The Adept Aphid

Mention the word ``aphid'' around avid gardeners and you are likely to send them scurrying off looking for their favorite brand of Bug-Be-Gone. Indeed, the bad reputation that these little sap-suckers have among gardeners and farmers is well deserved. For bug watchers, however, aphids are probably amongst the most fascinating of all the insects.

Aphids have complex life cycles. During the year, females, of at least a few generations, bear live young from unfertilized eggs. Unless a sexual generation is produced, the new offspring already have embryos ready to grow. As a result, aphid populations can build quickly. While the offspring are clones of their mother, mutations can change their genetic makeup. Under adverse conditions, particularly overcrowding, winged adults may develop allowing dispersal. Often adults, with and without wings, are present in a colony. Some species also have a winged sexual generation.

Over 90% of aphids are host specific, feeding on one, or perhaps a few, species of a particular plant genus. Some of the others alternate between two hosts, resulting in a seasonal movement between a primary woody host and a secondary herbaceous one.

Aphids feed on phloem sap. The feeding tube, or stylet bundle, rests on a thicker beak or proboscis which telescopes backwards when the stylets are inserted into the plant. Salivary enzymes disrupt the bonds joining the plant cells allowing the stylets to pass between the cells before piecing the phloem sieve tubes. Internal pressure on the sap forces it up the stylets. Because the concentration of proteins in phloem sap is low, a considerable amount must be ingested to satisfy the insect's protein requirements. The excess, mainly water and sugars, is excreted along with digested food as honeydew.

Honeydew is a favorite food of some ant species. These species have learned to encourage its release using physical stimulation. In return, the the presence of ants moving through the aphid colony provides protection, discouraging aphid predators, of which there are many. Ladybird beetles (ladybugs) and lacewings consume aphids throughout their lives. The larvae of some flies and wasps also feed on aphids.

Around the Nature Center, aphids, frequently attended by ants, are commonly found on mustard, telegraph weed, thistles and broom. Look for aphids on your walks. The insect-insect interactions you will observe are worth the effort.

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201