Aphids

Aphids life cycle

Aphids have a complex life cycle, with both winged and wingless forms of adults of the same species developing, depending on the conditions. Wingless adult aphids are known as apterous, and winged adults as alate. Alate aphids have two pairs of wings, one of which is much larger than the other.

For a large part of the season an aphid population consists of viviparous females. As reproduction is asexual, the progeny of a single female is genetically identical to the mother. In other words, they are clones. Because there is no genetic recombination, different characteristics, such as colour forms, or resistance to pesticides, remain unchanged and unsorted. The young aphids are born as developed nymphs and begin to feed instantly on plant sap. They grow rapidly and moult four times before becoming adult, with the conspicuous white cuticles, shed during each of these moults, betraying their presence in the crop.

There are two types of aphid species: those that change to other host plants in the winter, and those species that do not. Species that alternate between host plants reproduce asexually on their summer host plant, migrating to their winter host plant in autumn, where they reproduce sexually and lay eggs that overwinter. The summer host plants are either herbaceous or woody, while the winter host plants are hardy, woody perennials. Aphids that do not alternate between host plants also mate in the autumn and lay eggs which overwinter. Where mating occurs and eggs are laid, the life cycle is termed holocyclic (complete). In greenhouses, however, the alternation of host plant and egg-laying may not take place. In this case, reproduction continues through the winter by parthenogenesis, with viviparous unfertilized females continuing to produce new generations of females. This is known as an anholocyclic (incomplete) life cycle. However, in some greenhouse crops with very low winter temperatures (e.g. strawberries), aphid eggs have been observed.

Aphids damage

Aphids can cause damage to plants in several ways, and the damage can vary depending on the species of aphid and the type of plant being attacked. The following are common signs of aphid damage:

• Yellowing or distorted leaves: Aphids feed on the sap of plants, which can cause the leaves to turn yellow or become distorted. The damage is often more pronounced on the younger leaves of the plant.
• Stunted growth: Heavy infestations of aphids can cause plants to grow more slowly or become stunted. This is because the insects are drawing nutrients away from the plant as they feed on its sap.
• Sticky residue: Aphids excrete a sugary substance called honeydew, which can coat the leaves and stems of infested plants.
• Sooty mold: Honeydew can also promote the growth of a black, sooty mold that can cover the leaves and stems of infested plants. This mold can further reduce the plant's ability to photosynthesize and grow.
• Wilting or death of plant: In severe cases, aphid infestations can cause plants to wilt and die. This can occur when the insects are present in very large numbers and are feeding heavily on the plant.
• Curling or twisting of leaves: Some species of aphids can cause leaves to curl or twist as they feed on the plant sap.

How to prevent aphids

To prevent aphid infestations in crops, you can take several preventative measures:

Practice good crop hygiene

Keep the growing area clean and free from weeds, debris, and plant residues that can harbor aphids. Regularly remove and dispose of any infested plant material.

Monitor your crops regularly

Inspect plants for early signs of aphids, such as curled leaves, distorted growth, or the presence of the insects themselves. Early detection enables prompt action to prevent infestations from spreading. Sticky traps can be strategically placed throughout the growing area to capture flying aphids and provide an indication of their population levels.

By implementing these preventative measures, you can significantly reduce the risk of aphid infestations and maintain healthier crops.