Joy Michaud’s guide to control of aphids in pepper plants

by Joy Michaud on March 16, 2012 · 3 comments

in General Information

Aphids are serious pests of peppers. Colonies are capable of phenomenal growth, and can devastate a single plant or a whole crop in what seems 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 particular species are commonly found 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. Increasingly, another, slender, but longer bodied species is being found in greenhouses and polytunnels where they can have a devastating effect on pepper crops. This is the potato aphid (Macrosiphum euphorbiae). These are large (1.7–3.6 mm), most are green and some may have a dark 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– 3.0mm.

A colony of peach potato aphid on the underside of a pepper leaf, with adult and juvenile aphids.

A colony of peach potato aphid on the underside of a pepper leaf, with adult and juvenile aphids.

Life cycle

Aphids have a complicated lifecycle, 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 new-borns 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 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 the 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:

1. They feed by sucking the plant’s sap. Sap is a sugary solution that is passed around the plant. It is the plant’s food, and any loss will reduce growth.

2. The aphids suck up large quantities of sap, and excrete the excess as “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 mold growth, making the leaves turn black.

The effect of the dust and black covering of sooty mold 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 no light can reach the leaves and photosynthesis will be stopped altogether.

Blackened leaf from sooty mold on the fallen honeydew.

Blackened leaf from sooty mold on the fallen honeydew.

The sticky honeydew also reduces the quality of the fruit.

3. But this is not the whole story, aphid saliva is also bad news for the plants:

  • The saliva contains toxins that cause the emerging leaves to be deformed. This happens more with the glasshouse potato aphid than the other species.
  • The saliva can carry viruses, and an aphid can transfer a virus from an infected plant to a healthy one.

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 always indicate their presence:

  • Distorted leaves emerging from the growing point.
  • Scattering of white skin casings under the plant.
  • Ants very actively running over the plant.

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 18 years we have not yet had a whitefly problem on our peppers but we do regularly get whitefly on our tomatoes.

White skin casings that are often confused with whitefly.

White skin casings that are often confused with whitefly.

A simple test is shake the plant. Whitefly are actually white flies, and if the problem is whitefly the adults will fly up when the plant is shaken.

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

Control

An aphid infestation must never be ignored. Aphids can be controlled quite easy, 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
Adult hoverfly

Adult hoverfly

Biological control

There are several insects that prey on aphids. These include:

  • Ladybirds
  • Lacewings
  • Hoverflies
  • Parasitic wasps
Ladybird larvae

Ladybird larvae

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 adults 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!

Adult and larvae ladybird eating aphids on outdoor weed

Adult and larvae ladybird eating aphids on outdoor weed

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 have any insect predators occurring naturally, and chilli plants kept indoors as a house plant will have no defense against an aphid infestation. Still it never hurts bringing in any ladybird found outdoors to reside on a favourite plant!

Releasing parasitic wasps

Releasing parasitic wasps

It is possible to buy insect predators and parasites that once released into the polytunnel or greenhouse will provide aphid control. The website www.defenders.co.uk is a very good source for amateur gardeners. For commercial growers there are a number of companies that provide a complete control programme through biological control.

Aphid mummies caused by parasitic wasps

Aphid mummies caused by parasitic wasps

Growers who take their chilli growing very seriously are recommended to buy 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.

Water

Prevention is always better than a cure, and the predator option is undoubtedly the best method of aphid control, especially for growers with more than a couple of plants. However, if an infestation does occur than go for the water option before resorting to pesticides. Water is safe, effective and because of the sticky honeydew, aphid-infected plants will always need to be washed 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 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 the under 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. If the plants are in the ground then a brush down after the wash will help.

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.

Pesticides

Pesticides are substances or a concoction of substances – which can be either natural or synthetic – that kill pests. 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.

Chemical pesticides. 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 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 biobest side effects list).

Physically acting pesticides. These compounds are relatively benign to the environment and humans. The two most common for aphid control are:

  • Savona insecticidal soap
  • Majestik/Eradicoat

Savona is a preparation made from a soft soap. It is bought in a concentrated form and must be diluted before use. For best results soft water should be used to make the dilution. Any grower in an area with hard water will have to use collected rain water. Though completely safe to humans Savona should be used sparingly as too many applications will damage the waxy surface of pepper plant leaves. A search on the internet will reveal several companies that sell Savona, but two reliable ones are:
www.harrodhorticultural.com
www.organiccatalogue.com

Applying eradicoat to a aphid infested pepper crop

Applying eradicoat to a aphid infested pepper crop

Majestik and Eradicoat are different names for the same product, which is a sticky solution made from corn flour. It does not damage the plant, and its performance is not affected by the type of water used to make the dilution. It works through the physical action of blocking the aphids’ breathing pores (spiracles) causing them to suffocate. It is more effective when used on a sunny day as the solution dries faster. Unfortunately, Majestik/Eradicoat is not as widely available as Savona. The website www.defenders.co.uk used to sell Eradicoat, but it is not available from their website at the moment; it can, however, be bought from www.bcpcertis.com in commercial quantities. Majestik can be bought in commercial quantities from www.interhort.com

Majestik/Eradicoat and Savona are only effective with when a 100% coverage is achieved – this is so important the instructions on Eradicoat state the liquid should be applied until it drips off the plant. If the infected plants are in small pots then one way to guarantee full coverage is to make up a full bucket of the solution and submerge the plant in it, being sure to give a shake to remove all air pockets.

Unfortunately the physically acting pesticides kill many species of small insects and mites, including some 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.

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.

Summary

Healthy pepper plant var. Tequila

Healthy pepper plant var. Tequila

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.

Joy Michaud
Sea Spring Seeds
www.seaspringseeds.co.uk

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

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
http://www.bcpcertis.com
http://www.defenders.co.uk

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{ 1 comment… read it below or add one }

avatar Ross Mclardy March 16, 2012 at 11:46 am

Great article. Good resource for what to do against pesky bugs A quick question tho. Have you any experience or knowledge of SB Plant Invigorator? (Sbproducts.co.uk) its touted as a non biological pesticide and foliar nutrient and seems OK to eat fruits even straight after spraying. Just unsure how it works, or indeed if it does.?

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