Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

FNZ 29 - Cryptorhynchinae (Insecta: Coleoptera: Curculionidae) - Biology

Lyal, CHC 1993. Cryptorhynchinae (Insecta: Coleoptera: Curculionidae). Fauna of New Zealand 29, 308 pages.
( ISSN 0111-5383 (print), ; no. 29. ISBN 0-478-04518-2 (print), ). Published 02 Dec 1993


The larvae of New Zealand Cryptorhynchinae are almost certainly all endophytic, but adults are rarely so (an exception is the Psepholacini adults of species in this tribe spend part of their time inside the galleries excavated by and for the larvae). Most species appear to be more active (as adults) at night than during the day, and thus in their inactive period they must conceal themselves from predators. A number of species can be collected readily during the day by beating woody plants on which there is a proportion of dead wood, or dead leaves. These plants canno tbe assumed to be host plants in the sense of supporting larval (or adult) feeding, although the weevils clearly are exercising some choice, given that their distribution in a small area is often apparently governed by plant species rather than presence of dead plant tissue.

The most 'favoured' habitat for many New Zealand Cryptorhynchinae during the day is leaf litter, from which a high proportion of species have been collected. Again, it cannot be assumed that the adult or larval food is in the leaf litter, and records of adults from birds' burrows, seaweed, moss, lichen, under rocks, etc., when larval host records are none of these, indicate that it is the concealment afforded by the habitat that is important, not its food potential.

Many adult Cryptorhynchinae, if disturbed when on the aerial part of a plant, will fold their legs against their body and fold their head down so that the rostrum lies between the front legs. In this position they will drop to the ground and lie immobile. The lack of any long projection allows them to fall without getting caught on plants, and to slip into inaccessible crannies on the ground. Their cryptic patterns make them very difficult to see and, in their shape, they may resemble such natural objects as seeds (e.g., Ampagia) or broken twigs (e.g., Ectopsis).

All Cryptorhynchinae feed on plants or plant-associated material as larvae, as do the vast majority of other Curculionidae. A large proportion of New Zealand species have been collected or reared from dead wood. However, the larvae cannot necessarily be assumed to be feeding on the dead wood itself, since digestion of cellulose has not been demonstrated, and the wood will almost always contain fungi and bacteria which might be the true food. One observation on the galleries of Psepholax macleayi noted a black staining of the wood, which may have been caused by fungi. Few insects can in fact digest cellulose directly, although many utilise symbiont bacteria, fungi, or protozoans to gain nutrients f'rom wood. Among Cryptorhynchinae, Grinbergs (1962) has identified bacterial mycetomes at the base of the malpighian tubules in Psepholax (as Empleurodes) larvae in South America.

Apart from dead wood,other larval feeding records are from live wood (although this is rare, and frequently the plant is dying or stressed in some fashion), other dead angiosperm or gymnosperm plant tissue (e.g., leaf petioles), fern rhizomes (Agacalles species only) and leafmining (some undescribed Microcryptorhynchus). May (1993) states that many cryptorhynchine larvae, perhaps the majority, are primary invaders working initially in live tissue which degenerates as feeding progresses and is decayed by the time the adult emerges.

Adult feeding habits of New Zealand Cryptorhynchinae are unknown. Some at least probably feed on green leaves, but without the support of observational data this is mere guesswork. That they do feed is suggested by the presence of gut contents in males (female gut contents could have been derived from pre-oviposition drilling). However, light microscopy of gut contents has not yet indicated what the food might be.

All known plant associations are listed for each species, and are summariscd (by plant) in Appendix 2; occurrence of the species in leaf litter is also stated. Inferences as to host (i.e., feeding) associations from these data are to be drawn only cautiously, as many of the records for adults will be of chance relationships. Clearly, only rearing records, or adults or larvae being cut from the wood of a given plant, indicate a 'real' host association. Collection of adults from freshly cut wood (see collecting methods, below) suggests that the wood or associated biota is a larval food, but this is not conclusive.

Examination of the larval host records of New Zealand species (Appendix 2) reveals very few patterns, the association of Agacalles species with fern rhizomes being the clearest, though it is notable also that Mitrastethus baridioides develops only in gymnosperms (see also May 1993). At the species level, assignment of host specificity is difficult, owing in part to the low number of observations for most species (see discussion following Eutyrhinus in the main text). The dead wood of different species may be too uniform in terms of nutrient products to encourage much host specificity.

In the lists below the inclusion of a number in parentheses (e.g., 'W 71/85') refers to a wood sample collected by DSIR s taff; a full record of rearing from the sample, and the specimens reared, are preserved in the NZAC.

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