Evidence for trichromacy in the green peach aphid, Myzus persicae ...

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occidentalis (Pergande) (Matteson et al.,1992),and—as
the only phytophagous Homopteran species—the glasshouse
whitefly, Trialeurodes vaporariorum (Westwood)
(Mellor et al.,1997) have been examined. Especially,
there is no data regarding the spectral sensitivity of
aphids yet (Prokopy and Owens,1983; Gao et al.,2000;
Kelber et al.,2003). Therefore,the aim of this study was
to apply the electroretinogram (ERG) technique (Goldsmith
and Bernard,1974; Brown and Anderson,1996;
Scharstein and Stommel,1999) toM. persicae in order
to contribute to a better understanding of the spectral
properties of the eye and host finding of this agriculturally
most important aphid species.
2. Materials and methods
2.1. Electroretinogram recordings
A 100 W xenon arc lamp (Osram XBO100W/2 OFR)
was used as a light source in Xe-100 lamp housing device
(UV-Gro¨ bel,Ettlingen,Germany). Narrow bandpass
filters (10 nm half-width,from Ealing Davin Optronics,
Watford,UK) provided nearly monochromatic light in
the wavelength range 320–640 nm in 10 nm steps.
Neutral density filters (Ealing) allowed the intensity of
light passing through the bandpass filters to be varied
over approx. 2.5 log units. A manual shutter (T70
camera,Canon) produced 0.5 s light flashes as test
stimuli,which were transmitted through a 3 mm dia.
liquid light guide (UV-Gro¨ bel) into a light-proof Faraday
cage. The terminal end of the light quide was fixed
to a micromanipulator (Prior,Cambridge,UK) at about
1.5 cm distance to the aphid’s eye.
Summer migrants (winged virginoparae) were reared
on chili pepper plants (Capsicum annuum L.) under
crowded conditions in long days (16 h light,8 h dark at
20 and 16 1C,respectively) in an Environmental
Incubator (Inculight 400,Schu¨ tt,Go¨ ttingen,Germany).
The aphids were mounted on double-sided sticky tape
on a 10 10 mm platform. Under a stereomicroscope
(80 magnification,Leica S6E from Leica,Wetzlar,
Germany),legs,wings and antennae were gently secured
on the tape in order to immobilise the aphid. The
recording and indifferent electrodes were made from
electrolytically sharpened 0.2 mm diameter tungsten
wire (Goodfellow,Cambridge,UK). The recording
electrode was inserted into the left eye through the
cornea and the grounded reference electrode was placed
in the abdomen. Light stimuli were applied to the eye
and the response was displayed on a Digital Storage
Oscilloscope (HM205-3 from Hameg,Mainhausen,
Germany) via an AC amplifier equipped with low-pass
(300 Hz) filters and high-pass (0.1 Hz) filters (P-55 from
Grass Telefactor,West Warwick,USA). All recordings
took place at 20 1C.
ARTICLE IN PRESS
S.M. Kirchner et al. / Journal of Insect Physiology 51 (2005) 1255–1260
2.2. Experimental protocol
The optical system was calibrated at the position of
the aphid’s compound eye by measuring the emitted
light for all possible combinations of our neutral density
filters and bandpass filters with a spectrometer
(RAMSES ACC,from TriOs,Osnabru¨ ck,Germany)
in mW s 1 m 2 . These data were transformed to photon
flux equivalents by multiplication with the respective
wavelengths.
Pretests at 350,450,and 550 nm showed that
intensity–response curves followed log-linear functions.
An appropriate criterion response was selected to ensure
that over the whole wavelength range a constant
response could be produced by adjusting the light
intensity. For each wavelength,0.5 s flashes of 3–5
different light intensities were given to produce responses
near the selected criterion response. The photon
flux required to generate the exact criterion response
was found by calculating linear regression of the
response values as plotted against a log photon flux
scale. Spectral sensitivity was calculated as 1/photon flux
and values were normalised relative to the maximum
sensitivity for each individual. A mean curve was
produced from 7–9 individual aphids.
Spectral sensitivity was investigated under three
different adaptation conditions: (1) dark adapted,(2)
adapted with white UV-free light,and (3) yellow
adapted (590 nm). Adaptation light was produced by
LEDs and was spectrally characterised with a spectrometer
(TriOs RAMSES ARC) (Fig. 1). The adaptation
light source was fixed to the terminal end of the light
guide. Adaptation time prior to test flashes was 30 min.
The test flashes were delivered under continuous
presence of adaptation light. After pre-adaptation the
duration of the test series for each individual was
40–50 min.
Fig. 1. Normalised irradiance spectrum of LEDs for adaptation
(arbitrary units); UV-free white (continuous line) and yellow (dotted
line).