Provenance & quality
At each recording session, three palms were selected at random from within experimental plots. Random palm selection was done using the rnd function in excel before visiting the field, as each palm in the experimental plots has a unique identifying tag. In the replanted stands where palms are not tagged, the randomisation was done by counting steps from the corner of the plot, again using the rnd function in excel.
First the damage to the crown form herbivory was estimated visually from the ground. Then, counting from the youngest fully-opened frond, the 17th frond in the crown of the palm was cut and caught in a white cloth. Frond 17 represents a middle-aged frond (aged between 8.5 and 9 months) in mature palms and is commonly used by the oil palm industry as a standard to measure herbivory and herbivore load. The damage to the frond (in percent) was estimated visually. Then twenty paired leaflets, spaced evenly along the frond, were cut, placed flat on a white board and photographed (using a Nikon D90). Photography and visual damage estimates were always carried out by the same person, from the same angle (bottom left), to reduce sampling bias.
Photographs were pre-processed to remove noise and converted to binary images in Microsoft Paint. The total leaflet area was then measured in Fiji (https://fiji.sc/). A second image was prepared in Microsoft Paint, with all herbivory damage ‘filled in’, such that the leaflets appeared to be intact. Where leaflet tips were missing, a conservative estimate for the leaflet tip was drawn in, based on the length of neighbouring leaflets. This second image was measured in Fiji in the same way and the percent of the leaflet area consumed by herbivores was calculated.
Basic data checks were carried out (checks for impossible values, such as an increase in leaf area after processing or extreme discrepancy between visual estimate and image processing values).