Herbivore pressure by weevils associated with flower color polymorphism in Geranium thunbergii (Geraniaceae).

Although floral herbivory has recently received increased attention as an important factor influencing plant reproduction, relatively little is known about how its frequency and intensity vary depending on traits of host plants. Here we report that herbivore pressure by a weevil, Zacladus geranii, is associated with a flower color polymorphism of Geranium thunbergii (Geraniaceae). Pink and white flower color morphs have been reported in G. thunbergii, and we found in a three-year field survey in multiple populations that, generally, adult weevils more preferentially visited white flowers than pink flowers. Consistently, we found more severe damage by weevil larvae in white flowers. Overall herbivore pressure for G. thunbergii varied strongly between populations, and the difference seems to be partly explained by the co-occurrence of a related plant species, Geranium yezoense, in a population, as weevils preferred it to both color morphs of G. thunbergii, thereby relaxing overall herbivore pressure for G. thunbergii. Nonetheless, despite such high variability, the preference of weevils for white morphs over pink morphs of G. thunbergii was found across multiple populations. We discuss possible mechanisms causing the association between flower color and herbivore preference as well as its evolutionary consequences.

Larval Bradysia impatiens (Diptera: Sciaridae) potential for vectoring Pythium root rot pathogens.

A series of laboratory experiments were conducted to investigate the capacity of Bradysia impatiens (Johannsen) larvae to ingest propagules from two strains each of Pythium aphanidermatum (Edson) Fitzp. and P. ultimum Trow and transmit the pathogens to healthy geranium seedlings on a filter-paper substrate in petri dishes. The capacity of fungus gnat larvae to transmit P. aphanidermatum to seedlings rooted in a commercial peat-based potting mix and germination of Pythium oospores and hyphal swellings before and after passage through the guts of larval fungus gnats were also examined. Assays revealed that Pythium spp. transmission by larval fungus gnats varied greatly with the assay substrate and also with the number and nature of ingested propagules. Transmission was highest (65%) in the petri dish assays testing larvae fed P. aphanidermatum K-13, a strain that produced abundant oospores. Transmission of strain K-13 was much lower (<6%) in plug cells with potting mix. Larvae were less efficient at vectoring P. ultimum strain PSN-1, which produced few oospores, and no transmission was observed with two non-oospore-producing strains: P. aphanidermatum Pa58 and P. ultimum P4. Passage of P. aphanidermatum K-13 through larval guts significantly increased oospore germination. However, decreased germination of hyphal swellings was observed following larval gut passage for strains of P. ultimum. These results expand previous studies suggesting that larval fungus gnats may vector Pythium spp.

Fungus gnat feeding and mechanical wounding inhibit Pythium aphanidermatum infection of geranium seedlings.

ABSTRACT A series of laboratory tests were conducted to investigate potential effects of fungus gnat (Bradysia impatiens) feeding damage on susceptibility of geranium seedlings (Pelargonium x hortorum) to infection by the root rot pathogen Pythium aphanidermatum. Effects were compared with those from similar tests in which the seedlings were mechanically wounded by severing the root tip with a scalpel. Assays of geranium seedlings in petri dishes revealed a pronounced negative fungus gnat-Pythium interaction, with exposure to fungus gnat larvae 24 h prior to inoculation with P. aphanidermatum zoospores resulting in up to 47% fewer seedling deaths than would have been expected if the two agents had acted independently. Similar results were observed when seedlings were subjected to mechanical wounding 24 h prior to zoospore inoculation. In contrast, no interaction occurred when seedlings were mechanically wounded immediately prior to inoculation. The degree of plant damage inflicted by the feeding activities of the larval fungus gnats had no significant effect on the combined damage from fungus gnats and Pythium in petri dishes. Ancillary studies showed that Pythium development on V8 agar was not inhibited by the presence of fungus gnat-associated microorganisms, nor were seedlings inoculated with these microbes less susceptible to Pythium infection. The precise mechaism or mechanisms underlying the observed interactions were not elucidated; however, the results strongly suggest that both fungus gnat feeding and mechanical wounding activated systemic defenses that made the seedlings more resistant to Pythium infection.

Plant host effect on the development of Heliothis virescens F. (Lepidoptera: Noctuidae).

Heliothis virescens F. is an important polyphagous pest that can develop on >100 plant species, including 20 economic crops. Populations of this insect are believed to be locally maintained on a few crops and weed hosts in Washington County, MS. To find the intrinsic value of these plants for the development of H. virescens populations, we fed different laboratory and wild colonies with fresh and lyophilized plant tissue under a constant temperature. Development time of this insect under laboratory conditions varied up to 10 d between plant hosts and was dependent on the type of plant tissue provided: fresh or lyophilized. Life table parameters such as net reproductive rate, finite rate of increase, and generation time indicated that Trifolium repens, a wild host growing around agricultural fields year round, could be one of the most suitable local plant hosts for the development of H. virescens. Two species of Geranium, previously reported as the source of the first H. virescens generation in the region, had lower intrinsic value as a food source than did T. repens. Gossyipium hirsutum, perhaps the most important crop source of H. virescens in the region, produced low net reproductive rate and finite rate of increase parameters. Sampling conducted in agricultural fields during 2006 and 2007 found no larvae on the above mentioned wild hosts as it was previously reported. Results indicated that H. virescens populations in this region were not supported by the wild plant species growing around agricultural fields during the time when the survey took place.

Testing for non-target effects of spinosad on twospotted spider mites and their predator Phytoseiulus persimilis under greenhouse conditions.

The compatibility of the selective insecticide spinosad (Conserve SC), at rates recommended for thrips control in greenhouses, with release of the predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) to control spider mites, was investigated in a crop of ivy geranium Pelargonium peltatum, cultivar 'Amethyst 96.' Plants were inoculated with twospotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), 2 weeks before treatments were applied. There were three treatment variables, each at two levels: predators (released or not), spray application (water or Conserve SC at 2 ml/3.79 l), and timing of spray (1 day before or after predators were released). Twospotted spider mite populations then were sampled twice each week over a three-week period. The application or timing of spinosad had no effect on the ability of the predator to reduce the population of spider mites. Spider mite populations in the no-predator treatment continued to expand over the course of the experiment, while those in the predator-release treatment declined. We conclude that P. persimilis can be used in conjunction with spinosad on ivy geraniums without causing obvious detrimental effects to this predator or leading to a reduction in biological control.