Aphids in Greenhouse Crops
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Table of Contents
- Description and Life History
- Management Strategies
- Related Links
Aphids are small (2-3 mm), soft-bodied insects with long legs and antennae.
A pair of tube-like structures called cornicles project from the posterior
end. Several species may infest greenhouses, with colour patterns ranging
between black, grey, red, yellow and green. The species most commonly
found in greenhouse crops are the green peach aphid (Myzus persicae) (Figure
1), the cotton or melon aphid (Aphis gossypii) (Figure
2), the potato aphid (Macrosiphum euphorbiae) (Figure
3) and the foxglove aphid (Aulacorthum solani) (Figure
Adult aphids are predominantly wingless, although winged adults (Figure
5) can develop under conditions of high population density. This adaptation
serves as a dispersal mechanism, allowing aphids to move into a greenhouse
from outside, or to spread rapidly within a greenhouse.
Description and Life History
The aphid life cycle outdoors is quite complicated, at times involving
sexual reproduction and egg-laying, while at other times, only females
are present, reproduction is asexual and offspring are born live (Figure
6). In the greenhouse however, the life cycle is usually very simple.
All individuals are female. They give birth to live young, which in turn
can reproduce within 7-10 days. Individual aphids can give birth to 60-100
young (depending on host plants and nutritional status) over a 20-day
period. Aphid numbers can rapidly build up to very large populations.
Identification of aphid species is critical to successful control, especially
biological control, but also in some cases with chemical control. The
green peach aphid and the melon aphid used to be the most common species
found in Ontario greenhouses. However, since the late 1990s, the potato
aphid and the foxglove aphid have become much more common.
There are several thousand aphid species that have been described worldwide.
Positive identification can be very time consuming and complicated and
is best left to researchers or extension specialists. Because there are
only a few species that are most likely to be found in a greenhouse, there
are a number of things growers can look for to help distinguish between
them. A hand lens (10x) is needed to see some of these characteristics.
If in doubt, consult a specialist in your area for advice.
Colour and General Appearance
Be aware that colour on its own is not an accurate way to identify aphids.
While it may be useful in combination with other features (see below),
do not use colour as the only diagnostic tool. The green peach aphid is
commonly a light greenish-yellow, but it can be a darker green or sometimes
a pink/rose colour (Figure 1). Melon aphids are a
smaller species, and are often a very dark green (in Ontario, growers
often refer to them as 'black' aphids), but this too can vary, and it
is not uncommon to see 'black' aphids that are green, yellow or mottled
(Figure 2). The potato aphid is a large, very active
aphid, usually green, but can be variable in colour and often has a darker
stripe down the middle of its back (Figure 3). The
foxglove aphid is green, often quite shiny and with two darker patches
on its abdomen at the base of the cornicles (Figure 4).
Occasionally, less common aphids are also seen. Figure
7 shows the chrysanthemum aphid that is shiny and dark brown in colour.
The cornicles are two tubular structures that emerge from the end of the
abdomen. On green peach aphids, these are the same colour as the body,
slightly flared and darker at the tip. In melon aphid, the cornicles are
shorter, and dark throughout their length, regardless of their body colour.
In potato aphid, the cornicles are long and thin and may be curved outward
at the tips. They are the same colour as the body. The foxglove aphid
has cornicles of medium length, darkened at the tips.
Green peach aphid.
Figure 2. Cotton
(or melon) aphid (note black cornicles).
Figure 4. Foxglove
Figure 6. Foxglove
aphid giving birth to live young.
At the base of the antennae on aphids are two bumps called tubercles.
The shape of these tubercles is also used to determine identification;
however it is usually necessary to have a microscope to see them clearly.
Aphids pierce plant tissue with their mouth parts and suck out the sap,
causing deformed leaves and flowers (Figure 8a and
They excrete a sugary, sticky substance called honeydew, which promotes
the development of black, sooty mould fungus on the leaf surface. Sooty
mould is not pathogenic on the plant, but in severe infestations, it
can interfere with photosynthesis (Figure 9).
- Aphids can transmit plant virus diseases (e.g. cucumber mosaic virus).
- In ornamentals, the presence of the aphids themselves, their cast-off
skins, honeydew and sooty mould is largely responsible for their pest
status. For these reasons, they usually require control long before
actual plant damage occurs (Figure 9, Figure 10).
Distortion of young pepper leaves due to feeding by foxglove
Yellowing of older pepper leaves due to feeding by foxglove aphids.
Figure 9. Sooty
mould growth on leaves as a result of honey dew excretion by aphids.
Figure 10. Cast
off aphid skins on tomato.
The rapid reproductive rate of aphids and the potential damage that they
can cause dictate the need for early detection and timely implementation
of management strategies. Integration of several approaches or strategies
is often necessary for optimum suppression of aphid populations and avoidance
of resistance development. Important strategies for managing aphids are
Monitoring for Aphids in the Greenhouse
Monitoring for aphids involves two different strategies:
- Visual observation - It is important to carry out visual observation
of the crop on a regularly scheduled basis for wingless aphids on plants.
Look for insects and the small white flakes that are the cast-off skins
of aphids as they moult. Initial infestations are usually at isolated
points within the greenhouse, but can rapidly spread to other areas
if allowed to develop unchecked. Heavier infestations can result in
the presence of honeydew on the leaves, giving them a shiny appearance
and a sticky feel. Ants are often attracted to the honeydew, and their
presence can also suggest an aphid infestation. Different plant species
and varieties are more or less attractive to aphids. Use varieties or
crops known to be susceptible to aphids as indicators for early detection.
This can make monitoring more time-efficient and effective.
- Monitoring aphids using yellow sticky cards - This method is very
effective in detecting winged aphids. The presence of winged aphids
on sticky cards indicates one of two possibilities:
a. There has been movement into the greenhouse of aphids from outside.
This often happens in the
spring and fall as migrating aphids begin flying in warm weather.
b. An infestation within the greenhouse has developed to the point
where winged individuals are being produced and dispersing to find
new host plants (an effective crop inspection program should identify
these infestations before they reach this stage).
A number of biological control agents are readily available: the parasitic
wasps Aphidius spp. and Aphelinus abdominalis, the predatory midge Aphidoletes
aphidimyza, and ladybeetles (Hippodamia convergens, Harmonia axyridis)
are the most common. Lacewings and praying mantids are more generalist
predators and also available for aphid control. Aphidoletes and ladybeetles
are usually used to supplement the activity of Aphidius and for reducing
aphid populations in 'hot spot' areas.
Aphidius colemani on sticky card.
This parasitic wasp does not enter diapause and is usually more effective
during winter, early spring, and fall. During summer, other parasitic
wasp species can parasitize Aphidius reducing their impact on aphid populations.
Optimum conditions for Aphidius are 18-25oC and 80% RH. Aphidius completes
its development from egg to adult in about 10 days at 25oC, and 14 days
at 21oC. Three species of Aphidius are commercially available. One is
Aphidius matricariae, which can parasitize about 40 aphid species including
the green peach aphid. Aphidius matricariae has been largely replaced
by Aphidius colemani (Figure 11), which is effective
against both the green peach aphid and cotton aphid. Aphidius ervi is
a larger species used against potato and foxglove aphids. Aphidius wasps
lay their eggs inside the aphid. As the wasp develops, the aphid changes
colour and appearance, becoming swollen with a bronze colour and a papery
texture (Figure 12). This parasitized aphid is
known as a mummy. The new adult wasp emerges from the mummy (Figure
Aphidius emerging from an aphid mummy.
Banker plant in pot hanging from greenhouse support post.
Aphidius species are best used when aphid numbers are very low.
To facilitate continuous release of low numbers of these species, many
growers use "banker plants" that essentially consist of seedlings
of a cereal species (e.g. rye) (Figure 14). These
seedlings are host to cereal aphid species that do not attack non-cereal
crops, and the cereal aphids in turn are hosts or food for the parasitic
wasps. Research indicates that for optimum results, evenly distribute
banker plants throughout the greenhouse, with a distance of between each
banker plant ideally not greater than 40 m.
The main aphids that Aphelinus abdominalis (Figure
15) attacks in the greenhouse are potato and foxglove aphids. This
wasp prefers to parasitize the 2nd and 3rd nymphal stages while the 1st
and small 2nd nymphal stages are used for host-feeding (i.e. as food by
adults). To feed on an aphid, the wasp first pierces the aphid with its
stinger or egg-laying body part, and then feeds on the aphid's body fluid
through the tiny opening(s).
Aphelinus abdominalis adult.
In contrast to A. colemani, egg laying activity is low during the first
few days of this wasp's life. And then by the 4th day after emergence,
an adult female can lay 10-15 eggs per day for the rest of its life of
15-27 days. During this time, an adult female may parasitize more than
200 aphids and kill about 40 by host-feeding. Because adults prefer to
walk or hop rather than fly over the crop, they tend to remain localized.
Studies have shown that dispersal by this wasp is poor in the greenhouse,
and that most remain close to their points of release. This means these
wasps should be released as close as possible to aphid infestations for
best results. Note that aphids parasitized by A. abdominalis appear black
while those parasitized by Aphidius species are bronze.
Adult Aphidoletes aphidimyza resemble small mosquitoes whose larvae are
the predatory stage. Females lay eggs close to aphid colonies so that
upon hatching, the orange-coloured larvae have a readily available food
source (Figure 16). Eggs usually hatch after 2-3
days, the larval stage lasts 5-7 days after which they drop to the floor
to pupate. The pupal stage usually lasts about 8-10 days. Adult A. aphidimyza
feed on honeydew and are non-predatory. The larvae can kill between 10-100
aphids in total. A particularly positive characteristic of Aphidoletes
is that unlike parasitoids, it causes little disturbance in colonies.
Because of its furtive behaviour, it triggers little defensive reaction
by aphids. This means aphids attacked by Aphidoletes are less likely to
disperse, escape predation, and start new colonies. When aphids are attacked
by parasitoids, they defend themselves by kicking and producing alarm
pheromones (chemicals used for communication within a species), resulting
in their own escape, as well as many other members of their colony.
Aphidoletes larvae on aphid infested leaf.
Under natural daylengths, A. aphidimyza enters reproductive diapause between
September and March because the larvae require at least 15.5 hours of
light to prevent the pupae from diapausing. However, there is some evidence
to suggest that regular preventative releases of Aphidoletes can be made
throughout the winter to control aphids. The Aphidoletes adults lay eggs
and the larvae feed on aphids, however there is no second generation of
midges produced. If lighting can be supplemented, even low light intensities,
such as from incandescent bulbs, are sufficient to prevent diapause.
Adults are nocturnal and require a period of darkness for mating and
egg-laying. Therefore, continuous lighting from a bright source will prevent
reproduction. Likewise, lighting that eliminates dusk can also interrupt
mating. It is also important to note that larvae drop to the ground and
use grains of sand and possibly soil debris to form cocoons. If the larvae
fall on plastic or concrete that is dry and free of debris, mortality
of this predator will be high. Repeated or continuous release using banker
plants is necessary under such situations to achieve acceptable suppression
Two species of ladybeetles are commonly produced for control of aphids.
Harmonia axyridis (Figure 17), also known as
the multi-coloured Asian ladybeetle, is an introduced species that can
provide excellent control. However, it has developed a bad reputation
because of its establishment and development of huge populations in the
outdoor environment, its pest status on crops such as grapes, and its
displacement of native ladybird beetle populations. Some biocontrol producers
have stopped supplying this species because of its poor public image.
The second species, Hippodamia convergens (Figure
18), is a native North American species collected in the wild in California.
17. Multi-coloured Asian ladybeetle, Harmonia axyridis.
Convergent ladybeetle, Hippodamia convergens.
Both adult and larval ladybeetles feed on aphids. When daylengths are
suitable, ladybeetles must feed on aphids to maintain egg production.
Eggs are torpedo-shaped, orange-coloured, laid in circular clusters on
the underside of leaves, and hatch in 2-5 days. The larval stage lasts
for about three weeks after which they pupate. Adults emerge from pupal
cases after 3-5 days. To increase the percentage of ladybeetles remaining
in the greenhouse, make releases late in the evening, and sprinkle a sweet
liquid (e.g. diluted pop), over the beetles. The sweet liquid provides
an immediate energy and water source.
Lacewings (Chrysoperla spp.)
Lacewings also feed on a variety of prey including aphids, thrips, spider
mites, young caterpillars and moth eggs, mealybugs, scales, and whitefly
larvae and pupae. However, they do have a preference for aphids over thrips,
and then spider mites. Older larvae (3rd instar) are particularly voracious,
(Figure 19) and can eat unhatched eggs, other
larvae, and even adults if food is scarce. A larva can consume 300-400
aphids and are usually best suited for high aphid population situations.
If there is excess food, more prey will be killed than consumed. Adults,
on the other hand, feed only on honeydew, nectar and pollen.
19. Lacewing larva feeding on aphid.
Because aphid populations can build up so rapidly, a regular monitoring
program is essential for early detection and control. When detected early
(before winged adults are produced), control can often be achieved by
spot treatments, or removal of infested plants. There are a number of
registered pesticides for control of aphids in both vegetables and ornamental
greenhouses, and some of these are compatible with biological control.
Please refer to OMAFRA Publication 370, Production Recommendations for
Greenhouse Floriculture, and Publication 371, Growing Greenhouse Vegetables.
General Control Strategies
- Remove weeds from within, and immediately outside, the greenhouse.
Aphids can develop on many different species of weeds commonly found
in the greenhouse, and these can provide an ongoing source of infestation
that may never be covered by pesticide sprays.
- Spot treatments or removal of isolated infestations which are detected
early, can prevent their spread to the rest of the greenhouse.
- Make use of resistant varieties or crops if possible, or use susceptible
varieties as indicators for monitoring or as a check on the effectiveness
of control treatments.
- Consider the use of insect screening to eliminate the movement of
aphids from outside into the greenhouse. For more information see OMAFRA
Factsheet Order No. 00-021, Screening of Greenhouses for Insect Exclusion.