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Aphid control in peppers

Aphids are serious pests of peppers. Colonies are capable of phenomenal growth, and a single pepper plant or whole crop can be devestated seemingly overnight. As soon as an infection is noticed it should be dealt with – never delay and hope the aphids will go away, they won’t.

Aphids are small, oval-shaped sap-sucking insects. They normally have a green body, but they can be black – hence the common names “greenfly” and “blackfly” – and some species also occur in a red form. A large number of species are found in British gardens, and 14 of these have been identified on chilli and sweet pepper plants. However, three species are particularly common on pepper plants. The most widespread of these is the peach potato aphid (Myzus persicae). This species has a plump oval shape, and the adults are up to 2 mm long. They are normally green, but red versions do occur. Becoming increasingly common is the potato aphid (Macrosiphum euphorbiae). This species has a large (1.7–3.6 mm) and slender body, most are green and some may have a darker green strip running down the centre of their back. And finally, there is the glasshouse potato aphid (Aulacorthum solani), another large aphid measuring 1.8 – 3.0 mm.

Peach potato aphids on a pepper leaf

Peach potato aphids  (Myzus persicae) on the underside of a chilli pepper leaf.

The life cycle of an aphid

Aphids have a complicated life cycle, which includes both sexual and asexual reproduction, viviparous (giving birth to live young) and egg laying adults, and winged and unwinged forms. For the pepper grower it is important to understand how the pests overwinter, but a detailed understanding of their life cycle is only essential over the growing season.

In Britain the three main aphid species that attack peppers are polyphagous (feed on many different plant species). As long as temperatures are suitable, they may remain within the polytunnel or greenhouse residing on whatever plants are available – which often means using overlooked weeds. They also overwinter outdoors on whatever alternative host suits that particular species. For example adult peach potato aphids lay eggs on twigs of peach, plum and other related species.

During the growing season most greenhouse aphids are female and do not need a sexual encounter to reproduce. They do not lay eggs, but rather give birth to live young that are a perfect, though small, replicate of their mother. Not only are these newborns fully functional sap-sucking pests, but they are also pregnant – they actually have young developing inside them as they are being born themselves. In warm conditions the young aphids take about a week to grow and mature, before they too start giving birth to live, pregnant young.

Each aphid will give birth to about three to ten young every day for up to four weeks. Given these figures, in good conditions aphid populations can grow at alarming rates. When a grower notices signs of aphids on a pepper plant they should be dealt with immediately. If left alone it will not be long before there are thousands of aphids, and then hundreds of thousands.

The majority of aphids in a population are wingless, and they spread simply by walking from leaf to leaf and plant to plant. However, under certain conditions, particularly overcrowding, some aphids are born with wings. These can then fly away and infect a new plant elsewhere.

Damage caused by aphids

Aphids cause damage to the plants in several ways:

First and foremost, they weaken the plants by sucking the plant’s sap. Sap, a sugary solution that is passed around the plant, is the plant’s food, and any loss will reduce growth.

Secondly, as the aphids suck up large quantities of sap, they have to excrete the excess, which is a sticky solution called “honeydew”. The falling honeydew lands on the leaves and fruit below, which causes problems for the plants in two ways:

  • The honeydew is sticky, and when it covers the leaves they collect dust.
  • The honeydew is very sweet which attracts sooty mould growth, making the leaves turn black.

Black sooty mold caused by fungi growing on drippled honeydew

Black sooty mould growing on pepper leaf growing on honeydew dropped by aphids.

The effect of the dust and black covering of sooty mould on the leaves is to reduce the amount of light reaching the leaves. Without light photosynthesis cannot occur. Photosynthesis is the process within the leaves that makes the plants’ sugars, and is essential to the health and growth of the plants. In serious infestations little light reaches the leaves and photosynthesis will virtually be stopped.

The falling honeydew also makes the pepper fruit sticky and turn black, making them unappetising to the home grower and worthless for selling.

However, this is not the whole story; aphid saliva is also bad news for the plants:

  •  It contains toxins that cause the emerging leaves to be deformed. This happens more with the glasshouse potato aphid than with the other species.
  •  It carries viruses, and an aphid feeding on an infected plant will transfer the virus to all other plants it feeds on. 

 

Symptoms

Aphids congregate within the young leaves in the growing tips of peppers and on the underside of the mature leaves. This means they are well hidden, and go unnoticed when infestations are at an early stage. However, there are certain tell-tale signs that indicate their presence:

  • Distorted leaves emerging from the growing point.
  • Scattering of white skin casings under the plant.
  • Ants actively running over the plant.
  • Sticky leaves. In addition, the area around the plant can get sticky.
  • Leaves turn black from sooty mould.

The fallen white skin casings are a very clear sign of aphids; they are also the cause of a very common misdiagnosis. Young aphids shed their skins as they grow. Generally each juvenile will shed its skin four times before it reaches maturity. These skin casings fall to the leaves below, and as they dry they turn white. Many growers see these white forms on the plant and assume they have a whitefly problem. Though whitefly can attack peppers a serious infestation is rare – after growing chillies commercially for 22 years we have never had a whitefly problem on our peppers – even though we do regularly get whitefly on our tomatoes in neighbouring polytunnels.

A simple test is to shake the plant. Whitefly are literally white flies, and if the problem is whitefly the adults will fly up when the plant is shaken. Most aphids are wingless, and a hard shake may make them fall off, but they certainly will not fly.

Ants are often found near aphid colonies as they feed on the sweet honeydew. To protect their food source ants actively look after, or “farm”, the aphids, protecting them from insect predators.

Sticky leaves and sooty mould on the leaves are symptoms that occur when an infestion has become well established. For many growers the early tell-tale signs are harder to notice so these are often the first symptoms noticed. However, these symptoms will affect the plants' ability to grow and immediate action to control the aphids is essential.

Control

An aphid infestation must never be ignored. Aphids can be controlled quite easily, but it is an active process, they will not go away by “forgetting” they are there.

There are three approaches to control for a home chilli grower:

  1. Biological control
  2. Water
  3. Pesticides

 

Biological control

There are several insects that prey on aphids. The species that most gardeners are familiar with include:

  • Ladybirds
  • Lacewings
  • Hoverflies
  • Parasitic wasps

 

ladybird larvae eating an aphid
Ladybird larvae enjoying an aphid meal.

 Many of these “good guys” occur naturally in the garden, and a gardener should do their best to encourage them. It is well worth a gardener learning the life cycle of these insects, but most importantly, becoming familiar with what they look like. Of course, most people, easily recognise the adult stages of ladybirds, lacewings and hoverflies, but very few gardeners know what the eggs, pupa and juvenile stages of these species look like. An hour on the internet can easily remedy the situation, and would probably be one of the most valuable hours spent on the garden!

Hoverfly on a pepper leaf
Adult hoverfly

 

Ladybird larvae on a chilli pepper plant Ladybird pupae on a chilli pepper leaf
Ladybird larvae (left) and pupae (right) on a chilli pepper leaf.

Most outdoor areas will have some beneficial insects in residence that will maintain some level of aphid control without the gardener ever knowing. However, the indoor environment of a windowsill is unlikely to have any insect predators occurring naturally, and chilli plants kept indoors as a house plant will have no defence against an aphid infestation. Still it never hurts to bring in any ladybird found outdoors to reside on a favourite plant!

Adult and larvae ladybirds on fathen
Adult and juvenile ladybirds eating aphids on fathen, an outdoor weed.

As well as maintaining an envirionment that encourages the presence of naturally occuring insect predators, it is possible to buy-in predators and parasites. These are sent out through the post, and once released into the polytunnel or greenhouse, will provide aphid control and bred themselves thus increasing their populations. Though not cheap this is the method recommended for all gardeners with plants in a polytunnel or greenhouse.

For the home gardener and small commercial operations we recommend Agralan (www.agralan.co.uk), a company that is dedicated to protecting crops through biological control. For large commercial growers whose orders will satisfy the minimum order restrictions, there are a number of companies that provide a complete control programme through biological control.

 

Releasing parasitic wasps Releasing parasitc wasps
Releasing parasitic wasps into a polytunnel with a chilli pepper crop.

Aphid mummies Aphid mummy with the parasitic wasps escaped
Left: aphid mummies formed by the parasitic wasps laying an egg in the aphid.

Right: a single aphid mummy with with a round hole where the parasitic wasps escaped.

 

releasing ladybird larvae
Releasing commercially bought ladybird larvae into a crop of Hungarian Hot Wax chillies.

 

Growers who take their chilli growing very seriously are recommended to start buying in predators before they see any signs of aphids. This army of “good guys” will seek out the aphids and deal with them long before even an alert grower is likely to find them.

The mantra to follow for biological control is that it is a “numbers game”. If there are enough beneficial insects, aphids will be controlled. If predator numbers are too low, the aphid colonies will grow. The later you start using biological control the more predators or parasitoids will be needed.

Unfortunately, insect predators are not a realistic option if you keep just one or two chilli plants on a windowsill in the house or conservatory. See below for more information on controlling aphids on house plants.

 

  • Water

Prevention is always better than cure, and the predator option is undoubtedly the best method of aphid control. However, if an infestation does occur it means there has been an imbalance between predator and aphid populations, and a more proactive control method is necessary. In other words, once an infestation has become established, whether it is on a single plant in the house or in a large commercial greenhouse, will not clear up on its own, the plants will not recover without an intervention.

Before resorting to pesticides we recommend trying water as a control method. Water is safe, effective and because of the sticky honeydew, aphid-infected plants will always need to be washed down anyway. Aphids do not hold on tight and a blast of high pressure water – a finger on the end of a hosepipe or the cold water tap on full – will knock them off the plant. Most effort should be applied to the growing points and the underside of the leaves as this is where the aphids congregate. If the plant is in a pot lying it on the ground or turning it upside down will give full access to under the leaves. Water does not kill aphids and they will readily walk back onto the plant. So if the plant is in a pot do not wash it near where it is normally kept and wash down the area before returning it to its place. If the plants are in the ground this is not so easy, but if it is possible wash/brush down the area as well as the plant. 

It is unlikely that one wash, no matter how thorough, will remove all the aphids. So to get a complete control repeat the operation the next day....  and the next if necessary. Some customers say washing doesn't work; most likely the failure was due to a half-hearted wash; a thorough wash means THOROUGH.

 

  • Pesticides

Pesticides are substances or a concoction of substances that kill  pests. They can be either natural or synthetic and they work in different ways; some are potent poisons, others cause death through a physical process. Some have very specific target pests, others are generalists and an application will have a universal kill. All should be used carefully and with a full understanding of how to use the pesticide and the objective of the application.

When buying a pesticide always read the label. Be sure it is recommended for use on peppers, and if you keep the plant in the house, it must state that it is suitable for "houseplants". In addition, the pesticide must state it is to be used to kill aphids (the packaging may say "greenfly" or "blackfly"); remember whitefly is not another name for aphids. All retail stores selling garden pesticides to the home gardener must have someone at the available who can advice you on the use of pesticides. Commercial growers are governed by different laws, they can buy stronger pesticides from suppliers that are unavailable to home gardeners, but now must take specific pesticide application courses and then be registered.

Chemical pesticides: These are powerful, toxic chemical pesticides can be bought to kill aphids. A perusal in any garden centre will reveal a whole host of brands available to the home gardener. Many are systemic, which means they work by being absorbed into the plant so the aphids are killed when they digest the poisoned sap. Unfortunately, these pesticides (poisons) will be transported throughout the plant, including the parts of the plant we eat. It is essential the instructions are followed carefully, including the withdrawal period, i.e. the time that must elapse between applying the pesticide and consuming the peppers picked off the plant. The plus side of systemic pesticides is that they specifically target the sap-sucking pests. However, you should check the compatibility of individual pesticides with natural predators as some chemicals can kill natural enemies for up to 8 weeks (check the biopest side effects list).

Physically acting pesticides: There are several compounds that are relatively benign to the environment and humans. They work through a physical action taking advantage of the insects' small size or physical shape. Many are so safe they are considered acceptable for organic growers. A common active ingredient is a fatty acid, but there are others, e.g. SB Plant Invitorator, which kills simply through a physical action – the packaging states:  "aphids, juvenile whitefly and spide mite if directly hit are trapped by its wetness".

A search on the internet will reveal several companies that sell these pesticides, but the one we recommend is: www.harrodhorticultural.com

As these pesticides work through a physical action they can only be  effective when a 100% coverage is achieved – this is so important the instructions on some state the liquid should be applied until it drips off the plant.

 

Applying pesticide to kill aphids on chilli peppers

Applying Eradicoat, a sticky solution made from cornflour, to kill a bad infestation of aphids on a poblano chilli crop.

Unfortunately the physically acting pesticides kill all species of small insects and mites, including the beneficial species. Consequently, where beneficial insects are present in a crop it is often recommended to only apply the pesticide where the aphid colonies are worst. More beneficials can then be released as soon as the crop has dried after spraying physical products as there are no residual effects.

 

Aphids killed by eradicoat

Dead aphids after an application of Eradicoat; the "aphid mummies" are not hurt by the Eradicoat.

Some people make up their own home-made concoction to kill aphids. A common one is diluted washing up soap. Washing up liquid contains many more chemicals than just soap, some of which are bad news to a growing plant. So while such a solution may kill the aphids it may also kill the plant. In addition, under UK and EC law it is illegal to use any home-made mixture as a pesticide – the law states only preparations approved as a pesticide can be used specifically to kill pests.

 

The chilli plant as a house plant

Certain chilli species are very attractive when grown in pots, and as they like the warm, sunny conditions typical of a sunny windowsill or conservatory they do make excellent houseplants. Unfortunately, chilli house plants are not spared the risk of an aphid infection. As aphids reproduce asexually it only takes a single aphid to get into the house – maybe a winged one through an open window or possibly hitching a lift on a human or pet – to start an infestation. The home environment does not encourage the presence of natural preditors, so once an aphid colony has become established it is likely to grow at a frightening speed and quickly destroy a plant.

Chillies as attractive houseplants

Chillies make very attractive houseplants.

As a precaution it does not hurt putting ladybirds on a chilli house plant as they will seek out and eat any aphids that arrive. However, once an indoor plant is infected the best thing is to give the plant a thorough wash. It is unlikely all the aphids will be removed by a single wash, so repeat the wash every day for the next three or four days.

Summary

All pepper growers will experience an infestation of aphids on their plants at some time in their growing career. There is simply no way of avoiding the pest, and anyone who has not had a problem with aphids just haven’t YET had a problem.

But an aphid invasion is not the end of the world. Do not throw away your precious house plant – as some people do. Nor should you allow the aphid population to continue to grow. Deal with the aphids and reclaim the health of your plant.

 

References

  • Knowing and recognizing: The biology of glasshouse pests and their natural enemies. M.H. Malais and W.J. Ravensberg. Published by Koppert B.V. ISBN: 90 5439 126
  • www.agralan.com
  • www.defenders.co.uk

 

Thanks to Clare Sampson, consultant entomologist for BCP Certis., for reading this article and checking for accuracy.

 

© Joy Michaud