Naive females of Macrocentrus grandii Goidanich, a parasitoid of the European corn borer (ECB), Ostrinia nubilalis Hubner were attracted to some, but not all, host plant species of the latter; attraction being exceptionally strong for Zea mays L., progressively weaker for potato, snapbean and pepper, neutral for sunflower, with soybean being slightly repellent. Also, extracts of volatiles from soybean were repellent. The percent of females that responded to a given ECB host plant increased progressively with age of females to a maximum at 8 days. When various plant species were damaged by feeding activity of ECB, they became highly attractive to M. grandii females, and, subsequently: these experienced females were attracted to undamaged plants, even of plant species that were not attractive to naive females. Responsiveness of females to all plant species increased as a result of experience with ECB-damaged plant specimens of host species. Such experience with ECB-damaged potato increased attraction of females to sunflower by 4-fold, while experience with ECB-damaged sunflower increased attraction to potato by 2-fold. Loss of memory in Females of experience with ECB-damaged plants progressed with time and was total by the ninth day. Memory was induced primarily by frass, but not because Frass was derived from a particular plant species. Male M. grandii were not attracted to any host plants. Female M. grandii emit a series of 9,13 dienes of odd-numbered 27-41 carbon atom chains, and these, when oxidized by air, produced Z-4-tridecenal, the active volatile of the pheromone. The latter when emitted at 50 ng per hr proved to be a very effective lure. Further, we found that this aldehyde is synergized significantly by the lactone, (3S,5R,6S)-3,5-dimethyl-6-(methylethyl)-3,4,5,6-tetrahydropyran-2-one. It is noteworthy that this. lactone by itself is highly attractive when 1 to 500 ng are applied to the source. The lactone is produced by the mandibular gland of both males and females. Possibly the diene/aldehyde has arisen as an important intermediary evolutionary step, in a process wherein opportunistic males that recognize volatile chemicals unique to the female may have been selected. Natural selection then would favor females that produce more and more of the attractant precursor - a process that could lead eventually to selection for specialized cells and a pheromone gland.
