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
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.
1. Green peach aphid.
2. Cotton (or melon) aphid (note black cornicles).
3. Potato aphid.
4. Foxglove aphid.
5. Winged aphid.
6. Foxglove aphid giving birth to live young.
7. Chrysantheumum aphid.
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
Aphids pierce plant tissue with their mouth parts and suck out
the sap, causing deformed leaves and flowers (Figure
8a and 8b).
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
- Aphids can transmit plant virus diseases (e.g. cucumber mosaic
- 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
8a. Distortion of young pepper leaves due to feeding by
8b. Yellowing of older pepper leaves due to feeding by
9. Sooty mould growth on leaves as a result of honey dew
excretion by aphids.
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 outlined below.
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.
11. 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 13).
12. Aphid mummy.
13. Aphidius emerging from an aphid mummy.
14. Banker plant in pot hanging from greenhouse support
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).
15. 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.
16. 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 of aphids.
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.
18. 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
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.