Floral odors and the interaction between pollinating Ceratopogonid midges and Cacao

Most plant species depend upon insect pollination services, including many cash and subsistence crops. Plants compete to attract those insects using visual cues and floral odor which pollinators associate with a reward. The cacao tree, Theobroma cacao, has a highly specialized floral morphology permitting pollination primarily by Ceratopogonid midges. However, these insects do not depend upon cacao flowers for their life cycle, and can use other sugar sources. To understand how floral cues mediate pollination in cacao we developed a method for rearing Ceratopogonidae through several complete lifecycles to provide material for bioassays. We carried out collection and analysis of cacao floral volatiles, and identified a bouquet made up exclusively of saturated and unsaturated, straight-chain hydrocarbons, which is unusual among floral odors. The most abundant components were tridecane, pentadecane, (Z)-7-pentadecene and (Z)-8-heptadecene with a heptadecadiene and heptadecatriene as minor components. We presented adult midges, Forcipomyia sp. (subgen. Forcipomyia), Culicoides paraensis and Dasyhelea borgmeieri, with natural and synthetic cacao flower odors in choice assays. Midges showed weak attraction to the complete natural floral odor in the assay, with no significant evidence of interspecific differences. This suggests that cacao floral volatiles play a role in pollinator behavior. Midges were not attracted to a synthetic blend of the above four major components of cacao flower odor, indicating that a more complete blend is required for attraction. Our findings indicate that cacao pollination is likely facilitated by the volatile blend released by flowers, and that the system involves a generalized odor response common to different species of Ceratopogonidae

Low humidity as a cue for habitat preference in the parasitoid Lariophagus distinguendus

The larval and pupal parasitoid Lariophagus distinguendus Forster (Pteromalidae) is a potential candidate for the biological control of the granary weevil Sitophilus granarius L. and other stored product pests that mostly prefer the dry conditions of a storage environment. To predict whether parasitoids will remain within the dry storage environment after release, experiments with L. distinguendus were performed in a humidity gradient chamber. To our knowledge, this is the first time that humidity preference was examined directly for a parasitoid species. Regardless of experience, parasitoids significantly preferred drier areas in the order 32.5% (most preferred), 53%, 75.5%, and 97.5% r.h. (least preferred). This indicates that humidity is used as cue for host habitat preference. When used as natural enemy against stored product pests this preference probably will retain L. distinguendus within the storage environment after their release

Volatile cues from different host complexes used for host location by the generalist parasitoid Lariophagus distinguendus (Hymenoptera : Pteromalidae)

The ability of a generalist parasitoid to locate different non-related host species by volatile cues was examined in a static four chamber olfactometer with naive females of Lariophagus distinguendus Forster (Pteromalidae), a parasitoid of beetle larvae endophytic in seeds. The following seed-host complexes were tested: Sitophilus granarius in rice grains, Rhyzopertha dominica in wheat grains, and Callosobruchus maculatus in cowpeas. For comparison the non-host Sitotroga cerealella, a moth endophytic in wheat grains, was used. Healthy seeds from rice, wheat, and cowpea all had an arresting effect on L. distinguendus. Infested seeds from the complexes rice-S. granarius and wheat-R. dominica were significantly preferred over healthy seeds, but not from the complexes cowpea-C. maculatus and wheat-S. cerealella. Faeces from all beetle hosts, but not from the moth S. cerealella, had an arresting effect. These results indicate that L. distinguendus females innately react to volatile cues from different hosts and host plants. This is discussed with respect to current hypotheses on the use of chemical cues by generalist parasitoids