Author
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Vick, Brady |
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Charlet, Laurence |
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Submitted to: Journal of Kansas Entomological Society
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/4/1996 Publication Date: N/A Citation: N/A Interpretive Summary: Two species of sunflower seed weevils, the red and the gray, are serious pests of sunflower in the northern Great Plains. Both burrow inside the sunflower seed hull and feed there. They are called red and gray because of their colors in the adult stage. The two species are very closely related, and in the larval stage are impossible to distinguish visually. In this study we looked at the chemical composition of larvae of the two species to determine if differences could be detected. We found that, indeed, there were differences in the proportion of storage fat constituents deposited by each species. The gray sunflower seed weevil was composed of significant amounts (much more than 5%) of a fatty acid called palmitoleic acid, whereas the red sunflower seed weevil had only small amounts (less than 5%). The two species also differed in their preferred source of dietary nutrients. The red sunflower seed weevil consumed a large part of the sunflower kernel, and therefore the fats from the kernel were incorporated into the larval fat storage body. In contrast, the gray sunflower seed weevil did not eat the kernel. Instead, it positioned itself inside the hull near the bottom where nutrients normally flow into the developing kernel. It intercepted those nutrients for itself, and deprived the kernel of its food source. The gray sunflower seed weevil did not consume very much of the underdeveloped kernel, and so it incorporated only a minor amount of kernel fats into its fat storage body. In practice, these variations in fatty acid composition due to feeding behavior and biosynthetic differences can be used to reliably distinguish between the two species. Technical Abstract: Larvae of the red and gray sunflower seed weevils, Smicronyx fulvus LeConte and Smicronyx sordidus LeConte, respectively, are similar morphologically and cannot be easily distinguished. However, the larvae of the two species have important differences in lipid metabolism, primarily in the fatty acid composition of triacylglycerols, which is the major storage lipid class. The storage triacylglycerols of the two species are derived both by de novo synthesis from acetate and by modification of dietary lipids. In S. sordidus larvae, the concentration of palmitoleic acid was consistently higher than 5% (n=33) and always exceeded the linoleic acid content, which was usually less than 4%. In contrast, the palmitoleic acid content of S. fulvus was usually much lower than 5% and never exceeded linoleic acid. The amount of linoleic acid in the triacylglycerol of S. fulvus larvae was usually high, and correlated well with the linoleic acid content of the diet. In contrast, the linoleic acid content of S. sordidus was low, indicating that mature sunflower kernels, which are normally high in linoleic acid, were not the major nutrient source for S. sordidus. In both weevils, dietary triacylglycerols were not deposited intact into the fat bodies, but instead were processed by lipases to release the fatty acids, which in turn were reesterified to glycerol to form new triacylglycerol molecular species. There was no evidence that dietary fatty acids were catabolized to acetate for resynthesis into new fatty acids. The larvae of both species were able to synthesize myristic, palmitic, palmitoleic, stearic, and oleic acids in vivo from [1,2-14C]sodium acetate. Neither species synthesized linoleic acid. |
