Sublethal Effect of Plant Lower Nitrogen Level on Development and Feeding Preference of the Beet Armyworm, Spodoptera exigua (Lepidoptera: Noctuidae) Yigen Chen and John Ruberson Dept. of Entomology, UGA

Importance of N for insects Many herbivorous insects select high N plants Efficiency of conversion of ingested food (ECI) is low when feeding on low N diets N affects herbivore mortality incurred through natural enemies – plant indirect defense N affects plant direct defense

Study system Plant: Cotton (Gossypium hirsutum) Beet armyworm (BAW), Spodoptera exigua

Questions? How do plant biomass and nutritional quality change in response to N? Is development of BAW affected by host plant quality? Can BAW distinguish host plants with different nutritional quality? Is lifetime feeding damage per caterpillar affected by host plant quality?

Materials and Methods Cotton plants (var. FiberMax 989)  Grown with peat moss and top soil in flower pot, fertilized with chemical solution 4-5 times weekly; 100 ml each time  Plants had 4-5 mature leaves for experiments Cotton plant growing regimes 42, 112, 196, and 280 ppm N Beet armyworm from lab colony

Materials and methods — Question 1  Percentage of total N of dried leaf blade  Petiole NO 3 -N  Biomass measurement - Plants oven-dried and weighed ----How do plant biomass and nutritional quality change in response to N

Results — Question How do plant biomass and nutritional quality change in response to N

Results — Question How do plant biomass and nutritional quality change in response to N

Results — Question How do plant biomass and nutritional quality change in response to N

Materials and methods — Question 2  Bioassay with neonate BAW caterpillars in Petri dish with excised leaves  Bioassay with 3-d-old caterpillars in caged plants which allow intra-plant movement ----Is development of BAW affected by host plant quality?

Results — Question 2 Bioassay with neonate BAW ----Is development of BAW affected by host plant quality?

TreatmentCaterpillar weight (mg)Time to pupation (d) Time to adult emergence (d) 7-d-old10-d-old 42 ppm9.00±0.52 a42.34±2.48 a15.82±0.13 a23.04±0.25 a 112 ppm11.20±0.29 b63.34±1.60 b15.09±0.18 b22.28±0.20 b 196 ppm13.10±0.62 c82.19±5.54 c 14.72±0.16 bc 21.99±0.30 bc 280 ppm12.91±0.44 c93.26±8.65 c14.49±0.10 c21.59±0.19 c Note: Other variables such as pupa weight, % caterpillars pupated, and sex ratio were not affected by N treatment. Bioassay allowing intra-plant movement Results — Question Is development of BAW affected by host plant quality?

Materials and methods — Question 3  3-d-old caterpillar caged on leaves attached to live plants with different N contents  Bioassay starts with leaf node 1, 2, and so on upward  Measure leaf tissue eaten on D2, D4, D7, and after pupation ----Is lifetime feeding damage per caterpillar affected by host plant quality?

Results — Question Is lifetime feeding damage per caterpillar affected by host plant quality?

Results — Question Is lifetime feeding damage per caterpillar affected by host plant quality?

Materials and methods — Question 4  Eight 5-d-old caterpillars released in the center  Bioassay on 3 rd leaf ---- Can BAW distinguish host plants with different nutritional quality?

Results — Question Can BAW distinguish host plants with different nutritional quality?

Summary Low N level of host plants delays BAW development Low N level of host plants tends to increase BAW caterpillar lifetime feeding damage Lower N decreases cotton plant biomass and nutritional quality for BAW caterpillars BAW caterpillars feed preferentially on high N plants

Significance ? Trade-offs between low and high N cotton plants: But lower N may tend to increase plant indirect defense through natural enemies -  BAW may move more on low N plants to locate more suitable food, which would expose them to greater mortality  Natural enemies may be more effective because BAW feeding on low N plants have prolonged developmental time Low N decreases plant biomass and suitability for BAW, but the reduced suitability may be compensated for by increased feeding by herbivores

Acknowledgments  We thank Melissa Thompson for maintenance of BAW colony  The project was financially supported by GA Cotton Commission and Cotton Incorporated