Plant Source and Seed Parasitism Influence Seed Viability in Redbud (Cercis spp.)

Eastern redbud, Cercis canadensis L., trees are difficult to vegetatively propagate. Named cultivars of Eastern redbud do not grow “true” from seeds forcing growers to invest resources to graft buds or scions onto seed-grown liners. Whereas the unnamed North American native Eastern redbud trees produce large numbers of easily-collected seedpods that contain abundant numbers of seeds, these seeds are frequently infested with seed-parasitic beetles and associated wasp parasitoids. Of the redbuds investigated, we found that unnamed Eastern redbuds were preferred hosts for redbud seed beetles, Gibbobruchus mimus (Say). However, named and unnamed redbud types as well as C. chinensis ‘Avondale’ were suitable hosts for G. mimus. In addition, several new wasp parasitoids were recovered from seeds although it was not apparent whether these were attacking seed endosperm, G. mimus, or both. A float test successfully discriminated dead or infested seeds from seeds that were mostly viable. The majority of seeds that floated were empty, dead or infested. Growers can optimize seed collection efficiency by screening native Eastern redbud seeds for size and subjecting seeds to a float test thereby minimizing time, labor and land resources that would otherwise be expended for preparation and maintenance of fields planted with potentially nonviable seeds.

Seed parasitism redirects ovule development in Douglas fir

Many parasitic species of insects complete their entire development in seeds. They feed off storage reserves within the ovule. These reserves only normally accumulate in fertilized ovules. Consequently, female insects that oviposit their eggs directly into the plant ovule need to be able to select correctly, as unfertilized ovules of conifers normally become so-called empty seed. We provide clear evidence that in conifers, seed-parasitizing insects do not need to discriminate between fertilized and unfertilized plant ovules when ovipositing their eggs. A host-specific insect, the chalcid Megastigmus spermotrophus Wachtl (Hymenoptera: Torymidae), lays its eggs in ovules of Douglas fir ( Pseudotsuga menziesii (Mirbel) Franco) before fertilization has taken place in the plant. Oviposition not only prevents the expected degeneration and death of unfertilized ovules, but it induces energy reserve accumulation. Ovules that would otherwise develop as empty seed are redirected in their development by the insect to provide food for the developing larvae. Instead of the insect exploiting normal events during seed development, the insect manipulates seed development for its own reproductive advantage.