Aphids Can Cause Serious Damage to Plants
Sap Sucking Plant Lice Often Severely Deplete Nutrients
Apr 7, 2009
Gardeners often find tiny insects covering the flower buds or stems of their prized roses, peonies, hibiscus, or other plants. If green with a rounded abdomen, these insects are usually green peach aphids (Myzus persicae) or potato aphids (Macrosiphum uphorbiae), which might be green or pink. Although other aphid species have a wide range of host plants, these are two of the most widespread garden pests. Potato aphid eggs are laid in the fall on rose bushes, and this is usually the first garden species to appear in the spring.
It is unfortunate that the delicate plants the aphids seek out are the very ones the gardener enjoys most. A heavy crop of aphids can disfigure or even kill the flowers and sometimes the plants themselves. How aphids invariably find these plants in the indoor or outdoor garden seems a mystery until their feeding and dispersal methods are understood.
Aphids belong to a group of insects called Homoptera with piercing mouthparts. The aphids are usually wingless and have a distinctive tear-drop shaped body with the head at the narrow end. There are (usually) two long tubes, cornicles, projecting from the dorsal (upper) surface of the abdomen. Most aphids stand head down on the plant and periodically appear to perform push ups off the surface. Wings, when present, are roundish, transparent, and delicate. A few aphid species secrete a whitish waxy material that covers the abdomen in a cottony coating. Other aphids induce their host plant to produce galls inside of which the aphids are protected from discovery by predators.
How Aphids Eat
Like other Homopterans, aphids have a hollow stylus for feeding. Aphids use their stylus to penetrate the surface of a plant and puncture a single phloem cell in one of the plant's veins. The sap drawn from these phloem cells serves as the sole source of nutrition for the aphids. Since the stylus is smaller in diameter than a human hair, it is not able to penetrate the tough, corky bark of a mature woody plant. Thus, aphids feed on new growth of trees and herbs, on leaves, or inside galls that they induce the plants to produce.
Aphid Nutrition
The sap that aphids feed on is a thin syrup that contains:
- sugars and amino acids moving up the stems to stimulate new growth in the spring
- sugars moving down the stems to nourish the mature and growing tissues in stems and roots in the summer
- sugars and amino acids moving down the stem to be stored in the roots in the autumn
Since all plant tissues are capable of manufacturing their own amino acids, but only cells directly exposed to sunlight can manufacture sugars, sap provides aphids with minimal amounts of amino acids and high concentrations of sugar.
Being animals, aphids cannot produce their own amino acids. In order to obtain enough of these protein building blocks for growth and maintenance, aphids must move from plants that have stopped growing to plants that are actively growing or drink large volumes of sap. In doing so, they have to ingest far more sugar than they can process, and aphid tissues would be bathed in fluids so rich in sugars that their metabolic processes would shut down if they did not have a mechanism to prevent this.
How Aphids Prevent Sugar Overload
To prevent sugar overload, the cells lining the guts of aphids have many fewer sugar transport proteins than similar cells do in the guts of other animals. With a reduced means of transport into the cells, much of the sugar in sap is not absorbed and is expelled to the environment as honeydew. The expelled honeydew rapidly evaporates and becomes a sticky syrup that covers both the plant's leaves and stems, and the ground below. Automobiles parked beneath aphid infested trees during the growing season are often covered with this sticky evaporated honeydew.
Consequences of Honeydew Production
As a result of their need to expel large quantities of sugar during their quest for amino acids, sap removal by dense aggregations of aphids often exceeds the plant's ability to deliver nutrients to growing tissues and affected buds and twigs sometimes die. If enough aphids congregate on an herbaceous plant, the entire plant may wither and die.
Aphid honeydew is used by several organisms, some of which add further insult to the plants, while others may be beneficial to the plant.
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