Subject Editor: Thomas Schmitt
Feeding on rotting fruits, rather than nectar, is linked to high adult life-expectancy in certain butterflies, notably tropical
Regarding the sources of their reproductive investment, adult lepidopterans span the full range from capital to income breeders (
We here set out to experimentally investigate whether fitness benefits in terms of life-expectancy or fecundity also accrue to moths which feed on rotting fruits. In Europe, few moth species are specialist fruit feeders, but many opportunistically utilize such resources whenever available (
From a bionomics perspective, central European cold season noctuid species can be divided into autumn species and adult hibernators. Autumn species emerge from the pupa between late August and October. They soon mate and lay their eggs until the onset of true winter weather (usually in November) when all adults die and only the eggs overwinter. Typical representatives are genera such as
We sampled representatives of both life-cycle types from wild populations and subjected them to a feeding experiment in the laboratory. Specifically, we tested two hypotheses:
(a) When given access to a fruit resource of standardized quality in addition to pure carbohydrates, cold season noctuid moths survive for longer periods and lay more eggs than individuals offered sucrose solution only.
(b) Fitness benefits are more pronounced in noctuid species overwintering as adults which thus must survive longer, whereas benefits are smaller in more short-lived autumn species.
Most moths were attracted using a bait mixture of red wine and sucrose (1:1 weight relation; e.g.
Upon capture, moths were put in glass vials (50 ml, lined with a piece of filter paper, with air holes in the lid), transferred into a cool and dark bag, kept in a refrigerator overnight and on the next day transported into the laboratory. On the morning after capture, each moth was identified to species and placed into an individually marked rearing jar (transparent 500 ml plastic cups covered with gauze on top). The bottom of each jar was lined with moist filter paper to maintain sufficient humidity. All moths had access to water ad libitum and to cut twigs of their larval host plants for egg laying. Jars were positioned in a greenhouse on the roof of the university building. Temperature varied substantially, with average temperature during day-time being 18 °C (maximum: 26 °C) and 12 °C (minimum: 7 °C) during the nights.
Moths were randomly assigned to one of three feeding treatments: (1) sucrose solution (150 g sucrose in 1000 ml distilled water); (2) sucrose solution of the same concentration, enriched with 7.5 g vitamin mixture (Vanderzant vitamin mix for insects no. 300100, Dyets Inc.; for composition see:
Moths were inspected every 1–2 days, and it was noted if they were still alive. All eggs they had laid were removed and counted. Every individual was followed until its death. Dead moths were preserved in 70% ethanol and later dissected under a stereo microscope. Upon dissection, the number of mature oocytes was counted. The length of time between date of capture and death in captivity was taken as measure of the moth’s physiological life span. We used the sum of eggs laid in captivity plus the mature oocytes remaining in the abdomen after death as a measure of potential fecundity. Longevity and fecundity data were log-transformed to improve fit to normality and homoscedasticity assumptions.
For analysis, we partitioned our data into three subsets: (1) autumn species; (2) adult hibernators that remained in the experiment throughout winter; and (3) adult hibernators that had only been collected in early spring, after hibernation in the wild. Autumn moths comprised representatives of two genera (
In autumn moths, capture date had a clear effect on life-expectancy: the later individuals were caught in the season (standardized regression coefficient:
Results of GLMs (
df |
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---|---|---|---|
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Genus | 1 | 40.97 |
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Food type | 2 | 4.54 |
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Genus × Food type | 2 | 4.18 |
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Capture date | 1 | 9.94 |
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Residual | 82 | ||
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|||
Genus | 1 | 0.85 | 0.3627 |
Food type | 2 | 0.23 | 0.7957 |
Genus × Food type | 2 | 0.08 | 0.9231 |
Capture date | 1 | 0.18 | 0.6779 |
Residual | 30 | ||
|
|||
Genus | 1 | 0.01 | 0.9286 |
Food type | 2 | 3.69 |
|
Genus × Food type | 2 | 0.49 | 0.6142 |
Capture date | 1 | 8.32 |
|
Residual | 105 |
In adult hibernators overwintering in the lab, capture date was only weakly and not significantly associated with remaining life span (
Longevity of cold season noctuid moth females in captivity, according to the three experimental food treatments. ‘Banana’ refers to moths that had access to fruit slices in addition to sucrose solution as adult food. Given are means (controlling for capture date) and 95% confidence intervals.
Finally, among adult hibernators that had overwintered in the wild, capture date was again strongly and negatively associated with remaining life span (
Potential fecundity of cold season noctuid moths in captivity, according to the three experimental food treatments. ‘Banana’ refers to moths that had access to fruit slices in addition to sucrose solution as adult food. Given are means (controlling for life expectancy in the lab) and 95% confidence intervals.
Results of GLMs (
df |
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Genus | 1 | 11.71 |
|
Food type | 2 | 1.88 | 0.1599 |
Genus × Food type | 2 | 0.46 | 0.6317 |
Longevity | 1 | 33.91 |
|
Residual | 81 | ||
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Genus | 1 | 0.25 | 0.6217 |
Food type | 2 | 5.02 |
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Genus × Food type | 2 | 2.90 | 0.0711 |
Longevity | 1 | 0.47 | 0.4991 |
Residual | 29 | ||
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Genus | 1 | 89.76 |
|
Food type | 2 | 0.96 | 0.3878 |
Genus × Food type | 2 | 2.39 | 0.0967 |
Longevity | 1 | 19.64 |
|
Residual | 104 |
These results are surprising at a first glance, since other studies have established a clear link between longevity and fruit-feeding in butterflies (
Rather, our findings challenge the notion that fruit-feeding among adult
We attribute this lack of positive response to the digestive physiology of essentially nectarivorous moths. Licking at banana slices will likely yield dissolved mono- and disaccharides as well as other dissolved micro-nutrients. Yet, these nutrients did apparently not translate into longevity benefits. Moreover, fruit slices undergo microbial decay under the relatively warm and humid conditions in our experiment (E.-M. McMannis and K. Fiedler, unpublished observations). Even though we replaced fruit slices frequently this decay may have been the reason for premature death in some banana-fed individuals, especially in
In insects that specialize on rotting fruits, decomposition products, such as yeast amino acids, and the complex admixture of nutrients encountered in fruits are essential to achieve fitness benefits (
While we did not observe any increase of life span through access to fruit in addition to sucrose, clear positive effects of fruit-feeding on moth fecundity could be established. However, this increase in potential fecundity was confined to the two genera
Except for one single
All moths studied in our experiment are opportunistic feeders during their adult stage. They drink floral nectar whenever available (see
In the adult hibernators
In the analyses presented above, we focussed on potential fecundity, i.e. the sum of eggs laid in captivity plus the mature oocytes remaining in a female’s abdomen after its death. We feel this procedure to be justified since the mere number of eggs laid, viz. realized fecundity, can be misleading in experimental situations such as ours. For example, it was not always possible to offer high quality egg-laying substrates in such large amounts that females could have spread their eggs singly or in small groups, as they would do in nature. Hence, motivation to lay eggs may have been confounded by the circumstances applicable in captivity. Moreover, mating status and health of the moths may sometimes preclude that potential fecundity also translates into oviposition acts. However, analysis of realized fecundity yielded largely the same patterns as we present here with regard to potential fecundity (
In our experiments we offered banana as a standardized food. This is, of course, not a resource naturally available to any of our study species. We do not know whether decaying fruits that noctuid moths may encounter in autumn and winter in Central European woodlands, such as brambles, apples, plums or similar, may provide nutrients in different amounts than fresh banana slices. Hence, it remains to be seen if similar fecundity benefits might accrue in experiments employing more ‘natural’ food resources.
In summary, our experiments reveal that over a range of cold season moths, including two genera with unusually long adult life, there was no indication that facultative fruit-feeding, in comparison to nectar mimics, would enhance longevity. In contrast, beneficial effects on fecundity were pronounced in adult hibernators, which represent extreme cases of income breeders among the
We thank the authorities at Nationalpark Donau-Auen for granting the opportunity to sample moths for this study. The staff of the park, especially Christian Baumgartner, provided manifold logistic support during field work. Sabine Fischer, Claudia Huber, Christian Schulze, Florian Bodner and Martin Wiemers helped with sampling and/or keeping moths. Special thanks go to Jan Beck, Klaus Fischer, Andrea Grill and Florian Bodner for constructive comments on far earlier manuscript drafts and to Jörg Böckelmann for assistance with the graphics.