The effect of resource provisioning and sugar composition of foods on longevity of three Gonatocerus spp., egg parasitoids of Homalodisca vitripennis

Abstract

The effect of dietary supplements on the longevity of male and female Gonatocerus ashmeadi, G. triguttatus and G. fasciatus (Hymenoptera: Mymaridae), was determined in the laboratory. Treatments included: water only, 3:1 honey–water solution, floral and extra-floral nectars from five different plants (excised stems from Fagopyrum esculentum, Lobularia maritima, Phacelia tanacetifolia, Anethum graveolens and Vicia faba), honeydew from Coccus hesperidum and Homalodisca vitripennis (formally H. coagulata), a commercially available food supplement (Eliminade) and citrus foliage. Additionally, the sugar composition of each food resource was determined using HPLC and whole flower extracts. Honey–water and F. esculentum nectar significantly increased longevity of male and female G. ashmeadi, G. triguttatus, and G. fasciatus up to 1860%, 1323% and 1459%, respectively, when compared with water. For both sexes and all three parasitoid species, survival on citrus foliage, H. vitripennis excrement, and P. tanacetifolia flowers was equivalent to that on water only. The longevity of G. ashmeadi and G. triguttatus was up to 539% higher on Eliminade compared with water only, however there was no significant effect of Eliminade on survival of G. fasciatus. Coccus hesperidum honeydew increased survival times up to 665% for all mymarid species compared with citrus foliage alone. HPLC analysis indicated that food resources most beneficial to Gonatocerus parasitoids possessed a high proportion of glucose (up to 44%) and fructose (up to 53%), suggesting that sucrose may not be as important for parasitoid survival. Citrus and P. tanacetifolia flowers contained favorable proportions of glucose and fructose, but the inability of Gonatocerus spp. to benefit from this may be related to flower morphology which could prevent access to nectar.

Introduction

The glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), [formally H. coagulata (Say) (Takiya et al., 2006)] is native to the southeast United States and likely invaded California in the late 1980s possibly through movement of ornamental plants from Florida (Sorensen and Gill, 1996). It has since become a major threat to many Californian agricultural and ornamental plant industries due to its ability to vector the plant pathogenic bacterium, Xylella fastidiosa Wells et al. This pathogen resides exclusively in xylem tissue causing lethal scorch-like diseases in a wide range of host plants including commercial crops such as grapes, stone fruit, almonds and citrus, and many ornamental and native plants (Hopkins and Adlerz, 1988, Purcell and Saunders, 1999, Hopkins and Purcell, 2002).

Pierce’s Disease caused the rapid destruction of over 300 acres of vineyard in Temecula Valley during 1997–1999 (CDFA, 2002), and in 2000, a classical biological control program was initiated to reduce the density and spread of H. vitripennis and Xylella-related diseases. Three non-host feeding mymarid egg parasitoids, Gonatocerus ashmeadi Girault, G. triguttatus Girault, and G. fasciatus Girault (Hymenoptera: Mymaridae) have been released in California, and are being evaluated for their ability to suppress H. vitripennis populations. Gonatocerus ashmeadi is a solitary endoparasitoid that has been resident in California since 1978 (Huber, 1988). Genetic analyses indicate it is native to the southeast USA and probably invaded California with H. vitripennis (Vickerman et al., 2004). Gonatocerus triguttatus is a solitary endoparasitoid native to Texas and central Florida, and has been imported from Texas and released in California since 2001 (CDFA, 2003). Gonatocerus fasciatus is a gregarious parasitoid that was introduced to California from Texas in 2002 (CDFA, 2003). Widespread establishment of G. triguttatus and G. fasciatus in California has not been confirmed, but post-release recoveries of these two parasitoids have been made (Morgan, D., CDFA, personal communication).

Floral and extrafloral nectar are significant sources of nutrients for most adult Hymenoptera and can maximize parasitoid longevity, fecundity, searching activity and parasitism rates (Takasu and Lewis, 1995, Tylianakis et al., 2004, Irvin et al., 2006). Understorey management (i.e., the deliberate management of flowering plants beneath fruit-bearing plants in orchards and vineyards) is potentially one way to enhance parasitoid populations in agricultural systems thereby leading to improved pest control by natural enemies (Gurr et al., 2000, Landis et al., 2000, Gurr et al., 2004). Understorey management can decrease abundance and increase parasitism of leafhoppers in Californian vineyards (Daane and Costello, 1998, Nicholls et al., 2000) and this strategy should be evaluated for potential at controlling H. vitripennis. Plant candidates that have shown potential for enhancing parasitoid populations in orchards and vineyards include buckwheat (Fagopyrum esculentum Moench) (Irvin et al., 2000, Nicholls et al., 2000, Berndt et al., 2002, Tylianakis et al., 2004, Irvin et al., 2006), alyssum (Lobularia maritima L.) (Chaney, 1998, Irvin et al., 2006), Phacelia tanacetifolia Benth. (Baggen and Gurr, 1998), dill (Anethum graveolens L.) (Baggen and Gurr, 1998), and extrafloral nectaries of broad bean (Vicia faba L.) (Bugg et al., 1989).

Several studies have demonstrated that flower morphology and corolla length can prevent some parasitic hymenopterans from exploiting nectar (Patt et al., 1997) and not all nectars are beneficial to parasitoids (Wäckers, 2001). Additionally, some parasitoids benefit from feeding on honeydew excreted by homopterans (Johnson and Stafford, 1985, Miller, 1989). The benefit from floral and extrafloral resources and homopteran honeydew derived by Gonatocerus spp. released for H. vitripennis control in California is unknown. Consequently, the research undertaken here sought to determine whether food resources such as floral nectars, extrafloral nectar, arthropod waste products, honey–water solutions, and a commercially available insect food spray could enhance the longevity of male and female G. ashmeadi, G. triguttatus and G. fasciatus in the laboratory. Additionally, the sugar composition of each food resource was determined using high performance liquid chromatography (HPLC) analysis. The results of these studies will allow further identification of nutrition sources that may enhance mymarid parasitoid activity against H. vitripennis in commercial citrus orchards and vineyards.