5.1.2A Sensory organs

Bark beetle sensory organs, including visual, olfactory and gustatory receptors, are studied not only to understand the ecological interactions between host trees and bark beetles such as host finding and acceptance, but also many other behavioral processes such as communication, mating, feeding, and oviposition. The eyes of bark beetles are needed for flight, and in conjunction with antennae, for orientation toward or away from semiochemical sources. The eyes of bark beetles (e.g., Ips, Scolytus, and Pityogenes) have about 100-240 ommatidia, relatively less than many insects (Chapman, 1972; Byers et al., 1989a, Fig. 2). EM scan of Tomicus piniperda head
Fig. 2. Lateral view of the head of Tomicus piniperda seen through the scanning electron microscope showing eye with ommatidia and antenna with club containing the olfactory and tactile sensory hairs (height of head is 800 µm, preparation by E. Hallberg and J. Byers).
Two color receptor types in the eyes are indicated, based on electrophysiological recordings with a maximum at 450 nm (blue) and 520 nm (green)(Groberman and Borden, 1982). Observations of I. paraconfusus, I. typographus, D. brevicomis, P. chalcographus, and T. piniperda in flight chambers under dim red light or in complete darkness using an electronic vibration detector indicate they will not fly after dark (Lanne et al., 1987; Byers and Löfqvist, 1989; Byers unpublished). Bark beetles are attracted more so to traps baited with host odor or pheromone that are placed next to "tree trunk silhouettes" than to traps without such visual stimuli, indicating that beetles orient to the tree trunk during landing (Moser and Browne, 1978; Borden et al., 1982; Tilden et al., 1983; Lindgren et al., 1983; Bombosch et al., 1985; Ramisch, 1986; Chénier and Philogéne, 1989). Beetles of some species prefer to land on horizontal silhouettes rather than on vertical ones of the same size (Pitman and Vité, 1969). Bark beetles have relatively poor visual acuity, for example, my observations of T. piniperda indicate that males must walk within 1 cm of a female beginning her entrance hole before they can detect her and initiate guarding behavior. The same individuals as well as those of D. brevicomis can be induced to drop off the tree by movements of the human body about 2 m away (about the same angle of resolution and relative size).

Little is known about the sensilla on the maxillary and labial palpi as well as surrounding mouthparts in bark beetles except for morphological studies of D. ponderosae and I. typographus (Whitehead, 1981; Hallberg, 1982). In these species there is clearly a large number of chemosensilla (Fig. 3) and these appear important for host selection and food discrimination (as will be discussed in parts 5.2 and 5.3). EM scan of Tomicus piniperda mouthparts
Fig. 3. Ventral view of the mouthparts of Ips typographus seen through the scanning electron microscope showing the labial palpi (central pair) and maxillary palpi with their chemo- and mechanoreceptor hairs that are important in feeding behavior (maximum width is 700 µm, preparation by E. Hallberg and J. Byers).
The tarsi and ovipositor in other insects have chemosensilla (Städler, 1984), but these have not been studied in bark beetles; it is assumed that all important chemosensory functions involve the mouthparts and antennae. Most work has involved the antennae (Fig. 2), which are known to have sensilla responsive to volatile pheromone and host components as well as other air-borne chemostimulants (Borden and Wood, 1966; Payne et al., 1973; Payne, 1979; Mustaparta, 1984; Faucheux, 1989).
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Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.