Forestry alters foraging efficiency and crop contents of aphid-tending red wood ants, Formica aquilonia

Abstract

Forest management alters species behaviours, distributions and interactions. To evaluate forestry effects on ant foraging performance, we compared the quality and quantity of honeydew harvested by ants among clear-cuts, middle-aged and mature spruce-dominated stands in boreal forests in Sweden. Honeydew quality was examined using honeydew collected by squeezing the gasters of laden Formica aquilonia workers. We used fifteen laden individuals at each study site (four replicates of each stand age) and analysed honeydew chemical composition with gas chromatography-mass spectroscopy. To compare the quantity of honeydew collected by individual ants, we collected and weighed five ants moving up and five ants moving down each of ten trees at the twelve sites (totally 1200 ants). The concentration of trehalose in honeydew was lower in clear-cuts compared with middle aged and mature stands, and similar trends were shown for sucrose, raffinose and melezitose, indicating poorer honeydew quality on clear cuts. Concentrations of the amino acid serine were higher on clear-cuts. The same trend occurred for glutamine, suggesting that increased N-uptake by the trees after clear cutting is reflected in the honeydew of aphids. Ants in mature stands had larger heads and carried proportionally more honeydew and may therefore be more efficient foragers. Human alternation of habitats through clear-cutting thus affects food quality and worker condition in F. aquilonia. This is the first study to show that honeydew quality is affected by anthropogenic disturbances, likely contributing to the reduction in size and abundance of F. aquilonia workers and mounds after clear cutting.

Figure 1. Relationship between body mass and head width for ants ascending and descending trees in stands of different ages.

Line equations are: descending ants: clear-cut: y = −0.015+0.015x, R² = 0.65; middle-aged: y = −0.015+0.015x, R² = 0.64; mature: y = −0.017+0.015x, R² = 0.57; ascending ants: clear-cut: y = −0.012+0.012x, R² = 0.73; middle-aged: y = −0.011+0.012x, R² = 0.71; and mature: y = −0.012+0.012x, R² = 0.80.