The effect of varying alkaloid concentrations on the feeding behavior of gypsy moth larvae, Lymantria dispar (L.) (Lepidoptera: Lymantriidae)

Abstract

Nine alkaloids (acridine, aristolochic acid, atropine, berberine, caffeine, nicotine, scopolamine, sparteine, and strychnine) were evaluated as feeding deterrents for gypsy moth larvae (Lymantria dispar (L.); Lepidoptera: Lymantriidae). Our aim was to determine and compare the taste threshold concentrations, as well as the ED(50) values, of the nine alkaloids to determine their potency as feeding deterrents. The alkaloids were applied to disks cut from red oak leaves (Quercus rubra) (L.), a plant species highly favored by larvae of this polyphagous insect species. We used two-choice feeding bioassays to test a broad range of biologically relevant alkaloid concentrations spanning five logarithmetic steps. We observed increasing feeding deterrent responses for all the alkaloids tested and found that the alkaloids tested exhibited different deterrency threshold concentrations ranging from 0.1 mM to 10 mM. In conclusion, it appears that this generalist insect species bears a relatively high sensitivity to these alkaloids, which confirms behavioral observations that it avoids foliage containing alkaloids. Berberine and aristolochic acid were found to have the lowest ED(50) values and were the most potent antifeedants.

Figure 1A-C

Concentration response curves for A) berberine, B) aristolochic acid, and C) strychnine. The alkaloids were applied to red oak leaf disks in dual choice feeding bioassays between alkaloid-treated and control leaf disks. The plus sign indicates the deterrent threshold concentration at which feeding on an alkaloid-treated disk was significantly below that on a control disk. The concentrations that are statistically significant are indicated by the asterisks. Dashed lines and their corresponding point of intersection on each concentration-response curve represent the ED₅₀ values (approximate concentrations that reduced feeding by 50%) (0.66 mM berberine; 1.40 mM aristolochic acid; 1.89 mM strychnine). Results are derived from A, n = 9-12; B, n = 11-15; C, n = 13-36 larvae (i.e., number of replicates). Error bars represent S.E.

Figure 2A-C

Concentration response curves for A) caffeine, B) atropine, and C) sparteine. Details as in Fig. 1. Dashed lines and their corresponding point of intersection on each concentration-response curve represent the ED₅₀ values (approximate concentrations that reduced feeding by 50%) (2.61 mM caffeine; 4.39 mM atropine; 7.17 mM sparteine). Results are derived from A, n = 11-14; B, n = 10-25; C, n = 10-33 larvae or number of replicates. Error bars represent S.E.

Figure 3A-C

Concentration response curves for A) acridine, B) nicotine, and C) scopolamine. Details as in Fig. 1. Dashed lines and their corresponding point of intersection on each concentration-response curve represent the ED₅₀ values (approximate concentrations that reduced feeding by 50%) (11.9 mM acridine; 15.6 mM nicotine; 28.3 mM scopolamine. Results are derived from A, n = 10-35; B, n = 13-29; C, n = 10-40 larvae or number of replicates. Error bars represent S.E.