Category:Aphid

Insect (bioaggressor) Nitrogen excess Deficiency

Aphids are among the most destructive harmful insects for plants grown in regions with temperate climates. In addition to weakening the plant by sucking its sap, they act as vectors of plant viruses and disfigure ornamental plants.

INRAe has developed a large encyclopedia dedicated to aphids with a wealth of information: https://encyclopedie-pucerons.hub.inrae.fr/

Characteristics

Pest: small insects that suck sap.
Family: Aphididae.
Species: About 5,000 species. About 400 of them are found on food crops and fiber plants, and many are serious pests for agriculture and forestry, as well as a nuisance for gardeners.

Life cycle

Reproduction

They have the ability to rapidly increase their numbers by two modes of reproduction:

Asexual reproduction: Females, unable to fly, give birth to female nymphs without the participation of males (parthenogenesis).
Sexual reproduction: Reproduction being prolific, the number of these insects multiplies rapidly, and once the population is well established, winged females are born to colonize new environments.
In temperate regions, a phase of sexual reproduction occurs in autumn, with insects often overwintering as eggs.

Feeding

The life cycle of some aphids involves alternating between two host plant species, for example between a crop and a woody plant. Some species feed on a single type of plant, while others are generalists and colonize many plant groups.

Development

Some families of ants have developed a mutualistic relationship leading to coevolution with aphids, raising and protecting them from predators in order to harvest their honeydew.

Why do piercing-sucking insects proliferate on plants?

Amino acid nutrition

The plant: it mainly absorbs nitrogen in the form of nitrate or ammonium. Nitrates are reduced to ammonium then transformed into amino acids. These soluble amino acids are only converted into proteins if the plant has enough energy (photosynthesis) and especially if it has the necessary cofactor elements for protein synthesis. Since the plant needs energy and water to synthesize complex proteins from nitrogen in nitrate form, it will soak up water, and the turgor of cells will facilitate the penetration of the cuticle by piercing insects. Silica and calcium play an essential role as they are important elements for the synthesis of the plant cell's outer wall. Caution point: do not have an excess of potassium during the early stages of the plant as this hinders calcium absorption. Until tillering/early stem elongation, the plant will have a significant need for calcium. Too early potassium application can limit calcium absorption and have an undesirable effect on insect pressure.
Piercing-sucking insects: feed on free amino acids in the sap. The digestive tract of these insects is not developed; they lack the enzymes necessary to digest proteins and complex sugars. Moreover, many plants produce a “protease inhibitor” enzyme that prevents the insect from digesting proteins.

There is therefore a link between the free amino acid content in the sap and attacks by piercing-sucking insects.

Nutritional imbalance

The pressure can be increased by a nutritional imbalance in magnesium, molybdenum, sulfur, and boron: cofactors of enzymes involved in the conversion of nitrates into complex proteins. If there is an imbalance in the levels of these elements, protein conversion is less efficient. This results in a nitrate overload in the sap.

Under these conditions, aphids can easily develop, attracted by ammonium in the sap and fed by soluble amino acids.

Sensitivity to excess sugar in sap and redox

Piercing-sucking insects are sensitive to sugar content in sap: they cannot tolerate a high concentration. A quick solution can be to recommend a molasses application coupled with an insecticide as a lever to limit attacks by these insects (successful trials have been conducted by members of AgroLeague: 1L/ha on corn and on wheat at the 1-leaf stage). The advantage of this operation is its cost: about €0.02/ha.

Control methods

Preventive

Known preventive measures are no longer sufficient: delaying sowing dates, waiting for colder winter conditions to limit insect development, but mild temperatures in autumn and early winter, without marked cold periods, tend to induce a proliferation of autumn pests.

Using so-called more tolerant varieties to yellowing (virus transmitted by aphids) can help limit pressure but is not currently feasible for all crops.
Seed coating is a technique that allows stimulating the plant's immune defenses. It is essential to promote plant development from the start because the virus will have a greater impact if injected at the beginning of the crop's vegetative development. Hence the importance of plant nutrition.

These are highly effective preventive measures, but it is important to emphasize that one must still be very attentive, observe, and intervene chemically if necessary.

Curative

Insecticides do not always provide reliable results, given their resistance to several classes of insecticides and the fact that aphids often feed on the underside of leaves.
Aphids and leafhoppers prefer oxidized environments. The application of an antioxidant can help reduce pressure. This can concretely be done by applying vitamin C at low doses in the sprayer (10 g/ha).
The natural enemies of the aphid are predatory ladybugs, larvae of hoverflies, parasitoid wasps, larvae of aphid gall midges, crab spiders, neuropteran larvae, and entomopathogenic fungi. An integrated pest management strategy using biological means can work, but it is difficult to implement except in enclosed environments such as greenhouses.⁽⁾