Research Article |
Corresponding author: Jean-Luc Boevé ( jean-luc.boeve@naturalsciences.be ) Academic editor: Hannes Baur
© 2015 Jean-Luc Boevé.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Boevé J-L (2015) Multimodal defensive strategies in larvae of two Hemichroa sawfly species. Journal of Hymenoptera Research 46: 25-33. https://doi.org/10.3897/JHR.46.7064
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The two European sawfly species in the genus Hemichroa are a contrast in behaviour and appearance, since H. crocea is gregarious and brightly coloured, whereas H. australis is solitary and cryptic. Here, their defensive strategies are compared by integrating further components. In both species, ventral glands are minute, and no distinctive volatiles were detected by chemical analysis; hence, these exocrine glands are probably irrelevant in defence. Ethanol extracts of body parts were feeding deterrent to ant workers of Myrmica rubra, especially the integument of H. australis which was more deterrent than that of H. crocea. Single, living larvae of H. crocea were also attacked more frequently by ants. In contrast, single larvae of H. crocea are reluctantly taken by the bird Parus major that readily feeds on H. australis. The larvae of both species jerk their abdomen to physically defend themselves and/or to increase their (visual) warning signal (H. crocea). The larvae of H. crocea can scratch the host plant leaf with the tip of their abdomen to produce a sound assumed to convey information in intraspecific communication. However, this behaviour was also elicited from H. australis, when disturbed, which suggests that it may have another function. The defensive strategy is multimodal in both species. The principal differences are the reliance on gregariousness in H. crocea, as opposed to the use of integumental chemicals in H. australis.
Hymenoptera , Tenthredinidae , defence strategy, ants, behaviour, sounds, chemical ecology
The sawfly genus Hemichroa (Hymenoptera, Tenthredinidae, Nematinae) constitutes a small group of 13 recognized species (
Pictures of larvae of the two studied Hemichroa species. a, b H. australis, solitary (body length ca. 18 mm) c, d H. crocea, gregarious (body length ca. 20 mm). Field host-plant [sawfly collection reference number]: a Alnus glutinosa [P2553] b A. glutinosa [P3999] c Betula verrucosa [P3225] d A. glutinosa [P3230]
Nematinae larvae are characterized by the presence of ventro-abdominal exocrine glands which are turned inside out to emit volatiles used in defence (
It is likely that the defensive strategy of Hemichroa larvae is multimodal, combining behavioural, visual, chemical, and acoustic traits. This paper examines two aspects of their defence by using a comparative approach. The principal purpose was to determine whether or not H. australis and H. crocea – which display opposite appearance and gregariousness – also differ in other (behavioural and chemical) traits, and in the consequent effectiveness of their defensive strategies. Another aspect was to test whether and how acoustic cues are involved in defence.
Larvae of Hemichroa were collected in Belgium and identified following
Field observations were performed and documented with Pentax Optio W10 and Nikon Coolpix P300 cameras. An audio file was obtained in indoor conditions with a Zoom H4n digital recorder, its microphones being placed a few cm from a leaf harbouring a group of H. crocea larvae.
Ventral glands were dissected from larvae preserved in 70 % ethanol, then mounted between glass slides and plates. Glands were also dissected from larvae stored at -30 °C and thawed, to be analysed via solid sample injections by gas chromatography-ion-trap mass spectrometric detection (GC-ITD) as described in
Hemolymph was collected with glass capillaries from live larvae. Afterwards, the larvae were frozen and the thawed specimens dissected to isolate integument and internal organs (mainly the digestive tract). The three samples from a batch of larvae were extracted in ethanol, then filtered, dried, and dissolved in sugar water. The laboratory, dual-choice bioassay consisted of comparing the number of ant workers of Myrmica rubra feeding on sugar water versus sugar water plus extract. Another bioassay consisted of placing a single live larva in the presence of 20 ants; the number of ants attacking the larva was counted, and the behavioural interactions were noted. All experimental procedures are detailed in
The larvae of H. crocea and H. australis settle on the edge of a leaf, firmly gripping with their thoracic legs. Younger larvae make a hole in the leaf, thus feeding on the inner leaf edge, whereas older larvae feed on the outer edge, which is especially the case for H. crocea (
If disturbed, the larva places its body on the leaf side opposite to the source of disturbance, especially so in H. australis. Larvae can also perform defensive movements with the abdomen. These movements are either hearable scratch sequences (see Introduction; Suppl. material
The minute and flattened ventral glands of H. crocea and H. australis are associated with only one pair of retractor muscles. The secretory layer is composed of only about 25 glandular cells on each side of the pouch.
Only small amounts of chemical compounds were detected by analysing a whole ventral gland. These were alkanes with an odd number of carbon atoms from 23 to 27 in H. crocea, and 21 to 27 in H. australis.
All extracts at a starting-test concentration of 8 mg DW extract / ml sugar water significantly deterred ants (Tab.
Feeding deterrence rates of extracts of Hemichroa larvae against M. rubra ants.
Species | Extract | 8.0 mg DW/ml | 2.6 mg DW/ml | 0.8 mg DW/ml |
---|---|---|---|---|
H. australis | Hemolymph | 61** (93) | 7 (198) | 8 (141) |
Integument | 65** (126) | 24* (124) | 6 (128) | |
Internal organs | 87** (169) | 39** (154) | 9 (200) | |
H. crocea | Hemolymph | 57** (139) | 36** (115) | 3 (218) |
Integument | 31* (283) | -2 (182) | – | |
Internal organs | 91** (129) | 45** (131) | 6 (175) |
A single living larva of H. australis was significantly less likely to be attacked (by a mean ± SD of 3.5 ± 2.6 ants) than one of H. crocea (6.5 ± 1.8) (P < 0.05, Mann-Whitney test, two-tailed). Both sawfly species made violent body movements while being attacked. Conversely, H. australis provoked clearer signs of distress in the ants than H. crocea.
Anti-predator defensive mechanisms often act in concert, but are dynamically modulated so as to produce specific responses to threats that vary in type, time, and intensity (
Ventral glands are greatly reduced compared to other Nematinae species. The detected alkanes, from heneicosane to heptacosane, were not unique to Hemichroa or the Nematinae, but correspond to those hydrocarbons generally occurring on the cuticular surface of insects (
Both sawfly species exhibit similar abdominal movements. A larva can switch between jerking and scratching within a short period. The jerks are a common defensive behaviour among Nematinae larvae, and they can knock down a foraging ant or parasitoid, but are inefficient against birds as a physical defence (
Scratching is known to be a way of inter-individual communication in H. crocea. However, it is concluded here that the behaviour may be part of the defensive strategy in this gregarious species as well as in the solitary H. australis. There are gradual, behavioural responses to increasing levels of disturbance, with hiding (behind the leaf) followed by scratching, and finally jerking. The defensive arsenal is multimodal, involving behavioural traits as well as visual (gregariousness; brightly coloured versus cryptic integument), chemical (water-soluble chemical compounds), and acoustic (sounds by scratching) traits. The divergence between the two defensive strategies is gregariousness in H. crocea and integumental chemicals in H. australis. The identity of these chemicals remains unknown. They may be plant-derived since the digestive tract (as main part of the internal organs) was overall the most active extract tested. The comparison of the defensive strategies between the two Hemichroa species reveals, 1) obvious contrasts in larval appearance and gregariousness, 2) points of similarity in jerking, scratching and in the absence of functional ventral glands, and 3) different defensive efficiencies against ants and birds, with single larvae of H. crocea being better defended against birds, whereas H. australis against ants.
Jacques M. Pasteels (Brussels), Herbert R. Jacobson (Chico, California) and the reviewer Stefan Schmidt are warmly thanked for constructive comments on the manuscript.
Audio of scratch sequences performed by a group of larvae of H. crocea
Data type: audio
Explanation note: Audio (mp2; duration 3min25sec) of scratch sequences performed by a group of larvae of H. crocea. Larvae were collected in the field (Ave-et-Auffe, 08.X.2013) [P3799], but sounds recorded in indoor conditions (10.X.2013, between 9 and 10 PM). For clarity, the original audio was 20 dB amplified, followed by a 24dB background noise reduction. Note that the third scratch sequence is quite loud. Following
Group of larvae of H. crocea performing rather synchronized jerks with their abdomen
Data type: video
Explanation note: Video (m4v; duration 19sec) in field conditions of a group of larvae of H. crocea performing rather synchronized jerks with their abdomen. The larvae are those shown in Fig.
Larva of of H. australis performing the leaf scratching behaviour
Data type: video
Explanation note: Video (m4v; duration 40sec) in field conditions of a larva of H. australis performing the leaf scratching behaviour. Larva is the one shown in Fig.