Attack of the Clones: Aphids in Your Peach Orchard

Born Pregnant

Peculiar as it may seem, many aphid species do not have males at all, or have them for a very limited time of the year. In temperate growing regions like Ontario, aphids overwinter as eggs, and go through the standard developmental stages of egg, immature (nymph), and adult for that first generation in the spring. But during the growing season, most aphids reproduce parthenogenetically - a great 'ten dollar word' meaning basically 'without fertilization' (or 'without sex'). These aphids (all females) don't waste time on sex and they certainly don't waste time on development either. In fact, many aphids can be born pregnant. Think about that for a minute - that means that an aphids' granddaughter is already being formed inside her! Weird and twisted as that may seem from our mammal - centered view of the world, it is a great strategy for creating a large population quickly when resources are available.

Conquering Time

When you delve into this strategy a bit deeper, you also see that it is a very effective strategy for perpetuating your genes through the year. That's your genes, not some of your genes. No males and no sex mean that the female aphids are making virtually identical copies of themselves. Not a genetic copy shared or diluted by any useless male genes; but as close to a clone or actual copy of themselves as possible. (There is some genetic variability in parthenogenesis, but not nearly as much as with sexual reproduction). After all, in the great big world of biology, that's what it's all about -perpetuating your genetic material through the fourth dimension (time).

Defeated by Space

What this means from an integrated pest management (IPM) perspective is that monitoring for rapid increases in aphid numbers is vital to tell when the threshold for action is reached. Generally, all those daughter and granddaughter and great-granddaughter aphids stay close to the founding mother and create 'colonies' localized on the tree. However, the resources of a small area are finite, and those areas of crinkled, curled, distorted leaves are soon overcrowded. Aphids have developed ways to overcome crowding: they move away from mom and the gang to establish a new home.

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Fly or Die

Aphids have complex life cycles that often include the development of winged and wingless forms, dependent upon environmental and host plant conditions. Winged adults are usually associated with migration from one crop or host plant to another throughout the growing season; the development of winged forms is usually triggered by environmental changes, including decreasing photoperiod (daylength) or temperature, deterioration of the host plant, or overcrowding. That is, aphids develop winged forms when the going gets tough and it's time to move on.

Aphids in Your Peach Trees

Most aphid species live on one or a few related plants, however a few species spend winter, spring and fall on one plant type (primary host) and summer on unrelated plants (secondary hosts). In temperate regions such as Ontario, many aphid species overwinter in the egg stage on primary hosts. The green peach aphid (GPA), for example, overwinters as eggs on peaches and nectarines. GPA eggs hatch in the spring to form wingless females. In late spring and early summer (when primary hosts have hardened off), subsequent generations of winged GPA females leave their primary hosts and colonize secondary hosts, which include several hundred species of vegetable crops, ornamentals, and weeds. In the fall, when temperatures drop, day length shortens, and plants grow slowly or die due to unfavourable environmental conditions, winged aphids (males and females) are produced, fly back to primary hosts where they eventually mate, lay eggs and die.

Reaching Your Limit

Since green peach aphids move out of peach trees fairly early on, their numbers may not reach damaging levels in the orchard. What this highlights is a need to monitor populations. Thresholds have been developed for both peaches and nectarines. Peaches can tolerate infestations of up to 30% of their terminals or 20 colonies per tree. The threshold for nectarines is lower at 10% terminal infestation or 5-10 colonies per tree. The lower threshold on nectarines reflects the fact that the aphids will feed directly on nectarine fruit but rarely on peach fruit. Consider the presence of predators before spraying; one predator per colony may be sufficient to achieve acceptable levels of control. Predators of aphids include larval and adult lady beetles along with syrphid (hover) fly larvae. There are also parasitic wasps that attack aphids, making the aphids eventually turn into golden coloured "mummies". When monitoring, it is important to make sure aphid colonies are "active"; those that have been decimated by predators should not be counted in the total. Most growers choose not to manage aphids in bearing trees, except in extreme situations.

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