The suitability of field margin flowers as food source for Chrysoperla lacewings Paul C.J. van Rijn Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam, Science Park 904, Amsterdam, NL (
[email protected])
Abstract: Lacewings are among the most common natural enemies of pests in arable fields. Whereas the larvae are voracious predators of aphids and other insects, the adults are depending on nectar and pollen for survival and reproduction. In this study the suitability of flowers of 16 plant species is examined as food source for the common Chrysoperla carnea with non-choice survival tests. The results indicate that only umbellifers and other flowers with well exposed nectaries are suitable as sugar sources, allowing the lacewings to survive for more that 20 days and to produce eggs. On composits, even those with very short florets, survival and reproduction was clearly reduced. This indicates that their nectar was probably beyond reach, but that the pollen allowed the insects to survive longer that without food. Exceptions were composites with extrafloral nectar, as they allowed lacewings to survive much longer. The results are discussed in comparison with the results for hoverflies. These laboratory bioassays can, when performed for key natural enemies, be used to optimize the composition of field margins for sustaining natural pest control. Key words: natural pest control, conservation biological control, field margins, sustainable agriculture, flower morphology, nectar, pollen, longevity, reproduction, Neuroptera: Chrysopidae, Syrphidae
Introduction Many insect carnivores that play a role in the suppression of pests require nectar, and sometimes also pollen, during their adult life stage (Wäckers et al., 2005). The scarcity of flowering plants in modern agricultural fields may therefore prohibit an effective performance of these beneficial insects. In the second stage (2008-2011) of the Functional AgroBiodiversity (FAB2) project in the Netherlands (Van Rijn et al., 2008), both annual and perennial field margins have being developed. The botanical composition of these margins is chosen to optimize support for natural enemies of pests, without compromising other functions of arable field margins, such as aesthetic quality and the support of birds and bees and general biodiversity. In our region, aphids and thrips are the most important insect pests in arable crops such as potato, wheat and onion. Green lacewings (Chrysopidae) are, together with hoverflies, the most common natural enemies in these crops, especially Chrysoperla carnea s.l. Adult lacewings of the genus Chrysoperla require nectar and pollen as a food source, both for survival and for reproduction (Venzon et al., 2006). The short mouthparts of these insects may limit the range of flowers that can provide nectar even further than for hoverflies (Van Rijn & Wäckers, 2010). In related species pollen can however partly compensate for the lack of (accessible) nectar (Venzon et al., 2006). In order to learn which types of flowers are most suitable, we tested 16 flower species in laboratory bioassays with Chrysoperla carnea. To allow direct comparison with the flower range of hoverflies, a wide subset was selected among the flower species used in the study with hoverflies (Van Rijn & Wäckers, 2010).
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Material and methods To test the suitability of flowers for lacewings, both adult longevity and the ability to produce eggs by C. carnea were measured. The flowering plants were obtained from seed or from field margins and road sides, and were left for 2 days in cages before testing. Adult lacewings were obtained by isolating larvae, provided by Koppert B.V., in punched Eppendorf® 2 ml tubes which were supplied with eggs of the floor moth, Ephestia kuehniella twice a week until pupation. Adult lacewing longevity was measured in gauze cages (210 dm3) under controlled conditions (23 °C, 80% RH, 16L:8D). Each cage contained flowering plants of a single species and a 12 ml bottle with wet cotton wool for free water supply. In each cage 5 or 6 adults of C. carnea were released, within 48 hours after emergence from their pupae. The survival of the lacewings was checked at least every other day. The presence of eggs both on the cage and on the plants was checked once a week and after finishing the experiment. The stalked eggs remain visible long after hatching. Per plant species on average 14 lacewings were tested, with a minimum of 11, in at least two different years. The results were compared with data on hoverfly survival and nectar accessibility obtained in a previous study (Van Rijn & Wäckers, 2010).
Longevity lacewings (days) 36 R2 = 0.72 30 other
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Composites
18
with EFN Umbellifers
12
lacewings reproducing
6 0 0
3
6 9 12 Longevity hoverflies (days)
15
18
Figure 1. The mean adult longevity of the lacewing Chrysoperla carnea compared with the longevity of the hoverfly Episyrphus balteatus for 16 different flowering plant species. Symbols indicate the plant family and the presence of extrafloral nectar (EFN) and if the lacewings showed any reproduction.
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Results In absence of food the longevity of the adult lacewings is around 4 days. The presence of a non-flowering plant does hardly improve survival compared to a control with water only (3.5 (± 0.21) and 4.4 (± 0.48) days (p=0.58, t-test). In the presence of a 1M solution of sucrose the longevity was on average 18.3 (± 1.7) days, but no eggs were produced. With flowering plants the average longevity ranged from 5 to 30 days (see Figure 1). Average longevity of more than 19 day was observed with all three umbelliferous species (Apiaceae), with Baby’s Breath, Buckwheat, Cornflower and Sunflower. With these flower species the lacewings not only lived longer than with other food sources, they also consistently produced eggs. All these species have accessible floral nectar or –for the latter two composite species (Asteraceae)– extrafloral nectar on the outside of the flower head. All eight other composite species tested give rise to shorter longevities, albeit still longer that with water only. Neither of these species give rise to reproduction, with corn marigold as the only exception. The only flowering plant that did not increase longevity compared to the water control was Bird's-foot Trefoil, a legume species (Fabaceae).
Conclusions and Discussion The plant species tested were all included in the broader study with the hoverfly Episyrphus balteatus (Van Rijn & Wäckers, 2010), allowing a direct comparison between the two species. This comparison (Figure 1) shows that the results for the two species are highly correlated (R2=0.71), albeit with some obvious deviations. Both species clearly benefit most from umbelliferous species and other plant species with exposed nectaries. Both species can apparently not deal with leguminous flowers. However, with respect to the intermediately profitable composite species (Asteraceae) their responses seems to deviate. For the hoverflies the longevity showed large (4 to 5 fold) differences between composite species which were correlated with the depth of the tubular florets in the flowers (Van Rijn & Wäckers, 2010). For the lacewings longevity shows much smaller (2 fold) difference when feeding on composite species without EFN’s, and no correlation with floret depth is observed. These results are well explained by a combination of two observations: (1) The mouthparts of Chrysoperla lacewings do not show any adaptation towards feeding on floral nectar, such as elongation. This may prohibit entering the narrow tubular florets of these composites, which are moreover partly blocked by the anthers. (2) In absence of nectar Chrysoperla species can still partly survive by feeding on pollen (Venzon et al. 2006), whereas for hoverflies this supplementary effect of pollen is absent or at least much smaller (Van Rijn & Wäckers, 2010). To our knowledge, this is the first study comparing the suitability of various flowers to provide the necessary food sources for adult lacewings. Consequently, no direct comparisons can be made to literature data. Venzon et al. (2006) compared three plant species, but offered only the pollen, not the entire flower. Other studies assessed the flower choice from crop content analyses of field collected lacewings. Villenave et al. (2006) showed that Chrysoperla species feed on pollen from a wide range of flowers depending on the availability in the field, but that flowers that close at twilight, when the lacewings are active, are underrepresented. These studies however do not show on which plants they feed on nectar, nor how suitable the different flowers are in terms of longevity or reproduction. From our studies on hoverflies and lacewings we advise to dedicate a substantial part of the field margin vegetation to flowers with well exposed nectaries (depth < 1 mm), such as umbelliferous species and buckwheat. These flowers, as well as plants with extrafloral 215
nectaries, can be used by both groups of beneficial insect as a source of nectar. Moreover, parasitoids are likely to benefit from the same group of flowers (Wäckers & Van Rijn 2012). Since flowers with easily accessible nectar are much less common than those with accessible pollen, raising these flowers in or around agricultural fields will likely relieve an important limitation to natural pest control. The direct effects on potential pest species still need further investigation.
Acknowledgements The study is partly financed by the former Dutch Ministry of Agriculture, Nature Management, and Food Quality (LNV), the former Ministry of Housing, Spatial Planning and the Environment (VROM), the Dutch Federation of Agricultural and Horticultural Organisations (LTO), the province of Zuid-Holland and the Rabobank NL.
References Van Rijn, P., Van Alebeek, F., Den Belder, E., Wäckers, F., Buurma, J., Willemse, J. & Van Gurp, H. 2008: Functional agro biodiversity in Dutch arable farming: results of a three year pilot. IOBC/WPRS Bulletin Vol. 34: 125-128. Van Rijn, P.C.J. & Wäckers, F.L. 2010. The suitability of field margin flowers as food source for zoophagous hoverflies. IOBC/WPRS Bulletin Vol. 56: 125-128. Venzon, M., M. C. Rosado, D. E. Euzebio, B. Souza, and J. H. Schoereder. 2006. Suitability of leguminous cover crop pollens as food source for the green lacewing Chrysoperla externa (Hagen) (Neuroptera : Chrysopidae). Neotropical Entomology 35:371-376. Villenave, J., Deutsch, B., Lode, T. & Rat-Morris, E. 2006: Pollen preference of the Chrysoperla species (Neuroptera: Chrysopidae) occurring in the crop environment in western France. European Journal of Entomology 103: 771–777. Wäckers, F.L., Van Rijn, P.C.J. & Bruin, J. (Eds.) 2005: Plant-provided Food for Carnivorous Insects: A Protective Mutualism and its Applications. Cambridge University Press, UK. Wäckers, F.L. & Van Rijn, P.C.J. 2012: Pick and mix: selecting flowering plants to meet requirements of target biological control insects. In: Gurr, G.M., Wratten, S.D. & Snyder, B.E. (Eds). Biodiversity and Insect Pests: Key Issues for Sustainable Management. Wiley Blackwell: 139-165.
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