Fig. 9: FTIR spectra of additive-free LDPE films subjected to treatment with wax worm saliva (GmSal).

The spectra are for an untreated control film (grey), GmSal-treated PE (green), PE treated with heat-inactivated GmSal (red), and GmSal alone (orange). The spectra for treated PE and for GmSal alone show peaks corresponding to protein (N–H stretch around 3300 cm⁻¹, amide I and II peaks at 1650 and 1550 cm⁻¹), and a peak at ~1080 cm⁻¹, likely reflecting a hydrophobic contaminant(s) from the saliva. The peak at ≈ 1745 cm⁻¹ featured in the zoomed-in view and indicated with a red arrow is also present in all spectra except untreated PE and likely represents a saliva compound; see text for more details. 5 mm × 5 mm PE films were treated by nine consecutive applications of 5 µL of GmSal to approximately the same position in the centre of the film, with 90 min incubation at 24 °C between applications. To obtain the spectrum of GmSal alone, the liquid was allowed to air-dry, and the dry matter was deposited to the ATR sensor. Source data are provided as a Source Data file.