The Influence of Learning on Host Plant Preference in a Significant Phytopathogen Vector, <i>Diaphorina citri</i>

<div><p>Although specialist herbivorous insects are guided by innate responses to host plant cues, host plant preference may be influenced by experience and is not dictated by instinct alone. The effect of learning on host plant preference was examined in the Asian citrus psyllid, <i>Diaphorina citri</i>; vector of the causal agent of citrus greening disease or huanglongbing. We investigated: a) whether development on specific host plant species influenced host plant preference in mature <i>D</i>. <i>citri</i>; and b) the extent of associative learning in <i>D</i>. <i>citri</i> in the form of simple and compound conditioning. Learning was measured by cue selection in a 2-choice behavioral assay and compared to naïve controls. Our results showed that learned responses in <i>D</i>. <i>citri</i> are complex and diverse. The developmental host plant species influenced adult host plant preference, with female psyllids preferring the species on which they were reared. However, such preferences were subject to change with the introduction of an alternative host plant within 24–48 hrs, indicating a large degree of experience-dependent response plasticity. Additionally, learning occurred for multiple sensory modalities where novel olfactory and visual environmental cues were associated with the host plant. However, males and females displayed differing discriminatory abilities. In compound conditioning tasks, males exhibited recognition of a compound stimulus alone while females were capable of learning the individual components. These findings suggest <i>D</i>. <i>citri</i> are dynamic animals that demonstrate host plant preference based on developmental and adult experience and can learn to recognize olfactory and visual host plant stimuli in ways that may be sex specific. These experience-based associations are likely used by adults to locate and select suitable host plants for feeding and reproduction and may suggest the need for more tailored lures and traps, which reflect region-specific cultivars or predominate Rutaceae in the area being monitored.</p></div

Experience dependent host plant preference.

<p>Shifts in preference after short-term adult experience feeding on the alternative host plant species. The labels “Kept on SO” (sour orange) or “Kept on OJ” (orange jasmine) represent the control groups—<i>D</i>. <i>citri</i> maintained on a single host plant species for the duration of the study. The experimental groups are labeled as “Moved to SO” or “Moved to OJ.” Significant differences between groups at each time point are indicated by different letters (χ² test, α≤0.05).</p

Developmental host plant preference.

<p>Differences in female <i>D</i>. <i>citri</i> preference based on natal host plant species. Asterisks within bars indicate statistically significant differences within groups, while asterisks associated with a bracket indicate differences between groups (χ² test, *: < 0.05, **: < 0.01).</p

Learned response to a novel olfactory stimulus (a) and a visual stimulus (b).

<p>(a) Light gray bars indicate responses to a novel olfactory stimulus (vanillin odor). (b) Blue bars indicate the responses to a novel visual stimulus (blue light). (a, b) White bars indicate responses to the blank control. Asterisks (*) within bars indicate statistically significant differences within groups selecting arm A or arm B, while asterisks (*) outside bars indicate differences between naïve and experienced insects (χ² test, *: < 0.05, **: < 0.01, ***: <0.001).</p

Latency to selection for <i>D</i>. <i>citri</i>.

<p>Differences in selection time in female (gray bars) and male (white bars) psyllids based on (a) the stimulus modality of the test (olfactory or visual test) and (b) the learning treatment applied to the insects (naïve or experienced). Asterisks (*) indicate significant differences *: < 0.05, **: < 0.01, ***: < 0.001).</p

Compound conditioning experimental test groups.

<p>Each group differs only in stimulus presentation during testing. Each single stimulus is the presentation of the olfactory or visual stimuli alone. The compound stimulus is presentation of the olfactory and visual stimuli simultaneously.</p

Experimental conditioning procedure.

<p>In single stimulus tests, either blue light or vanillin were paired with the host plant. In compound conditioning, all test groups received simultaneous presentation of the visual and olfactory stimuli.</p

<i>D</i>. <i>citri</i> responses after compound conditioning to six test conditions (a-f).

<p>Gray bars represent the olfactory stimulus (vanillin odor), blue bars represent the visual stimulus (blue light), striped blue and gray bars represent the compound stimulus (vanillin odor + blue light), and white bars indicated a blank control. Asterisks (*) within the bars indicate statistically significant differences within a group selecting arm A or arm B (χ² test, *: < 0.05, **: < 0.01, ***: < 0.001), ****: <0.0001. Asterisks (*) outside the bars indicate differences between a set of naïve and experienced insects for a given test condition (χ² test).</p