Complex feeding tracks of the sessile herbivorous insect Ophiomyia maura as a function of the defense against insect parasitoids

Abstract

Because insect herbivores generally suffer from high mortality due to their natural enemies, reducing the risk of being located by natural enemies is of critical importance for them, forcing them to develop a variety of defensive measures. Larvae of leaf-mining insects lead a sedentary life inside a leaf and make conspicuous feeding tracks called mines, exposing themselves to the potential risk of parasitism. We investigated the defense strategy of the linear leafminer Ophiomyia maura Meigen (Diptera: Agromyzidae), by focusing on its mining patterns. We examined whether the leafminer could reduce the risk of being parasitized (1) by making cross structures in the inner area of a leaf to deter parasitoids from tracking the mines due to complex pathways, and (2) by mining along the edge of a leaf to hinder visually searching parasitoids from finding mined leaves due to effective background matching of the mined leaves among intact leaves. We quantified fractal dimension as mine complexity and area of mine in the inner area of the leaf as interior mine density for each sample mine, and analyzed whether these mine traits affected the susceptibility of O. maura to parasitism. Our results have shown that an increase in mine complexity with the development of occupying larvae decreases the probability of being parasitized, while interior mine density has no influence on parasitism. These results suggest that the larval development increases the host defense ability through increasing mine complexity. Thus the feeding pattern of these sessile insects has a defensive function by reducing the risk of parasitism.

Figure 1. Two example digital images of Ophiomyia maura pupal mines.

Ophiomyia maura frequently feed on at the leaf margin early in its development and the inner part of the lamina later. Mining pattern characteristics: the upper; leaf area = 16.42 cm², total mine area = 1.13 cm², interior mine area = 0.26 cm² and fractal value = 1.13: the lower; leaf area = 9.63 cm², total mine area = 1.67 cm², interior mine area = 1.26 cm² and fractal value = 1.46. The images were digitally scanned and manipulated in color (see the Materials and Methods for details on the manipulation).

Figure 2. Relative state ratio of Ophiomyia maura larvae at the margin/inner part of the leaf.

Figure 3. The relationships of Opiomyia maura (a) larval and (b) pupal parasitism with mining pattern traits.

Mining pattern traits are represented by PC1 scores. A high PC1 score means a long (well developed), high interior-density and highly complex mine. Solid and dashed lines indicate lowess and logistic curves, respectively.