In the March 1998 issue of Environment Hawai`i, Teresa Menard questioned the stability of the host preference of Phyllonorycter myricae Dechka, an exotic gracillariid moth proposed for introduction into Hawai`i as a biological control agent against fayatree, or Myrica faya (Myricaceae). The question is important because an insect having a tendency to undergo host shifts would be unsuitable as a biological control agent.
A “host shift” is the adoption of a host that formerly was rejected (or, at least, would have been rejected had the plant been exposed to the insect). Silverleaf whitefly (Bemisia argentifolii) is an example of an insect that is notorious for undergoing host shifts. Insects may also adopt new hosts to which they are pre-adapted when circumstance brings insects and plants together for the first time. It is important to distinguish between these two situations. True host shifts stem from genetic changes that are relatively uncommon (at least in most taxonomic groups) and difficult to predict, whereas adoption of new hosts already within the host range is relatively frequent and easily predictable on the basis of host-specificity tests (the preference of Rhinocyllus conicus beetles for Cirsium thistles is a rather famous example of the latter).
Menard offered the following facts as indirect evidence that P. myricae might use new hosts after its introduction into Hawai`i:
* P. leucographella in Denmark prefers Pyracantha but feeds occasionally on other Rosaeae (Buhl et al., 1994).
* P. messaniella attacks oaks (Quercus spp.) and beeches (Fagus spp.) in its native home in continental Europe. After southern beech (Nothofagus sp. was introduced into the British Isles, P. messaniella also attacked it (Young 1997).
* P. maestingella ordinarily attacks a beech (Fagus sylvatica) (Fagaceae), but Groenen (1996) reported that the species completed development on a new host plant, Wisteria floribunda DC (Fabaceae), that was growing near an infested beech tree.
P. leucographella
In fact, the wide host range of P. leucographella implies nothing about stability of host preference in P. myricae. Phyllonorycter is only one of a great many genera composed of species having different levels of host specificity. Some genera tend toward polyphagy, others tend toward monophagy, while many have both monophagous and polyphagous members. Polyphagy in a few members of a genus is no cause to suspect host shifts throughout the genus. Apion ulicis and A. fuscrirostre are biological control agents that have long been reported only from gorse and Scotch broom, respectively, even though a close relative, A. impunctistriatum, attacks plants in many families.
Nevertheless, if a single type of food habit strongly predominates in a genus, one might reasonably wonder about the stability of the minority members. To determine the predominant host range within the genus Phyllonorycter, 38 species of Phyllonorycter were checked in the data base of the Center for Agriculture and Biosciences International (CABI) for the period 1989-May 1998. P. myricae is the 39th species for which host records were easily available. The following results were obtained:
* Thirty species (77 percent) attacked plants in a single genus.
* Four species (10 percent) attacked plants in two genera of the same family.
* Three species (8 percent) attacked plants in more than two genera of the same family.
* Two species (5 percent) attacked plants in two families (however, only one genus in each family was attacked).
* No species attacked plants in more than two families.
These data indicate a strong tendency toward host specificity in the genus Phyllonorycter. P. myricae seems to be typical in this regard.
P. messaniella
The simplest interpretation of the facts as presented by Young (1996) is that southern beech, a fagaceous tree, was always within the host range of P. messaniella. The moth, a native of continental Europe, was already known to feed on plants in two other genera of Fagaceae. It was not known that southern beech was in the insect’s host range until man brought moth and tree together (southern beech is native to Australia and New Zealand). This was also the substance of Young’s interpretation, although he employed different language.
Similarly, Phyllonorycter myricae, as we know from host-specificity tests, attacks two species of Myrica — M. faya and M. cerifera. But that does not mean that no other species of Myrica are within the moth’s host range. If man ever brings the moth together with other species of Myrica, the moth may well attack them. In fact, it may even attack other genera of Myricaceae because the host specificity tests conducted on P. myricae were not designed to explore the moth’s limits within that plant family (no native Myricaceae occur in Hawai`i). If and when the moth does attack Myricaceae to which it has never before been exposed, there will be no need to postulate a host-shift with all that implies about underlying genetic changes. Occam’s razor, the Law of Parsimony, applies: The simplest explanation is the one most likely to be correct.
P. maestingella
The rearing of P. maestingella from wisteria was a new record. Since the wisteria plant was growing adjacent to a beech tree infested with P. maestingella, the wisteria may have been exposed many times to ovipositing adults that had recently left the usual host. Only a single larva was reared from the wisteria. Development of a single larva on an anomalous host in these conditions does not seem to be cause for concern. Had a lengthy series of moths been reared from the wisteria, or had even a single individual been reared from a wisteria plant that was remote from any infested beech, the matter might require further study.
Finally, the slight possibility of a host shift in P. myricae must be conceded because that possibility exists for all insects, including the hundreds of phytophagous insects native to Hawai`i.
Whether or not P. myricae will be successful in controlling fayatree is an open question, but present evidence does not cast doubt on the stability of its host specificity.
Ron Hennessey
Literature cited:
Buhl, O., P. Falck, B. Jorgensen, O. Karsholt, K. Larsen, and F. Vilhelmsen. 1994. Records of microlepidoptera from Denmark in 1993. Entomologiske Meddelelser 62: 105-115.
Groenen, Fl. 1996. Wisteria floribunda, a new host plant for Phyllonorycter maestingella (Lipidoptera: Gracillariidae). Entomologische Berichten 56(3): 60.
Menard, T. 1998. USDA will re-evaluate proposal to release moth for fayatree control. Environment Hawai`i, March 1998, pp. 6-9.
Young, M. 1997. The Natural History of Moths. T. & A.D. Poyser, Ltd., University Press, Cambridge, Great Britain.
Ron Hennessey is an entomologist with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS), Riverdale, Maryland (e-mail: Ronald.D.Hennessey @usda.gov). The views expressed in this article are the author’s and are not an official position of APHIS.
Volume 9, Number 8 February 1999