Date of Award

12-23-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Janet Lanza

Abstract

This thesis represents a combination of three projects, all related to cotton agriculture, their pests, and natural enemies of those pests. In the first project, I compared the abundance of soil-surface arthropods in cotton fields with three different tillage methods (conventional and two conservation tillage methods). Cotton plants produce extrafloral nectar, an adaptation of many plants that attracts natural enemies of plant pests and reduces feeding injury from herbivores. Therefore, in the second project, I analyzed the sugar and amino acid concentrations of the extrafloral nectar of five lines of cotton. Green lacewings are natural enemies that are often used in biological control of agricultural pests and are known to feed on extrafloral nectar. In the third project, I tested preferences of green lacewings (Chrysoperla rufilabris) for artificial mimics of two cotton extrafloral nectars and the nutritional effects of five diets on adult green lacewings. Adoption of conservation tillage systems, including cover crop and no-till practices, have reduced environmental and economic costs compared to conventional tillage systems. Impact of conservation tillage on conservation biological control and insect pests in mid-south cotton (Gossypium hirsutum L.) is largely unknown. I examined effects of three tillage treatments (conventional tillage, cover crop, and no-till) on cotton arthropod herbivores and their predators in a two year study, using pitfall traps to survey soil-surface arthropods. In the first year (2010), three tillage systems with three pesticide treatments were used. In the second year (2011), these tillage systems with two different fertilizers at different application times were used. Repeated Measures Analysis of Variance was used for statistical analyses. The conservation tillage systems did not increase cotton pest populations on the soil surface. Conservation tillage systems resulted in significantly higher predator populations (carabids in Year 1 and spiders in Year 2) than conventional tillage. The abundance of both herbivores and predators changed between years, possibly because of different weather conditions or because of differences in the cotton cultivars grown in the two years. Conservation tillage did not lead to increase in pest arthropods. Pest risks should not pose a barrier to adoption of conservation tillage practices for farmers in northeast Arkansas. Extrafloral nectar produced by many plants attracts natural enemies (predators and parasitoids) that then reduce herbivore abundance. The two most concentrated solutes of extrafloral nectar are sugars and amino acids, both compounds contribute to natural enemy attraction and nutrition. I measured the sugar and amino acid composition of extrafloral nectar of five different cotton lines, including three conventional lines (Ark 0102-48, Ark 0222-12, and Ark 0219-15), and two Roundup Ready®, Bt transgenic varieties, Stoneville 5288 B2F and Deltapine 0912 B2RF. Extrafloral nectar sugar and amino acid compositions and concentrations were analyzed by gas chromatography-mass spectrometry (GC-MS). Sugars and amino acids were quantified by constructing a calibration curve for each individual chemical using linear regression. The sugar concentrations of the lines differed significantly. The highest and lowest sugar concentrations were measured in the two commercial cultivars. Stoneville 5288 B2F produced nectar with the highest sugar concentrations while Deltapine 0912 B2RF produced nectar with the lowest sugar concentrations. The three conventional lines (Ark 0102-48, Ark 0222-12, and Ark 0219-15) were intermediate in sugar concentrations. Although six amino acids (alanine, proline, valine, serine, leucine, and glutamic acid) were identified and quantified in the extrafloral nectars of all five cotton lines, no significant differences occurred in the concentrations of individual amino acids among lines. The differences in sugars among cotton lines were genetic because all were grown simultaneously under the same conditions. If extrafloral nectar can be manipulated genetically, we might be able to improve the attraction and nutrition of natural enemies in the field and improve natural biological control. Biological control, the use of natural enemies, is widely recommended to decrease the need for pesticides. Because green lacewings (Chrysoperla rufilabris) can play an important role in biological control, they are of great interest to many farmers and scientists. In order to produce baseline data that could improve the reproductive output of green lacewings in the field, I experimentally tested the ability of C. rufalabris to detect amino acids in extrafloral nectars, their preferences for extrafloral nectars of different cotton cultivars Stoneville 5288 B2F and Deltapine 0912 B2RF, and the nutritional value of five different adult diets. Using mimics of an ecologically unrealistic extrafloral nectar with a high concentration of amino acids (Passiflora menispermifolia, a tropical canopy vine), both male and female C. rufilabris preferred sugar-amino acid nectars over sugar-only nectars. Thus, amino acid-rich nectars could be important in attracting green lacewing adults. However, neither male nor female C. rufilabris discriminated between sugar-only and sugar-amino acid mimics of cotton extrafloral nectars (Stoneville 5288 B2F and Deltapine 0912 B2RF), probably because the amino acid content of the nectar mimics was too low. In nutrition tests of five adult diets (Stoneville 5288 B2F extrafloral nectar only, Deltapine 0912 B2RF extrafloral nectar only, Stoneville 5288 B2F extrafloral nectar plus pollen, Deltapine 0912 B2RF extrafloral nectar plus pollen, and an artificial diet), females fed only nectar produced the fewest number of eggs. The highest fecundities were produced when adults received high-protein diets (artificial diet or nectar plus pollen). If cotton cultivars can be developed with high levels of amino acids in the extrafloral nectars, they might contribute to increased reproduction by C. rufalabris. Alternatively, planting pollen sources on the edges of agricultural fields could also help improve reproduction by C. rufalabris.

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