Oviposition site-selection by Bactrocera dorsalis is mediated through an innate recognition template tuned to γ-octalactone.

Innate recognition templates (IRTs) in insects are developed through many years of evolution. Here we investigated olfactory cues mediating oviposition behavior in the oriental fruit fly, Bactrocera dorsalis, and their role in triggering an IRT for oviposition site recognition. Behavioral assays with electrophysiologically active compounds from a preferred host, mango, revealed that one of the volatiles tested, γ-octalactone, had a powerful effect in eliciting oviposition by gravid B. dorsalis females. Electrophysiological responses were obtained and flies clearly differentiated between treated and untreated substrates over a wide range of concentrations of γ-octalactone. It triggered an innate response in flies, overriding inputs from other modalities required for oviposition site evaluation. A complex blend of mango volatiles not containing γ-octalactone elicited low levels of oviposition, whereas γ-octalactone alone elicited more oviposition response. Naïve flies with different rearing histories showed similar responses to γ-octalactone. Taken together, these results indicate that oviposition site selection in B. dorsalis is mediated through an IRT tuned to γ-octalactone. Our study provides empirical data on a cue underpinning innate behavior and may also find use in control operations against this invasive horticultural pest

γ-Octalactone is an oviposition stimulant and an oviposition site recognition cue.

<p>(a) The clear discrimination of the untreated (No OL, γ-octalactone not present) half (A) and γ-octalactone treated (OL, γ-octalactone present) half (B) by <i>B. dorsalis</i> in a single plate two-choice assay. (b) Number of eggs laid into treated and untreated pulp (one-tailed paired <i>t</i> test, <i>n</i> = 30, <i>t = </i>11.27, <i>df</i> = 19, <i>P</i><0.0001). Error bars = standard error of mean.</p

Schematic representation of the response of flies with different rearing histories towards γ-octalactone. <i>B. dorsalis</i> was reared on 3 different hosts.

<p>The final (third) generation gravid females were used in oviposition bioassays. A one-tailed <i>t</i> test revealed a significant difference (<i>P</i><0.0001) between Test (OL, γ-octalactone present) and Control (No OL, γ-octalactone not present). One-way ANOVA between the tests of group 1 (mango), 2 (banana) and 3 (guava) showed no significant difference in the mean no. of eggs laid.</p

Behavior and egg-laying response of <i>B. dorsalis</i>.

<p>(a) Behaviour assay where <i>B. dorsalis</i> were presented with a filter paper disc containing γ-octalactone. Arrows show the hallmark oviposition behavior of extending the ovipositor and probing action (for better view watch <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085764#pone.0085764.s003" target="_blank">video S1</a>). (b & c) Egg-laying response of <i>B. dorsalis</i> to a pulp disc with γ-octalactone (Test) or without (Control) in a 24-h oviposition bioassay. More eggs were laid on the test disc (<i>P</i><0.0001, one-tailed paired <i>t</i> test of non-transformed data). Error bars = Standard error of mean.</p

γ-Octalactone is a crucial cue that elicits oviposition in <i>B. dorsalis</i>.

<p>(a) Flies were allowed to oviposit into an empty agar disc. Area A contained all EAD active mango cues (cv. Alphonso) at natural ratios except γ-octalactone; Area B contained γ-octalactone alone. Flies could clearly distinguish between the untreated area (A) and the γ-octalactone treated area (B) (One-tailed <i>t</i> test, <i>t</i> = 6.610, <i>df</i> = 8, <i>P</i><0.0001). (b) The mean number of eggs laid into A or B. Oviposition in <i>B. dorsalis</i> was mediated by γ-octalactone. Error bars = Standard error of mean.</p

Effect of SA post treatment interval (days) on the oviposition of gravid female <i>B</i>. <i>dorsalis</i>.

<p>Repeated measures (mixed model) two-way ANOVA with Bonferroni post test (a) punctures (<i>F</i> = 6.86; <i>edf</i> = 20; <i>P</i> = 0.03) (b) Eggs per puncture (<i>F</i> = 130.20; <i>edf</i> = 20; <i>P</i> < 0.0001). Error bars = Standard error of mean.</p

EAG and ovipositional response of <i>Bactrocera dorsalis</i> to varying concentrations of γ-octalactone.

<p>Control = Hexane; SEM: Standard error of mean; Means within a column followed by the same letters ^a or ^b are not significantly different at <i>P</i><0.001 (ANOVA); ppm: parts per million.</p

Effect of salicylic acid (SA) on the oviposition of <i>B</i>. <i>dorsalis</i>.

<p><b>(a)</b> punctures (b) Eggs per puncture. Decreased number of punctures and eggs were recorded in SA treated fruit when compared to the control fruit. (paired <i>t</i> test, ovipunctures (0.5 mM: <i>t</i> = 7.59, <i>df</i> = 4, <i>P</i> = 0.002; 1.0 mM: <i>t</i> = 3.81, <i>df</i> = 4, <i>P</i> = 0.02; 2.0mM: <i>t</i> = 11.49, <i>df</i> = 4, <i>P</i> = 0.0003; 5mM: <i>t</i> = 6.89, <i>df</i> = 4, <i>P</i> = 0.002); eggs per puncture (0.5 mM: <i>t</i> = 5.15, <i>df</i> = 4, <i>P</i> = 0.01; 1.0 mM: <i>t</i> = 9.57, <i>df</i> = 4, <i>P</i> = 0.001; 2.0mM: <i>t</i> = 5.49, <i>df</i> = 4, <i>P</i> = 0.01; 5mM: <i>t</i> = 4.78, <i>df</i> = 4, <i>P</i> = 0.01). Error bars = Standard error of mean.</p