A new approach to quantify semiochemical effects on insects based on energy landscapes.

Our ability to document insect preference for semiochemicals is pivotal in pest control as these agents can improve monitoring and be deployed within integrated pest management programmes for more efficacious control of pest species. However, methods used to date have drawbacks that limit their utility. We present and test a new concept for determining insect motivation to move towards, or away from, semiochemicals by noting direction and speed of movement as animals work against a defined energy landscape (environmentally dependent variation in the cost of transport) requiring different powers to negotiate. We conducted trials with the pine weevils Hylobius abietis and peach-potato aphids Myzus persicae exposed to various attractants and repellents and placed so that they either moved up defined slopes against gravity or had to travel over variously rough surfaces.Linear Mixed Models demonstrated clear reductions in travel speed by insects moving along increasingly energetically taxing energy landscapes but also that responses varied according to different semiochemicals, thus highlighting the value of energy landscapes as a new concept to help measure insect motivation to access or avoid different attractants or repellents across individuals.New sensitive, detailed indicators of insect motivation derived from this approach should prove important in pest control across the world

Schematic diagram of the incline energy landscape olfactometer showing the major features and dimensions.

<p>Note that the apparatus rests on a board (coloured grey) and that the angle of this board can be varied by rotation about a pivot point.</p

Test chemicals used in the Y-tube and energy landscape olfactometry studies.

<p>Test chemicals used in the Y-tube and energy landscape olfactometry studies.</p

Examples of frequency histograms (expressed as a percentage of the total number of trials conducted for that condition) of the speeds at which pine weevils walked up specified inclines in response to selected semio-chemicals.

<p>Each condition consisted of 36 trials for weevils (see text).</p

Schematic relationship between speed, incline and power of an insect climbing up 3 slopes of different inclines (after Full & Tullis 1990), showing how the results gained within the energy landscape olfactometer relate to energy expenditure.

<p>The grey box shows the proposed operational area of subjects. Walking towards an attractive semio-chemical, insects can either maintain power use at a constant level for the varying inclines, in which case speed is expected to drop with increasing incline (arrows terminating at A and B), or they can maintain speed, in which case power requirements increase with increasing incline (arrows terminating at C and D). In reality (red lines), animals are likely to operate somewhere between these two extremes (see arrows terminating in ‘e’ and ‘f’ for speed and ‘g’ and ‘h’ for power) with the more motivated subjects tending to maintain speed and incur increased power use with increasing incline.</p

Y-tube olfactometer results for numbers of weevils choosing to walk towards either the semiochemical or the control for; (A) control [for examining arm preferences], (B) water [for examining differences between our control in A and water], (C) ethanol [for examining differences between our control in A and ethanol], (D) α-pinene, (E) β-pinene, (F) 3-carene and (G) garlic metabolic solution [GMS], all against the control for their respective solvents.

<p>Significant differences are indicated by (*).</p

Speed at which aphids and weevils responded to different chemicals and the manner in which this changes with incline, represented by box and whisker plots indicating median (bold line), inter-quartile ranges (box), 95% confidence intervals (whiskers), and outliers (*).

<p>(<b>A</b>) Speed with which weevils move towards different chemical attractants (the control shown is the blank - Fig. 2a). The type of chemicals to which weevils were exposed significantly altered subjects approach speed (LMM: Wald = 10.86, df = 4, P = 0.028), with subjects moving faster towards α-pinene than all other treatments (significant pairwise differences (P<0.05) across chemical trials are indicated by different letters). (<b>B</b>) Speed with which weevils moved at different inclines (LMM: Wald = 75.21, df = 1, P<0.001). (<b>C</b>) Speed with which aphids move away from different chemical repellents (the control shown is the blank cf. Fig. 2a), which significantly affected aphid speed (LMM: Wald = 105.09 df = 5, P<0.001). Significant pairwise differences (P<0.05) across chemical trials are indicated by different letters. (<b>D</b>) Speed with which aphids move at different inclines(LMM: Wald = 75.21, df = 1, P<0.001). See methods for more details of models.</p

Changes in aphid movement speed as a function of incline and surface roughness.

<p>(<b>A</b>) The predicted effect of incline upon aphid speed to move away from chemicals, from a model in which the intercept and slope of the effect of incline is allowed to vary with respect to chemical (i.e. a random slope, random intercept model), and in which our control condition is used as the reference category. See methods for further details. (<b>B</b>) Box and whisker plot showing the overall effect of surface type upon speed (LMM: Wald = 93.94, df = 1, P<0.001); shown is the median (bold line), inter-quartile ranges (box), 95% confidence intervals (whiskers), and outliers (*).</p

Model results for how incline is predicted to affect weevil speed.

<p>In this model, the intercept and slope of the effect of incline were allowed to vary with respect to chemical (i.e. a random slope, random intercept model) and our control condition was used as the reference category. See methods for further details.</p