Hymenoptera and Conservation

T. R. New

• Publisher: Wiley-Blackwell
• Year: 2012
• ISBN:
  9781118381328
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Hymenoptera, the bees, wasps and ant, are one of the largest insect orders, and have massive ecological importance as pollinators and as predators or parasitoids of other insects. These roles have brought them forcefully to human notice, as governors of some key ecological services that strongly influence human food supply. Recent declines of pollinators and introductions of alien pests or biological control agents are only part of the current concerns for conservation of Hymenoptera, and of the interactions in which they participate in almost all terrestrial ecosystems. Both pests and beneficial species abound within the order, sometimes closely related within the same families. Many taxa are both difficult to identify, and very poorly known. This global overview, the first such account for the whole of the Hymenoptera, discusses a broad range of themes to introduce the insects and their conservation roles and needs, and how their wellbeing may be approached. The book is intended as a source of information for research workers, students, conservation managers and naturalists as an introduction to the importance of this dominant insect order.

1

Introducing Hymenoptera and their Conservation

Perspective

Hymenoptera have many influences on the well-being of natural communities and of people. Perhaps best known to many lay people either as stinging or nuisance pests (wasps, ants), or providers of honey (bees), their complex ecological roles give them central importance in the maintenance of ecological processes and systems. Pollination by bees and wasps is critical in both crop production and floral maintenance in nature, and the complex interactions of numerous hymenopteran predators and parasitoids with prey and hosts are integral components of some pest management programmes and of natural food webs, in which such species are commonly amongst the most diverse and influential taxa present. Yet defining and categorizing these influences, recognizing and enumerating the insects involved and evaluating their ecological roles and the ways in which these can be sustained and the agents themselves conserved are all complex exercises. Perceptions of Hymenoptera thus span the range from being essential and highly beneficial to being serious pests, affecting human welfare and ecological systems in many ways – from being essential to sustaining them to serious agents of change or loss and threats to other biota. Those widely polarized views can sometimes apply even to the same species in different contexts. In particular, conflicts over the roles and impacts of introduced honeybees, bumblebees, and some classical biological control agents have stimulated much debate on the impacts of alien species and the needs to monitor and screen them carefully.

Classification and Diversity

Very broadly, the Hymenoptera conventionally comprises two suborders of insects, with one divided into two large sections. The suborder Symphyta is regarded as the more primitive group of Hymenoptera, and comprises the sawflies and woodwasps, all plant-feeding species on foliage and wood, respectively, and some being serious pests of forestry. The sole exception to this herbivorous habit is the family Orussidae, often included within the Symphyta and the only non-apocritan wasp parasitoids, attacking the larvae of wood-boring insects, although the hosts of only a few of the approximately 75 species are known (Vilhelmsen 2003). The primary feature for the recognition of Symphyta is that they lack the ‘wasp waist’ of more advanced Hymenoptera, so that the thorax and abdomen are joined broadly, without any constriction between them (or, more accurately, between the first and second abdominal segments). Relationships between the various families of Symphyta – globally 14 families are recognized – are still debated, but the group comprises several distinct lineages. This is by far the smaller suborder, with fewer than 10 000 species described throughout the world, and comprising well fewer than 10% of described species within the order. By far the larger suborder, Apocrita, are fundamentally carnivorous, with many of the species being predators or parasitoids, but some have secondarily reverted to plant-feeding habits, as in the gall wasps (of markedly different families), sometimes causing abundant spherical galls on wattles in Australia or oak trees in the northern hemisphere, or as nectar feeders (bees, some ants). However, around 75–80% of species are parasitoids, even though the adult wasps may feed on pollen or nectar or other plant products. With the unifying structural feature of the waist, Apocrita are divided into two major groupings, both taxonomically complex and with an array of rather different groups. The Aculeata are those in which the ovipositor has been modified as an envenomating sting, and the ‘Parasitica’ (sometimes ‘Terebrantia’, but with this name applied more formally to a suborder of thrips) retain a conventional ovipositor. More familiarly, the Aculeata are the conventional ants, bees and wider array of wasps, and so, the Hymenoptera of public perception, and the Parasitica almost wholly parasitoid wasps, depending on other insects and related arthropods as hosts for their survival. The ‘stinging Hymenoptera’ (including stinging parasitoids such as Pompilidae, the spider wasps) are far better known than the non-stinging parasitoids, and contain far fewer species. Unlike the caterpillar-like larvae of Symphyta, the typical larva of Apocrita is grublike, being legless, lacking eyes and, in most, also without antennae. However, the evolutionary unity of these major groups, whilst accepting the use of their names as broad descriptors, is by no means universally accepted amongst hymenopterists, and much of the intraordinal arrangement remains unclear.

Thus, whilst there is little doubt that the Apocrita are a natural group of insects, with a single origin (possibly through the group of ectoparasitoid Symphyta, the Orussoidea, Orussidae), relationships within it are more open to debate. If, as commonly thought, orussids are the sister group to Apocrita, the latter are founded in the parasitoid lifestyle. Different authorities cleave to slightly different taxonomic arrangements and the boundaries between some families are not wholly settled, so that the concept and scope of any large family of Hymenoptera used in publications may need to be defined carefully if comparative appraisals of diversity or abundance are to be made. Grissell’s (2010) refreshing comment on the enormous parasitoid family known as the Pteromalidae is highly pertinent. He wrote: ‘The family…with 39 subfamilies, is actually an aggregation of genera and species, some of which may belong in 10 different other families, but we don’t yet know what those families might be, whether we should name some new families to solve the situation, or even if we should combine all the families in Chalcidoidea into one…. So we just talk about the family Pteromalidae as if it actually existed’. Parallel dilemmas occur elsewhere, as amongst the bees and related wasp groups. Some of the largest families are themselves very complex. Ichneumonidae contains some 35 subfamilies and the related Braconidae 29 subfamilies, for example, with the precise number of such groups depending on the opinion of the individual specialist providing that figure.

With few exceptions, Hymenoptera are not well understood, and members of most non-aculeate groups, in particular, are difficult for non-specialists to identify to genus or species levels, and many specialists also encounter difficulties in this. As Huber (2009) put it, ‘The order Hymenoptera contains far more, and more diverse, species than simply ants, bees and wasps’ and ‘Most Hymenoptera belong to groups unknown to the general public’. Many groups lack non-technical common names. Vast numbers of species remain undescribed, and estimates of richness in many taxa are very variable, with suggestions of total species of Hymenoptera ranging as high as a million (Ulrich 1999) and with around 150 000 so far formally named. Whereas at least half of the Symphyta and Aculeata species have probably been described, perhaps fewer than 10% of Parasitica yet have names (Huber 2009) and, even for most of the named species, biological knowledge (such as of host ranges) does not exist. Even formal names may not represent real species, because of earlier propensity to erect new taxa on small differences of colour or structure, without appreciating the variations within species, and many taxonomic revisions not only add new species but also eliminate many of those described earlier as synonyms. Thus, for the ‘tarantula hawks’ noted in the preface, Vardy (2000) found an initial total of 612 species names, of which 546 remained in the genus Pepsis, but 419 (77%) were considered synonyms after his study. With other changes, including new species, his revised total of these spider wasps was 133 species.

This taxonomic and ecological abundance is largely based in terrestrial biomes, with Hymenoptera virtually ubiquitous wherever any exploitable resources occur. However, Hymenoptera have also developed aquatic associations, with 150 species (representing 11 families) of parasitoids occurring in freshwater environments (Bennett 2008) and the habit apparently originating independently some 50 times. Three major categories are involved: (i) species in which females enter water to seek aquatic hosts; (ii) species with endoparasitoid larvae in aquatic larval hosts, even if oviposition is terrestrial; and (iii) species in which newly emerged adults must travel to the water surface after pupation. Bennett considered his enumeration likely to be minimal because of lack of detailed knowledge for many regions. Ichneumonoidea were the most diverse records (39 species of Ichneumonidae, 26 of Braconidae), and the only apocritan reported is a pompilid (spider wasp), Anoplius depressipes, that captures aquatic spiders and moves them onto land before oviposition (Roble 1985). Collectively, aquatic parasitoid hymenopterans have been reported from at least 25 host families across seven insect orders. Bennett (2008) suggested that the habit may have evolved through wasps parasitizing semiaquatic hosts around the water surface. The variety indicated so far is yet further evidence of the evolutionary exuberance within the order.

Beetles (Coleoptera) have traditionally been considered the group of animals with the most species, but many entomologists feel that Hymenoptera may in fact be leading contenders for this status as more information accumulates. Both orders are hyperdiverse and appear to be well ahead of the other two large holometabolous orders, Lepidoptera and Diptera, in numbers of species. Historically, Coleoptera have been better documented as the subjects of more assiduous collector attention, albeit based largely on the more spectacular groups of beetles. Historical interest in Hymenoptera has been much more uneven, and also biased largely towards the larger and more conspicuous life forms. Groups such as bumblebees and ants are relatively well known, but the enormous array of tiny parasitoids remains one of the most daunting ‘black holes’ in insect documentation. The title of ‘most diverse insect group’ must remain conjectural for the time being, but the ambivalence emphasizes how little we know about the diversity of our predominant animal groups. In addition, quoting from Hawkins (1993), ‘the stunning variety of parasitoids in general and parasitic Hymenoptera in particular, as well as that of their insect hosts almost certainly precludes our ever having a complete record of all the species involved’. Searches for ecological patterns to aid predictions of their diversity and distribution continue, but generality and accuracy are both difficult to achieve at any global scale. As with other hyperdiverse and poorly known invertebrate groups, many approaches to estimating richness have been advanced based on extrapolations from various assumptions or correlations. Dolphin and Quicke (2001) examined some of these for Braconidae, and the various shortcomings can commonly involve regional collecting bias and uncertainty over species integrity and identity; whilst valiant, many such cases still leave much uncertainty over the central question of species numbers. Likewise, the use of biodiversity databases, increasing rapidly in complexity and importance, depends on the reliability and completeness of the information they contain (see approach by Santos et al. 2010 for Ichneumonidae). Any such ‘data-mining’ exercises may be informative, but their limitations must be assessed carefully.

For many Hymenoptera the basic templates are not yet sufficiently complete to form an effective substitute for original investigations. Gauld (1991) suggested, for example, that diversity of tropical Ichneumonidae remains underappreciated because ‘scores of sympatric species’ look very similar whilst flying, and many others are small and inconspicuous. His studies (Gauld 1991 and later volumes) on the Ichneumonidae of Costa Rica imply that this single country has an immensely diverse array of the family; he also commented that if some earlier estimates of the magnitude of insect diversity are correct, this family alone could include more than a million species, but he regarded this as very unlikely.

In contrast to tropical Lepidoptera and Coleoptera, Hymenoptera – particularly many of the small parasitoid groups – were not accumulated abundantly during the nineteenth-century exploration era, so that the bulk of foundation knowledge of their systematics arose largely from studies on temperate region faunas. Even since then, the great majority of Hymenoptera have not been attractive to hobbyists, in part reflecting their small size, difficulties of preservation and study, and inability to identify them without good microscopical equipment, considerable preparation and access to first-class institutional collections and library facilities. The problems have been exacerbated by reared parasitoids commonly being ‘unwelcome’; hobbyists rearing Lepidoptera have frequently been disappointed to find small wasps rather than the butterflies or moths they expected and, historically, many such specimens have been discarded without their importance to documentation being recognized. In short, many parasitoid groups have tended to remain in the domain of the specialist, of whom there are far too few. Some very large families of wasps, for example, of massive taxonomic and ecological complexity are studied by only a handful of specialists throughout the world at any time, and many smaller groups are essentially ‘orphaned’ other than from sporadic attention. Yet some of these insects are amongst the most numerous animals in many terrestrial biomes, and many have complex and often highly specific interactions – for example, as pollinators, predators, parasitoids or competitors – vital to the continuation of other species within those communities. Entomologists seeking to document and understand these processes and the influences of Hymenoptera in natural ecosystems over much of the world must inevitably seek guidance from the perspective gained from study of the best-documented faunas, those of Britain and Western Europe. Even there, however, significant problems remain in identifying species. Representative comments on the British parasitoid wasps (from Barnard 1999) include them being still ‘extremely poorly known’ (British Ichneumonidae), having ‘numerous cryptic species’ (British aphid parasitoids), ‘difficult to identify’ (British Figitidae), ‘frequently posing problems with their identification’ (British Trichogrammatidae) and so on. Collectively, such hymenopterous parasitoids were described in a recent text (Foottit and Adler 2009) as ‘exhibiting incredible levels of species richness, accompanied by an equally high level of diversity in biological habits’.

Evolutionary radiations within parasitoid groups of wasps can become immensely complex to interpret as measures of ‘real diversity’, and are perhaps particularly difficult amongst some of the taxa that have become phytophagous and their parasitoid complexes. Initially, host plant species and associations (such as form of galls induced by the wasps) are often highly specific, and each may then found a unique partnership or community. Specific mutualisms of pollinating fig wasps (Agaonidae) and figs (Ficus) or the community of gall-forming cynipoids on oak trees (Quercus) are two such examples. Both have for long attracted the attention of ecologists, and their study has provided pivotal points in understanding evolutionary processes and some of the factors generating diversity, but both still have many questions of detail unanswered. As examples, recent molecular appraisals have revealed previously unsuspected diversity amongst species of a major genus of fig wasps, Pleistodontes. For Pleistodontes imperialis in Australia, Haine et al. (2006) found four major clades that overlapped in distribution along the eastern border of the continent. They inferred that many fig species host two wasp species as pollinators, so that fig wasp speciation may have proceeded more rapidly than fig speciation, countering the ‘one-to-one’ reciprocal relationship for long traditionally accepted. In some cases, wasp speciation seems to have occurred without a shift in host species. Many such instances of ‘cryptic species’ may occur, with the conse...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• Acknowledgements
• 1 Introducing Hymenoptera and their Conservation
• 2 Alien Hymenoptera in Classical Biological Control
• 3 The Junction of Biological Control and Conservation: Conservation Biological Control and Cultural Control
• 4 Introduced Bees: Threats or Benefits?
• 5 Social Wasps and Ants as Aliens
• 6 Pollinator Declines
• 7 Levels of Conservation Concern and the Shortcomings of Current Practice
• 8 Habitat Parameters and Manipulation
• 9 Species Case Histories
• 10 Assessing Conservation Progress and Priorities for the Future
• References
• Index