Aphids and Enemies

You really don't want to be an enemy of the aphids - two papers today! The first is quite straightforward:

Aphids Make 'Chemical Weapons' To Fight Off Killer Ladybirds:

Cabbage aphids have developed an internal chemical defence system which enables them to disable attacking predators by setting off a mustard oil 'bomb', says new research. The study shows for the first time how aphids use a chemical found in the plants they eat to emit a deadly burst of mustard oil when they're attacked by a predator, for example a ladybird. This mustard oil kills, injures or repels the ladybird, which then saves the colony of aphids from attack, although the individual aphid involved usually dies in the process.

So, these aphids directly defend themselves against their own enemies by using the chemicals they derives from the plants they eat. But the next study introduces more complexity - several levels of the food web (i.e., tri-trophic relationship);

High Susceptibility of Bt Maize to Aphids Enhances the Performance of Parasitoids of Lepidopteran Pests:

Concerns about possible undesired environmental effects of transgenic crops have prompted numerous evaluations of such crops. So-called Bt crops receive particular attention because they carry bacteria-derived genes coding for insecticidal proteins that might negatively affect non-target arthropods. Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects. Indeed, Cotesia marginiventris, a parasitoid of lepidopteran pests, lived longer and parasitized more pest caterpillars in the presence of aphid-infested Bt maize than in the presence of aphid-infested isogenic maize. Hence, depending on aphid pest thresholds, the observed increased susceptibility of Bt maize to aphids may be either a welcome or an undesirable side effect.

Translation: transgenic corn has somewhat more nutritional value for the aphids. Thus, there are more aphids (per plant) on such corn. Thus, there is more "honeydew" (per acre) that they produce. Thus, there is more food (per acre) for the wasp. Thus, there are more wasps in the field. Thus, they are better able to control the population of moth caterpillars. Thus, there are fewer caterpillars to eat the corn. Final result: the farmer is happy. Now go to the paper itself and add comments, annotations and ratings to it.