Terrestrial Insects: a Hidden Biodiversity Crisis? 1

Chapter 7—Terrestrial Insects: A Hidden Biodiversity Crisis? 1 Chapter 7 Terrestrial Insects: A Hidden Biodiversity Crisis? C.H. Dietrich Illinois Natural History Survey OBJECTIVES Like most other elements of the biota, the terrestrial insect fauna of Illinois has undergone drastic change since European colonization of the state. Although data are sparse or entirely lacking for most species, it is clear that many formerly abundant native species are now exceedingly rare while a few previously uncommon or undocumented species, both native and exotic, are now abundant. Much of this change may be attributable to fragmentation and loss of native habitats (e.g., deforestation, draining of wetlands, agricultural conversion and intensification, urbanization), although other factors such as invasion by exotic species (including plants, insects and pathogens), misuse of pesticides, and improper management of native ecosystems have probably also been involved. Data from Illinois and elsewhere in the north temperate zone provide evidence that at least some groups of terrestrial insects have undergone dramatic declines over the past several decades, suggesting that insects are no less vulnerable to anthropogenic environmental change than other groups of organisms Yet, insects continue to be under-represented on official lists of threatened or endangered species and conservation programs focus primarily on vertebrates and plants. This chapter summarizes available information on long-term changes in the terrestrial insect fauna of Illinois, reviews possible causes for these changes, highlights some urgent research needs, and provides recommendations for conservation and management of terrestrial insect communities. INTRODUCTION Insects are among the most important “little things that run the world” (1). They comprise approximately 55% of known species (Fig. 1) and many are regarded as “ecosystem engineers” due to their influence on vegetation structure and nutrient availability (2). The ecological services provided by terrestrial insects, which include nutrient cycling and pollination, were recently valued at more than $50 billion to the U.S. economy annually (3). On the other hand, certain insect species in their roles as agricultural pests and vectors of animal and plant pathogens are capable of inflicting severe economic losses on human society. Insect herbivores are estimated to reduce crop yields by 18% on average worldwide (4). As early as 1861, Benjamin Walsh (later to become Illinois’ first State Figure 7.1. Pie chart showing proportion of known species represented by different major Entomologist; Fig. 2) estimated the annual groups of organisms based on data from Wilson (80). losses due to injurious insects in Illinois at not less than $20 million (ca. $433 million in today’s dollars) and, if one takes into account the losses recognized for aquatic insects, which are routinely used as as well as the costs of pest management, the costs to human bioindicators of water quality (see Chapter 10) , but insects society are much higher today. are also increasingly being used as indicators of biodiversity Because of their diversity and richness in species and ecological integrity in terrestrial ecosystems (5). and functional groups (including herbivores, detritivores, Nevertheless, criteria for characterizing natural areas and predators, and parasites) and sensitivity to various kinds of selecting priority areas for conservation continue to focus environmental change, insects are useful indicators of both almost entirely on vascular plants and vertebrate animals (6), biodiversity and ecological integrity. This has long been and monitoring of terrestrial ecosystem health traditionally 2 INHS Special Publication 30: Canaries in the Catbird Seat the ecological associations of the vast majority of species. For some insect groups, this is nearly as true for Illinois as it is for the relatively unexplored regions of the tropics. Available data indicate that at least 33,000 species of insects inhabited Illinois just prior to European settlement (10 and unpublished data), but by the time the insect fauna began to be documented in earnest in the mid-1800s, the landscape of Illinois had already been altered dramatically (see Chapters 3 and 4). Studies of recent fossil insect faunas indicate that the effects of agricultural conversion of the upper Midwest following European colonization were indeed dramatic (11). However, given the extreme paucity of fossil evidence and the mostly nonrandom, opportunistic means by which knowledge of the extant fauna has been assembled, the gaps in our knowledge of the species composition and long-term trends in the Illinois fauna remain large. Nevertheless, the few detailed studies that have been published as well as anecdotal reports and unpublished data provide valuable clues regarding the changes that have occurred over the past 150 years. ENVIRONMentaL CHANGES INFLUENCING TERRESTRIAL INSECT Faunas Figure 7.2. Benjamin D. Walsh (1808–1869), Illinois’ first State Entomologist, was the first professional entomologist AgrICUlTUrE. to attempt to document the insect fauna of Illinois, and one Since at least the late-1800s, most of the land area of Illinois of the first to estimate the economic costs of insect pests. has been devoted to agriculture. Thus, although influence Unfortunately, his insect collection was destroyed in the of other processes such as urbanization and invasions by 1871 Chicago Fire. exotic species should not be discounted, shifting agricultural practices have been the predominant force influencing has focused mostly on these groups of organisms (7). terrestrial insect communities in the state for more than 100 Terrestrial insect communities are more difficult to monitor years. The most detailed and reliable data on long-term because insects are less conspicuous, may be difficult to trends in populations of Illinois insects are found in the vast identify, and their populations may fluctuate dramatically economic entomological literature, but this literature has from year to year (8). On the other hand, because insects are focused almost entirely on insect pests and, to a lesser extent, the most diverse components of terrestrial ecosystems and their natural enemies, which make up <1% of the known often respond rapidly to environmental change, they provide species. Entomologists, amateur naturalists, and lay persons more detailed information than other, less speciose, groups alike have published observations of trends in populations of (9). Their responses to disturbance, including common agricultural pests like the Chinch Bug and Armyworm (Fig. vegetation management practices, may also be quite different 7.3) dating back to the early 1800s. Citing such reports, from those of plants or vertebrate animals. Use of terrestrial Decker (12) provided an overview of changes associated insects as indicators requires detailed knowledge of their with shifting agricultural practices in Illinois from the early distributions and life histories, and acquiring such knowledge 1800s to the 1950s: has been a major focus of Illinois Natural History Survey “Before extensive agricultural development of the (INHS) entomologists over the past 150 years. state, a large part of Illinois consisted of broad expanses Unfortunately, despite their ecological and of prairie grass, much of which was replaced by timothy economic importance, knowledge of terrestrial insects and other tame [exotic] grass or cereal crops planted by remains fragmentary. At the most basic level, we do not farmers. Insects preferring these crops became notorious have a precise estimate of the number of species either pests, but as the acreage of grasses was reduced as a result worldwide (realistic estimates range from 2 to 10 million) of increased legume production, certain insects began to or in Illinois. By some estimates, fewer than 10% of insect decline in importance. These included the white grubs, the species extant worldwide have been described and named billbugs, the armyworms, the sod webworms, and the corn and species new to science continue to be discovered, even root aphid. The burrowing webworm and the cutworm… in relatively well-studied regions like Illinois. Even for most have all but disappeared… As the rail fence was replaced by named species, the extent of our knowledge is embodied the wire fence, and roadsides and ditch banks were graded by a few museum specimens, the associated data labels or otherwise cleaned up, the amount of giant ragweed and recording the place and date of collection, and a published elderberry available to insects was greatly reduced, so that description of the morphological features distinguishing the common stalk borer became less important and the old them from related species. Little or nothing is known about spindleworm was practically exterminated. Likewise, as Chapter 7—Terrestrial Insects: A Hidden Biodiversity Crisis? 3 the pot holes and low spots were drained, wireworm damage in those areas declined steadily. Conversely, in certain dry, sandy areas which were brought under irrigation wireworm damage increased.” Decker’s description mentions only a few economically important species, but the dramatic physical alterations to the Illinois landscape he describes presumably affected many nonpest insect species as well. Changes in the Illinois landscape since the 1950s have been at least as dramatic as those described by Decker (12) for the preceding 100 years, particularly involving expanded production

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