Why aphid-hunting is important

Some aphid species are very important pests, whether of agricultural crops, ornamental plants or forestry trees. Accordingly, we know a great deal more about those few pest species than we do about the others, and our perceptions are biased accordingly.
 

Aphid pest-species - need to be controlled effectively

One obvious reason for looking for aphids is to reduce their numbers on our crops and garden flowers, or to "nip outbreaks in the bud".

Owing to their parthenogenetic reproduction and winged adults, and consequent high reproductive and ability to invade temporary habitats, aphids are known as classic 'r-strategists'.

• The 'r' refers to the rate of increase of the population.

This view is true of many polyphagous aphid pest species which do especially well in disturbed transitory habitats - such as farmers' fields. Black bean aphids Aphis fabae (pictured below) are a good example.

Similarly, each summer innumerable winged cereal aphids Sitobion avenae, transported to the upper atmosphere by thunderstorms on the continent, arrive in the UK and rapidly reproduce. Such aphids are common because they exploit a specially favourable environment - large numbers of young rapidly growing plants. Other commonly observed aphids have an abundant but genetically non-diverse host such as sycamore aphids, nettle aphids and rose aphids (pictured below).

Several highly polyphagous greenhouse pest aphids such as Aulacorthum solani and Macrosiphum euphorbiae exploit a specially favourable environment. Yet outside a greenhouse environment they seldom attain such a predominant position.

Given their potential for rapid dispersal and reproduction, many people assume the only reason to search for aphids is in order to destroy them - so they treat all aphids in similar fashion. That simple approach can easily make the pest situation worse....

For example in UK four aphid species occur commonly on apple: 

1. Woolly apple aphids Eriosoma lanigerum (below) are a potentially damaging pest.

   

   They are not as easy to control as some aphids, and insecticide control should only be applied if this pest is known to be present on at least 4% of trees. In gardens a better option is to encourage their natural enemies.

2. Apple-grass aphids Rhopalosiphum oxyacanthae (below) are common early-spring, but usually leave by mid-May and cause little if any damage.

   

   Spraying them is probably pointless or counter-productive - the insecticide kills insects that naturally control aphids.

3. Green apple aphids Aphis pomi (below) look rather like R. oxyacanthae but have longer black siphunculi.

   

   Small colonies cause little damage. Large dense colonies on young trees should be controlled by sprays that do not harm natural enemies.

4. Rosy apple aphids Dysaphis plantaginea (below) are the most serious aphid pest on apple.

   

   They feed on young foliage and the developing fruit and cause leaf galls. They must be spotted early for effective control - spraying late in the season will be ineffective.

In other words to know what to do when you have 'apple aphids', you need to know which apple aphid. Spraying insecticide to get rid of the (relatively) harmless apple-grass aphid can cause much bigger problems later in the season if you kill off the natural enemies.

Out of about 600 aphid species in the UK (plus an ever increasing number of introduced or invasive species), a mere 20-30 of those species are regarded as serious 'pests' - in other words, under favourable conditions, those species may cause noticeable economic damage. Globally there are about 5000 species of which only about 250 are pests. Paradoxically, most aphids are quite rare, cause no economic damage and contribute greatly to our biodiversity.

Aphids - rarities, biodiversity and conservation

Of the aphids in Britain, a few can be pests, some are invaders, but others are rare and unusual, and even useful.

• As Heie, 2009 says, although most people probably believe that all aphids are common, the truth is that normally they are rather difficult to find.

Many aphid species are little noticed. One such aphid, the large oak aphid Lachnus roboris is pictured below. This species lives on oak and amongst aphids is much more of a 'k-strategist'. The 'k' refers to the so-called carrying capacity at which the population is more or less stable. Such aphids tend to be larger, have a lower reproductive rate and can be strikingly slow to recolonize degraded and restored habitats, such as replanted woodland.

Some of these k-strategists are very rare, such as Lachnus pallipes which live on beech. Only our older fragments of ancient woodland host such rarities of the aphid world.

Most species of aphids of course fall somewhere between the r and K extremes. One should also point out that dividing aphid species into r and k-strategists is simplistic and only reflects part of their life history strategies. But it does highlight the diversity within the group.

Apart from the general benefits of biodiversity in the environment, some aphids bring specific benefits to man. Consider these three species:

1. Green-striped spruce aphids Cinara piceicola (below) are not especially rare, but are very important for ants (some of which are keystone species in woodland).

These aphids cause little damage, and in Europe bees feeding on their honeydew produced much-prized honey.

2. The ragwort aphid Aphis jacobaeae feeds upon ragwort, a plant poisonous to livestock.

Research has shown that ragwort plants infested by Aphis jacobaeae are less likely to be eaten by livestock. The aphid has also been considered for biological control of this toxic weed.

3. Some studies suggest nettles infested with common nettle aphids Microlophium carnosum (below) provide a reservoir for natural enemies of crop pests.

Aphids 'support' many species of predators (such as ladybirds and hover fly larvae) and parasitoids (internal parasites that kill the host in order to mature) as well many 'mutualists' (where each organism derives benefit from the association). Most ant species have a mutualistic relationship with aphids. They consume the high-energy sugary honeydew which aphids produce, in return for providing aphids a measure of defense from predators. In forestry honeydew may make an important contribution to nutrient cycling, and honeydew can be highly-valued by bees and beekepers.

At present we probably know less about which species of aphids are found in different parts of the country than any other major insect group. There is quite a high chance of finding a species that has never been recorded before in Britain.

References  Heie, O.E. (2009). Aphid mysteries not yet solved/Hemiptera:Aphidomorpha. Monograph: Aphids and other hemipterous insects 15, 31-48.  Full text