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FOLIVORE FEEDING ON MALE CONIFER FLOWERS °
DEFENCE AVOIDANCE OR BET-HEDGING?
THOMAS SECHER JENSEN
Institute of Biological Sciences, Department of Zoology
Aarhus University, DK-8000 Aarhus C, Denmark
INTRODUCTION
In the fifties and sixties a group of British researchers led by G.C.Varley made pioneering studies on the life history
of the winter moth (Varley and Gradwell 1970). Among many other things, they concluded that the key to understanding the
main mortality in this species was the timing between egg hatch and bud-burst. Larvae which hatch on trees, having buds
firmly closed, fail to find food there. Instead such larvae disperse by spinning a thread of silk, and are blown away to an
uncertain destiny. Larvae that hatch when buds have just burst, survive well. This first model was further developed by
Feeny (1970) who in another pioneering study argued further that larvae which hatch after bud-burst face certain difficulties,
this time in relation to nutritional conditions, mainly decreased nitrogen and water, and increased tannin concentrations.
In summary these factors indicate that in Quercus robur, there is a "phenological window" represented by recently
burst, highly nutritious and poorly-defended leaves. In oak many defoliators, especially lepidopteran species, take advantage
of this window and often reach high densities, even outbreak levels, at that time. Also species richness is highest in the early
spring. However, this situation in Quercus robur does not seem to apply in a number of other tree species. The period of
shoot growth in oak is rather short, and Niemel/i and Haukioja (1982) demonstrated that in many other deciduous trees the
period of shoot growth is much longer and seems to give defoliators a chance to find a much broader phenological window.
In this paper the nutritional status of two coniferous genera, pine, Pinus, and spruce, Picea, are considered in relation
to the phenological window, including the situation when alternative food, especially flowering buds are available. The
insect species studied has mainly been the nun moth, Lymantria monacha, a close relative of the gypsy moth. The nun moth
is a polyphagous species that often defoliates large areas of conifer forest in Central Europe (Jensen 1991).
METHODS
Rearings and experiments of nun moth larvae and other insects were performed in a laboratory at constant 20°C.
Eggs were collected in the forest and brought to the laboratory during winter. In growth and survival experiments nun moths
were kept in batches of ten in the first three instars; fourth and fifth instars were reared singly. Larvae were fed needles and
flowers attached to excised twigs put in water.
Chemical analysis were performed by means of micro-Kjeldahl analysis (nitrogen) and gas-chromatography (carbohydrates,
cyclites and phenolic acids). For details see Jensen (I988).
RESULTS
Spruce
The genus Picea comprises about 30 species in the northern hemisphere. In parts of Europe, Norway Spruce, Picea
abies, is the native species, but it is also widely planted, along with the introduced North American species Sitka spruce,
Picea sitchensis. The buds burst rather early in spring along with a lot of deciduous trees, and the new shoots and needles are
Mattson, W.J., Niemel_i, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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