Sunflower, insect resistance

Approximately half of the fifty species of Helianthus have shown some resistance to the major insect pests of cultivated sunflower in greenhouse and field trials (6-12). We began our studies by chemically investigating a small group of Helianthus species which were reported to be especially insect resistant. The major lipophilic secondary metabolites of the aerial parts of these plants proved to be terpenoid compound of two types sesquiterpene lactones and diterpenes. We have now studied 17 species of Helianthus chemically and have found high levels of sesquiterpene lactones and diterpenes in most of these (13-25). Both sesquiterpene lactones and diterpenes have been isolated from species of Helianthus by other workers (26-44). 

Terpenoids isolated in large quantities from resistant species were tested with several sunflower insect pests. Since sesquiterpene lactones were present in much higher concentrations than diterpenes in the most resistant species studied, our investigations emphasized sesquiter-pene lactones. Three species of insects were used in these studies Spodoptera eridania, Melanoplus sanquinipes and Homoeosoma electellum. 

The next section, AUelochenUcals for Control of Insects and Other Arumals, discusses insect resistance factors in petunias, geraniums, com, centipede grass, sunflowers, and neem, as well as insecticidal activity of monoterpenoids and fish toxins from mangrove plants. 

In this chapter, we first review the terpenoid constituents isolated from resistant wild species of Helianthus and present the results of some insect bioassays with these compounds. Then we discuss the evidence which implicates terpenoids in the resistance to the sunflower moth (Homoeosoma electellum), the most destructive insect pest of cultivated sunflower in the United States (4), and describe how this information might be exploited in reducing insect damage to cultivated sunflower. 

Homoeosoma electellum (Lepidoptera Pyralidae) - the sunflower moth. In contrast to L eridania and II. sanguinipes, H. electellum is a specialist which feeds on the inflorescences of a few species of the Asteraceae (4,. 51). Since sesquiterpene lactones are found in several parts of the infloresences of Helianthus species which are considered to be resistant to the sunflower moth (52). it was thought that these compounds might serve to limit the damage caused by this insect. 

Labdanes.—The cis- and trans-isomers of (-)-ozic acid (9) have been isolated from the wild sunflower, Helianthus occidentalis (Compositae). Their presence in this plant may be associated with resistance to insect attack. The diene... 

Homeosoma electellum) contain high concentrations of tra-chyloban-19-oic acid (70) and ( —)-16-kauren-19-oic acid (71) in their florets. As sunflower florets that contain only small amounts of these compounds are a major portion of the diet of first instar larvae of this insect, it is likely that these acids serve as feeding inhibitors. At the 1% level, both kaurenoic and trachylobanoic acids decreased the growth of sunflower moth larvae and tobacco budworm (Heliothis virescens) by about 50%. At the 0.5% level, both reduced larval growth of the cotton bollworm and the pink bollworm to less than 5% (Fig. 22.23) (Mabry and Gill, 1979). The Z-and -isomers of (- )-ozic acid (72) have been isolated from Helianthus occidentalis and may be associated with resistance to insect attack (Stipanovic et al., 1979). 

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