Aphids are among the most destructive pests on cultivated plants in temperate regions. They are the fear of many growers, as they can cause yellowing, mottled leaves, stunted growth, curled leaves, browning, low yields and even death in plants.
Part I - Scroll down for the first part of the article which featured in CANNAtalk issue 14. In this part we discuss the different kinds of aphids.
Part II - If you've already read part I, please click here to continue the article on page 2 and read what kind of damage aphids can do.
Aphids, Damage and Control - Part I
By Iñaki García
When we refer to aphids, or plant lice, we usually mean a super family of insects which includes over 4,000 species of plant-specific parasites. They are not longer than about 0.16 inch, have a bulbous abdomen and can be many different colors. Many species bear a common or scientific name that indicates their favourite host plant, either for food or for raising their offspring, or to some of their distinctive characteristics. So, for example, Hyadaphis coriandri prefers coriander, the cotton aphid (Aphis gossypii) prefers cotton, the black peach aphid (Brachycaudus persicae) prefers peach and so on.
Syringe needle
All aphids are characterized by a stylus (a kind of syringe needle) that is used to pierce and suck the sap from the plant. Also, they have a couple of tubes in the back called cornicles or siphunculi through which the animals excrete a kind of honeydew called cornicle wax. For nutrition, aphids usually feed on the plant’s phloem sap, which is rich in sugars, minerals and other elements. The phloem is responsible for distributing this kind of sap throughout the plant. For water, aphids draw fluid from xylem, where raw sap runs directly from the roots. This allows them to stay hydrated during hot or dry periods.
Ladybird feeding on an aphid.
Life cycle
A generation of aphids survives the winter as eggs, which allows them to withstand extreme environmental conditions of temperature and moisture. In spring the eggs on the plant (primary host) hatch, leading to the first generation of aphids. All the aphids born from the winter eggs are females. Several more generations of female aphids are born during the spring and summer. A female can live for 25 days, during which time she can produce up to 80 new aphids. Spring and summer reproduction occurs asexually – without males. In these cases, the resulting aphids are basically clones of the mother. In addition, the young are born live rather than as eggs. When the fall approaches, there is a generation that grow into both male and female individuals. Females fertilized by the males lay winter eggs on the plant where they are, closing the cycle.
Winged aphids
Aphids can be winged or wingless. Usually the first generation to emerge from the winter egg are wingless. However, after several generations there can be a lack of space on the host plant. This triggers the birth of a generation of winged aphids, which can migrate to other hosts.
Some species of aphid develop only on plants of a particular species. These types of aphids are called monoecius. The most common species that attack crops are heteroecious aphids. Heteroecious means that they feed on different plant species. Heteroecious aphids that reproduce sexually at least once in their lives start their cycle when the winter eggs hatch on the primary host. The primary hosts are usually annual weeds, shrubs or trees.
A couple of parthenocarpic generations (reproduced without fertilization) then give rise to a generation of winged females that migrate to the secondary host. This is usually a cultivated plant. In this new environment the aphids reproduce asexually for several generations of females until the arrival of autumn, when there is a generation of winged males and females which return to the initial host plant and lay fertilized winter eggs, closing the cycle again.