Contributions to Zoology, 82 (1) – 2013Andrea Grill; Andrea Cerny; Konrad Fiedler: Hot summers, long life: egg laying strategies of Maniola butterflies are affected by geographic provenance rather than adult diet
Discussion

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How does geographic provenance relate to oviposition strategy and longevity?

Only in one population the timing of oviposition was weakly affected by the addition of amino acids. Pannonian females oviposited earlier when they received amino acids in their nectar substitute (Fig. 1c). However, these females did not oviposit earlier than those of other populations, rather the sucrose-fed females from Pannonia oviposited later than the amino acid-fed individuals of the same population. In the other populations there was no detectable effect of amino acids on the timing of oviposition. Similarly, the duration of the reproductive period and total lifespan were not significantly affected by the addition of amino acids.

Development hold-up before oviposition was very pronounced in Sardinian Maniola (M. nurag and M. jurtina) which showed a clear delay in egg-laying with respect to all other M. jurtina. These observations are completely in line with earlier investigations on Ma­niola jurtina from Mediterranean origin (e.g. Scali, 1971; Grill et al., 2006). As individuals from Krk were not collected immediately after eclosion, but towards the end of August, many of them oviposited immediately after capture, which is rather an artefact than a result of the experiment. Very interestingly, Pannonian individuals significantly differed from their relatives from nearby Austrian mountains in lifespan (see Figs 3b, 4b), and their egg-laying curve (Fig. 2) resembled that of Mediterranean individuals. This may follow from the climatic circumstances in Pannonian landscapes, where summers are considerably hotter, drier and last longer than in the adjacent alpine upland and winters are only moderately cold (Neuwirth, 1976; Köllner, 1983; Harlfinger and Knees, 1999). We do not exclude that also Pannonian individuals experience an (at least short) dormancy period before egg-laying, similar to Mediterranean ones.

Compared to the aforementioned species of Pieris which are known migrants, Maniola butterflies are quite sedentary. Very few individuals move more than a hundred meters within their lifetime (Grill et al., 2006). Moreover, they are restricted to one adult generation per year. This obliges these butterflies to match their survival and reproductive strategy with the climatic conditions at the location where they have grown up.

Delayed oviposition is an ecological trait of various Lepidoptera species (see for example García-Barros, 1988, 1992; Grüner and Sauer, 1988; Sauer and Grüner, 1988) and has been known from M. jurtina butterflies since the 1970s (Scali, 1971). It has also been observed in other species of the genus Maniola occurring in Mediterranean habitats, like the island endemic M. nurag (Grill et al., 2006). Even though Meadow Browns have been studied by various authors (Brakefield, 1982a, b; Goulson, 1993), it remains unclear what proximate cues trigger the butterflies to deposit their eggs immediately after mating, or keep them in store for ecologically better times.

Developmental hold-ups are usually under endogenous control in univoltine insect species (Košťál, 2006). Multivoltine species, on the other hand, like the Large White butterfly Pieris brassicae, are known to react more plastically to exogenous factors (Spieth et al., 2011 and references therein), giving them the advantage to finely adjust the number of generations per year depending on environmental conditions. For univoltine Maniola we would thus expect endogenous factors rather than actual habitat conditions to regulate egg-laying strategies. These could be genetic or influenced by the conditions the butterflies had been exposed to in an earlier developmental stage than the imago. If only the photoperiod during the flight period of the adults were to influence oviposition and induce aestivation, as has been shown for a number of other butterfly species like Pieris brassicae (Held and Spieth, 1999), P. melete (Xiao et al., 2009), or Polygonia c-aureum (Fujita et al., 2009), all butterflies in our experiment should have oviposited more or less synchronously, as they were kept under identical light conditions. But they did not. Photoperiod will surely have a certain effect but is definitely not the only regulatory mechanism for the great variation in oviposition behaviour observed in Maniola.

Maniola butterflies spend the winter in the larval stage. Day-lengths during winter time do not vary substantially between, say, Sardinia and Central Europe, but average temperatures do. These larvae hibernate in strict dormancy under cold climatic conditions, while they remain active also during wintertime in warmer regions. In our experiment we saw that even individuals from geographically very close populations can have different oviposition strategies if these populations live in areas with differing local climate.

We consequently suggest that temperature regimes experienced during the larval stage could be a crucial factor determining the oviposition strategy of a Maniola female for the following summer. If this is true, it would also allow a certain developmental flexibility as a response to variation in weather conditions between years and may be good news for aestivating butterflies in a climate warming scenario.

It has been shown that individuals of Danaus plexipus enter reproductive diapause in the adult stage when as caterpillars they are exposed to short day photoperiods and low temperature conditions (Barker and Herman, 1976). Generally, short-day photoperiods and low-temperature regimes encountered during embryonic or post-embryonic stages often determine whether an insect species undergoes a diapause during one of its development phases, be that egg, larvae, pupae or imago (e.g. Danks, 1987).

We conclude that timing of oviposition and lifespan of the adult female depend on geographic provenance rather than on the nutrition the adult butterfly receives or the conditions it is kept in. Meadow Brown butterflies from Mediterranean regions live much longer and have considerably longer reproductive periods than butterflies from Central European mountain areas, but deposit similar numbers of eggs through their lifetimes.