of the plant to lay their eggs.

Whitefly can over-winter in a greenhouse on crops or weeds and the scales can withstand the odd frost.

Biological Control of Whitefly

During the warmer months whitefly can be controlled biologically in greenhouses or conservatories by using their natural enemies such as Encarsia formosa or other specialist whitefly parasites and predators. 

You can monitor whitefly populations with Yellow Sticky Traps.

Pest Problems Explained – VINE WEEVIL

Vine Weevil Description

The adult beetle is flightless, it is dark brown or black in colour and has fine yellow speckles on its back, and appears dusty. The adult is nocturnal so rarely seen during the day.

Most commonly found are the larvae (grubs), these are found in soil or compost around the roots of plants. The larvae are creamy-white in colour with a brown head, they curl into a “C” shape when touched. Pupae may also be found amongst the plant roots. These are also creamy-white in colour but have started to produce legs and antennae.

Black Vine Weevil (Otiorhynchus sulcatus)

Vine Weevil Damage

Adults feed on the edges of leaves during the summer, generally April to August, causing notching. The beetles walk and climb exceedingly well, making plants in hanging baskets and wall boxes easy prey.

The eggs laid throughout the summer develop into larvae (grubs) which feed on plant roots, this is the stage in which the insect causes most damage.

Symptoms range from lack of growth, dying leaves or branches right through to sudden wilting and death. The latter case being seen when plants have single tap roots or have been attacked at the main stem.

Grubs can also bore into corms and either kill by eating the inside away or by allowing infection to enter and cause rot.

Black vine weevil

Vine Weevil Life Cycle

All vine weevil are female, and so all adults are capable of laying eggs. The adults are nocturnal, hiding during the day at the base of plants, under plant pots or hidden in debris. They can often be found inside houses.

The adults lay their eggs around the base of a plant, each adult will lay on average 500-600 eggs throughout the summer, although as many as 1,500 has been recorded.

Newly laid eggs are white turning brown as they mature. Potentially 40% of eggs can make it to the larval stage although dry or very wet soil conditions will take their toll.

The eggs take between 8.4 days at 27ºC and 56 days at 9ºC to hatch into larvae. At this stage the larvae are about 1mm in length, creamy white with a dark head. Eventually these grubs can grow up to 13mm in length after feeding.

When the grubs are fully grown they burrow 15-20cm into the soil to pupate before hatching as adults.

In warm greenhouses, all stages of the vine weevil can be found at any time. Outside, in pots and containers, the adults usually start to hatch in April, this can be earlier in warmer years.

The adults feed throughout the summer, lay eggs from late August and die out during the autumn. The grubs in the soil then over-winter before pupating and hatching in the spring.

Biological Control of Vine Weevil

Vine weevil can be controlled biologically by using their natural enemy, the parasitic nematode, Steinernema kraussei also known as Nemasys.

Pest Problems Explained – THRIPS (Thunder Flies)

Thrip Description

Small, slender insects about 2-3mm long when fully grown. The adults have two pairs of narrow wings fringed with long hairs, the wings are held along the back when at rest.

Colours range from pale yellow to black, dependant on species.

Thrip Damage

Thrips feed on cell sap by piercing the leaf or flower bud with their mouthparts. These tissues then become mottled or flecked and are subject to dehydration, in severe cases they appear to have been scorched.

When leaves or buds expand they may become distorted or torn, forming “Windows” in the tissue.

Thrips can often be found in large groups near the veins on the under-side of leaves.

Thrip Life Cycle

A single female can lay about 60 eggs throughout the summer. Eggs are laid on the underside of leaves.

The larval stage lasts for about 10-14 days, after which the larva falls to the ground to pupate beneath the soil surface.

The pupal stage lasts between 4 to 7 days.

Biological Control of Thrips

Thrips can be controlled biologically by using their natural enemies such as the predatory mite Amblyseius cucumeris and several other species.

Pest Problems Explained – SLUGS

Slug Description

The adult slug is familiar to most gardeners! They come in a range of sizes and colours depending on the species.

The field slug is small (2cm) and grey, while the round back slugs can be relatively large (5-10cm) and black or brown in colour. in some cases round black slugs can have vibrant orange colouring on their foot.

Slug Damage

 Slugs are capable of feeding on flowers, leaves, stems, roots and seeds.

Leaf damage is usually shown as leaf shredding or severe notching.

Young plants are most at risk as the leaf feeding can be so severe that the plants die.

A Hosta with extensive slug damage

What is seen less often is the damage they cause below the soil surface to seeds, roots etc. This damage can result in seed that does not germinate, and in the case of potatoes, a very poor and damaged crop (see picture above).

Slug Life Cycle

Slugs are hermaphrodite, so every individual can lay eggs – up to 300 each slug. Eggs are laid in batches, usually 10-50, in moist but not waterlogged soil.

Most species found in gardens have an annual life cycle lasting less than a year, and lay eggs in any month providing conditions are suitable.

For example, field slugs hatching from eggs laid in the spring will become adults and lay eggs in the autumn. Eggs laid in the autumn will develop into adults the following summer. Because its generations overlap, all stages of the field slug are present throughout the year.

Many slugs spend most of their life below the soil surface tunnelling, rather like earthworms. Those seen on the soil surface represent only a small part of the total slug population.

They have a remarkable ability to survive during dry and cold periods by remaining deep in the soil.

Biological Control of Slugs

Slugs can be controlled biologically by using their natural enemy, the nematode Phasmarhabditis also known as Nemaslug.

Pest Problems Explained – SCIARID FLIES (Fungus Gnats)

Sciarid Fly Description

The young are small, 4-6mm long, white larvae (maggots) with black heads, these can be found in the top few millimetres of soil or compost.

Adult Sciarids are 3-4mm long, black, midge like flies that can be seen jumping or hovering over the soil surface.

They are likely to be found in the greenhouse or around houseplants where compost is moist and warm, and where algae may have developed on the soil surface.

Sciarid Fly Damage

Seedlings are nibbled off at the base of the stem or just below the soil level. Roots of cuttings are eaten away. Damaged plants may succumb to rot.

Sciarid Fly Life Cycle

Adult females lay their eggs in the surface layer of the soil, and although short-lived they can lay between 100 & 300 eggs during a week.

The eggs hatch into maggots, which will eat rotting algae, compost, seedlings and cuttings.

Sciarid Fly Larva

The whole life cycle takes about 4 weeks at 20ºC (68ºF).

Scairids can be present in the greenhouse or conservatory at any time of the year.

Biological Control of Sciarid Flies

Sciarid flies can be controlled biologically inside by using their natural enemy, the predatory mite Hypoaspis miles.

Pest Problems Explained – HARD SCALE INSECT

Hard Scale Insect Description

Hard scale insects are small pests that also consume plant juices but have no legs so cannot move.  They secrete a waxy material mixed with waste products as a kind of shell over the top of their body adding to it around the edge on daily basis. They do not produce any sticky honeydew.

Hard Scale Insects on a Plum Tree

Hard Scale Insect Life Cycle

There are many species of hard scales and they usually cause severe damage and dieback of infested shoots. When full grown one can distinguish the elongate males that develop into winged insects from the more rounded females. 

Females produce eggs underneath the protective scale cover where they remain until hatching.  These crawler stages then spread out over the plant or get carried in the wind to new hosts.  Once settled they moult into legless individuals and start to build their waxy cover.

Pest Problems Explained – SOFT SCALE INSECT

Scale Insect (Soft Scale) Description

There are two distinct groups of scale insects – soft scale and hard scale.

The most commonly found soft scales are Coccus hesperidum – the adults are 3-5mm long, green to brown in colour, oval- shaped and appear slightly flattened, and Saisettia coffeae – the adults are 5mm or longer in length, deep brown in colour and dome-shaped.  Most soft scales have 6 legs and can move around the plant as they grow

Scale Insect (Soft Scale) Damage

The symptoms are similar to those of whitefly or aphids.

Soft scales are normally found congregating along leaf veins or stems. The scales feed on plant sap and produce large amounts of sticky honeydew.

Large populations will cause yellowing of the plant and defoliation. The honeydew results in the growth of black sooty moulds which ultimately kill the plant.

Soft Scale Life Cycle

Soft scales produce a number of eggs over several days. The eggs are laid under the cover of the adult “scales” which offer protection during development. The adult dies once it has laid its eggs.

Eggs hatch into “crawlers”, which are tiny legged creatures which disperse all over the plant and its foliage in search of a suitable site to settle down and become an immobile scale.

Scales grow relatively slowly and have a long life cycle. Saisettia coffeae takes about 95 days at 18ºC (64ºF) to complete it’s life cycle

Pest Problems Explained – RED SPIDER MITE

Red Spider Mite Description

Small yellow/olive mites which have dark patches on either side of the body, red spider mite are less than 1mm long. Also known as “two-spotted mite” they can be found in large numbers on the underside of leaves.

Super macro photo group of Red Spider Mite infestation on vegetable. Insect concept.

In bad infestations they can produce fine webs.

As day lengths reduce in the autumn, they become deep red in colour and leave the plants to overwinter.

Red Spider Mite Damage:

Red spider mites (Tetranychus urticae) and Cyclamen mite (Phytonemus pallidus) on damaged strawberry leaf.

The mites suck sap from cells on the underside of plant leaves, in the early stages, characteristic white speckles can be seen from the upper leaf surface.

As mite numbers increase these white speckles will increase in number, the leaf will take on a bleached appearance and die.

The mites are found in highest numbers on the underside of leaves although you may need a magnifying glass to see them!

As the population builds you will start to see webs and aggregations of mites at certain sites, usually the growing points of the plant.

Cucumber leaf affected by Red Spider Mites.

Red Spider Mite Life Cycle:

The mites go through 5 development stages. Egg to adult takes about 14 days at 21ºC, or less than a week at 30ºC.

Eggs are laid on the underside of leaves. Each adult female can produce more than 100 eggs in 3 weeks. They reproduce at alarming rates – 10 spider mite in May are capable of becoming 1,000 by June & 100,000 by July!

Red Spider Mite and eggs

High humidities can reduce the egg laying rate of the mites.

During the autumn, when day lengths shorten the mites turn deep red in colour and migrate from the plants to hibernate in crevices within the glasshouse structure.

Red spider mite can overwinter without feeding and re-emerge in the spring and summer to re-infest plants.

NB. Artificial lighting may stop the mites from hibernating.

Biological Control of Red Spider Mite

Red spider mite can be controlled biologically indoors or outside by using their natural enemy, the predatory mite Phytoseiulus persimilis which is unable to control any other mite species.

Pest Problems Explained – MEALYBUGS

Mealybug Description

Mealybugs are small, soft-bodied insects which have sucking mouthparts. The females are oval in shape and can be up to 5mm long. They are white or whitish-pink in colour and are generally covered in a white waxy material. They have filaments made of wax around the edge of their bodies and tails.

The most common species found in glasshouses are the citrus mealybug which has very short tail filaments and a central grey stripe, the vine or glasshouse mealybug which has a pair of tail filaments about half the body length and no stripe (see right) and the long-tailed mealybug which has waxy tail filaments as long as the body and a grey stripe.

Mealybugs (Planococcus Citrus) on an orchid leaf.

The males, if produced by the species, are small and elongate in the young stage and develop into delicate winged insects.

Mealybug Damage

Mealybugs are often found in clusters on stems and leaf whorls. They produce honeydew which often leads to sooty mould growth.

Large colonies can weaken the plant because of the amount of sap being taken which can result in yellowing leaves & defoliation.

Mealybugs feeding from the stem of a plant

Root-feeding species are seen as white patches among the roots when re-potting, especially around the sides of the root ball.  Care is needed to distinguish them from root aphids

Mealybug Life Cycle:

Most mealybug species lay eggs. However, the long-tailed mealybug gives birth to live young, whilst the citrus & vine mealybug produce quantities of “waxy wool” in which they can lay up to 500 eggs.

Egg laying may take up to 10 days and reduces the size of the female mealybug considerably. The female dies once she has completed laying her eggs.

Once hatched the young, or “crawlers” as they are sometimes called, are very mobile. They disperse rapidly and find suitable sites in which to feed and settle. The females continue to feed until they are mature enough to lay eggs.

The complete life cycle takes approximately 50 days at 20ºC (68ºF), this is reduced to 25 days at 30ºC, higher temperatures may inhibit egg laying.

Biological Control of Mealybugs.

Mealybugs can be controlled biologically in greenhouses or conservatories by using their natural enemies Cryptolaemus or Parasitoids or a combination of both.

Pest Problems Explained – APHIDS (Greenfly & Blackfly)

Aphids Description

Aphids are small soft bodied insects, which range in colour from yellow-green to dark-green, purple, brown and black dependant on species and the time of year, their bodies are pear shaped.  There are around 400 species in the UK each with their own range of host plants and usually requiring particular species of predators and parasitoids to control them. 

They live on plants in dense colonies and have both wingless and winged forms.

NB. Aphid cast skins are sometimes mistaken for whitefly! However, whitefly will fly away when disturbed, these cast skins will fall off or not move. Just shake the plant to find out which you have!

Aphid Damage

Aphids feed on plant sap and excrete a sticky “honeydew” on which growths such as sooty mould often develop.

They normally feed at shoot tips, which restricts and deforms plant growth.

Sooty mould reduces the amount of light reaching the plant leading to leaf yellowing and defoliation.

Aphids around the new growth on a plant.

Aphid Life Cycle:

Any aphid about to give birth is effectively three generations in one! Not only is the adult aphid about to give birth to immature aphids, but these already have the next generation of young developing inside them.

Young are produced at a rate of about 3 to 6 per day for several weeks. When aphid colonies become dense some wingless aphids will move off to find new places to produce their young.

The aphid colony will also start to produce a much higher proportion of winged aphids which are capable of finding new breeding sites further afield.

Aphids can generally over-winter in a greenhouse on weeds or crops.

Biological Control of Aphids

Aphids can be controlled biologically in greenhouses or conservatories by using their natural enemies such as Aphidius or Aphidoletes or a combination of both.

A Ladybird, Lacewing, Garden Predators Box can help to encourage beneficial insects to stay in your garden to help control aphids outside.