APHIDS

Most of the technical tools (keys) used for aphid identification are based on wingless adults (apterae), although there are some based on winged forms (alates). Immature stages (nymphs) are usually much harder to correctly identify to species level. 

Species recognition usually depends on a combination of host plant information plus several morphological characters, including the shape and length of the siphunculi (paired tubular structures towards the rear of the abdomen; also known as cornicles)(see picture, right), the shape and length of the cauda (the short projection between the 2 siphunculi), the size and shape of the frontal tubercles (projections on the inner surface of the base of the first antennal segment), and other characters such as the sensillae (sensory receptors) on the antennae, the relative lengths of the two parts of the final antennal segment, and various others. Clearing and slide-mounting of specimens and a good compound microscope are usually needed to see most of the latter. Please see our handy guide to help you locate these key characters!

However, on most commercial crops, only a limited number of aphid species are usually encountered, which makes identification much simpler: please see our color guide to some of these species.  Sierra Biological is happy to help our regular customers with aphid identification if you have any additional questions.

As far as selecting appropriate biological controls is concerned, the parasitoids of aphids (Aphidius and Aphelinus species) are more specialized than are the predators (e.g. Chrysoperla and Aphidoletes), because the parasitoid larvae must develop within the living host and must therefore be able to overcome the aphid’s internal defences (immune system). 

In selecting a suitable parasitoid species, it should be kept in mind that most crop aphids fall within two tribes of the same sub-family (Aphidinae): the tribes Aphidini and Macrosiphini (pictured below). Of these two tribes, the first (Aphidini) tend to be smaller and rounder in shape than the Macrosiphini, with much shorter siphunculi. Examples of aphids within the tribe Aphidini include the cotton-melon aphid (Aphis gossypii) and other Aphis species, while the Macrosiphini includes the green peach aphid (Myzus persicae), the cannabis aphid (Phorodon cannabis), the foxglove aphid (Aulacorthum solani) and various others.

Compared to aphids in the tribe Aphidini, those in the tribe Macrosiphini are larger, have more spindle-shaped bodies (i.e., broadly oval, tapering at both ends), and usually much longer siphunculi and a more prominent/longer cauda.

Of the two Aphidius species we offer, Aphidius colemani is typically the best choice for aphids in the tribe Aphidini, while Aphidius ervi tends to prefer aphids in the tribe Macrosiphini. Aphelinus abdominalis will attack species in both tribes, but is generally considered to prefer aphids in the tribe Macrosiphini. It can therefore be complementary in action to the two Aphidius species.

 Aphids are small, soft bodied insects that feed by sucking plant sap. Their feeding cannot make visible holes in plant tissues, but because they ingest more sugars than proteins, excess plant sugars are excreted in the form of sticky ‘honeydew’ that is flicked away onto surrounding leaves.

When aphids are present in large numbers, their honeydew may attract ants (as pictured), which in turn defend the aphids from natural enemies. If ants are present in a crop with aphids, they can disrupt biological control. Ant control or exclusion may be needed in such circumstances.

Under conditions of high humidity, black ‘sooty mold’ may also grow on any honeydew coating the leaves, reducing the amount of light reaching the leaf surface and hence reducing photosynthesis as well.

Under open-field conditions, most aphids alternate between herbaceous (i.e., non-woody) summer host plants and one or more woody winter hosts (trees or shrubs). The species of plant hosts attacked by individual aphids is characteristic of each aphid species, with some being highly specialized (e.g. cannabis aphid (Phorodon cannabis)) and others (e.g. green peach aphid (Myzus persicae)) much less so. 

In the summer months, under outdoor conditions, the aphid life-cycle takes the form of a repeated asexual phase, in which the entire population is all female and reproduction is by the production of live female offspring; no mating or egg production occurs until the autumn, when decreasing temperatures and daylength stimulate the production of specialized sexual forms (both male and female), which migrate to the winter woody host(s), where mating and egg-laying occur. The egg stage is the overwintering form under outdoor conditions. The eggs hatch in spring to give rise to a new, all-female population, the offspring of which eventually disperse to the summer host plant(s).

However, under indoor conditions in which constant ‘summer’ temperature and light conditions are maintained, the sexual phase does not occur, and the aphids remain in the asexual phase, with the consequent ability to rapidly increase in numbers if left unchecked. 

In the early stages of an aphid infestation, almost all offspring develop into wingless forms (known as apterae). However, when aphid population densities increase, the quality of the host plant tends to decline, and this, together with the increasingly crowded conditions in the aphid colonies, causes the production of offspring that mature into winged (alate) forms. These alates are specialized for dispersal, and each one has the potential to start a new colony on a new plant.

For this reason, successful biological control depends very much on good, regular monitoring, so that aphid colonies are detected early enough to allow sufficient time to select, purchase, and release appropriate natural enemies before alate production begins. 

Regular visual inspection of the crop should focus on the young growing tips and the underside of leaves close to those points, as these tissues tend to be the highest in nitrogen and hence are preferred feeding sites for aphids and other sucking pests. 

Yellow sticky cards are useful for detecting winged forms, but will not reveal the presence of apterae. 

When aphid colonies are actively increasing, white flecks may be seen on the leaves; these are the remains of the outer cuticles shed by growing aphids when they molt to the next stage.