Science
Insects make up the bulk of terrestrial diversity (1). Reports of insect declines, best documented in Europe and North America, suggest that 40% of insect species in temperate countries may face extinction over the next few decades (2), although this figure is probably inflated (3). Other studies have highlighted falling insect biomass in Germany and Puerto Rico (4, 5), as well as threats to many insect taxa in Europe (5) and insect pollinators worldwide (6) that support food production (7). To protect insects, it is crucial that they are considered as separate species with distinct responses to threats, with particular attention to tropical insects and their habitats. Bees and butterflies may serve as an initial focus, but conservation efforts must go far beyond these iconic species. Halting habitat loss and fragmentation, reducing pesticide use, and limiting climate change are all required if insect populations are to be preserved.
The Main Threats
Trends in biodiversity decline are more severe for invertebrates than for vertebrates (4), because the former are highly specialized in terms of food resources and microhabitats. About half of insect species are herbivores and have intimate relationships with their host plants; the slightest alteration to plant abundance or phenology may therefore have severe consequences for insect populations. Multiple interacting threats affect insects, often with negative consequences not just for the insect species themselves but also for other species that rely on them and for overall ecosystem functioning. However, little is known about the identity, genomics, or ecological role of most insect species.
Habitat loss and fragmentation are probably the most serious threats to temperate and tropical insects, particularly to rare, endemic, and specialized species, resulting in reduced and homogeneous assemblages of generalist species across space (8). Habitat loss is fueled by agricultural expansion and intensification, which involves substantial use of chemical pesticides (insecticides and herbicides). The latter are another substantial threat to insect species; insecticides have been linked to insect decline in temperate countries (2, 4) and to global pollinator decline (6). The increasing introduction of large-scale agriculture in the tropics may similarly cause substantial harm to insect populations through the impacts of pesticides beyond agricultural systems (9). The use of fertilizers and herbicides may also shift plant composition, altering the population dynamics of host plants and dependent insects (3).
Climate change, and especially the frequency of extreme climatic anomalies, may be especially detrimental to tropical insects, which tend to have narrow geographic ranges and low tolerance to changes in temperature and rainfall (5, 10). Invasive species and pathogens may also threaten local populations, as can light pollution (2, 3).
Improving Knowledge
Insects are the central component of the living world, and their protection is crucial to maintaining functioning ecosystems and ensuring food security (4, 7). However, scientific knowledge is limited because of insufficient funding for entomological science and the resulting scarcity of adequate field studies. Many past studies have relied on overall insect biomass measurements, which are relatively easy to conduct (2, 5). However, insect biomass greatly varies in space and time and provides little information about the population dynamics of specific species. Instead, population trends can be summarized by combining insect species into different functional groups (10), which may help to identify which species are coping better or worse with anthropogenic changes (3).
Furthermore, many studies are resurveys—that is, snapshots taken at specific time intervals rather than continuous monitoring. The latter is crucial for evaluating how insects respond to individual threats. Comparison of snapshots is further complicated by habitat changes, does not accurately capture which species are present or absent, and may yield misleading trends (3).
Assemblages monitored in the long term must be representative of local insect populations and reasonably diverse. Findings of low insect densities and rates of local extinction must be corroborated with independent studies, particularly in the tropics, where many species subsist at low densities (10). Further, contrasting insect responses to threats must be acknowledged and scrutinized (3, 10). For example, many native species may be declining in temperate forests, but several pest species are expanding their geographical range in response to climate change (7). Efficient monitoring programs can benefit from recently developed technologies involving molecular methods (11) or bioacoustics, as well as from citizen participation (6).
Conservation efforts cannot succeed without sound ecological knowledge of the role of insects in ecosystem maintenance and functioning and of the complex processes, such as adaptive strategies, food behavior, or cascading trophic interactions, that may be disrupted by threats (5). Because even small ecosystem fragments have conservation value for insect biodiversity and ecosystem services, studies should focus on how to preserve forest heterogeneity, enhance the values of fragments by increasing forest connectivity, and promote habitat restoration favorable to insects. Experiments should investigate the consequences of extreme temperatures, which may reduce the fitness of predatory and parasitoid species. A better understanding and delineation of the species that need to be protected is also important. Taxonomic knowledge can be advanced by training more taxonomists and by developing DNA barcode libraries, which provide tractable and testable taxonomic frameworks (11).
Protection Measures
Insects are of crucial importance for ecosystem functioning (including pollination and forest regeneration), for mitigation of pests, and as a source of protein for animals and humans (7). Effective protection measures can be implemented now to mitigate insect decline by examining the evidence available for temperate insects. If decision-makers fulfill their commitments toward the implementation of the 2015 Paris Agreement to mitigate global warming, threats to insect populations resulting directly from global climate change will be alleviated. In urban areas, policies that favor organic agriculture and insect-friendly gardens can greatly support insect species (12). Planting native species in urban environments such as parks, roofs, and backyards can also help to protect insect populations and deliver pollination services.
In rural areas, insect species would benefit from support for organic agriculture and permaculture, the reduction and more efficient use of pesticides, use of integrated pest management (7), and local-scale farming practices that nurture insect populations. Boosting the abundance, diversity, and continuity of floral resources and providing nesting sites are efficient ways to mitigate pollinator decline (6).
Efficient, appropriate, and permanent conservation measures for natural habitats (such as old-growth forests) and human-influenced areas of even very small sizes can support high insect diversity (3). National coordination, informed by scientific results, can lead to better conservation management, such as supporting effective landscape-scale ecological networks (13). Funding of long-term research activities on habitat conservation in general, and specifically on insect science and taxonomy, is especially important to evaluate and mitigate future changes in insect communities, obtain reliable insect time series, and discover species before they go extinct (1).
Engaging the Public
In general, the public tends to appreciate aesthetic insects such as butterflies and the beneficial role of pollinators (6). These perceptions can be used to strengthen the conservation value of insects. However, bee and butterfly species represent only <4% of the insect species described worldwide (1). Many people have negative perceptions of insects in general and do not perceive them as separate species (14). Further, the roles of insects in ecosystem services can be difficult to comprehend (except for pollinators), as are the consequences of insect species loss and overall attrition of biodiversity.
Although public interest in insects varies from one country to another, biological education about the conservation of insects and their natural habitats is urgently needed at all levels of society, starting with field education programs (14). The extraordinary natural history of insects offers many opportunities in biological education and citizen science (14). Field surveys and experiments help the public to appreciate the importance of insects in terrestrial biodiversity (14). Such activities may promote greater empathy and curiosity toward insects and their habitats. Finally, promoting science through traditional and social media can spread enthusiasm and respect for insects and those who study them.
A male weevil (Rhinostomus barbirostris) protects an egg-laying female in Panama.
PHOTO: YVES BASSET
Tropical Data Gaps
In the tropics, where most insect species live, circumstantial data exist, but long-term records are too sparse to support the conclusion of a global insect decline. Most tropical datasets (see supplementary materials) were collected in locations buffered from the effects of agricultural practices and habitat disturbance. Most of these studies do not unequivocally suggest a decline in insect abundance or species richness; rather, they point to contrasting patterns in population dynamics and to the possible impact of climate change. This may reflect an initial positive effect of rising temperatures or merely the dynamics of common species (see fig. S1 in supplementary materials). For example, the species richness of a community of leaf litter ants in Ecuador remained constant for a study period of 11 years, with little or no evidence of directional change toward a new community (15).
Longer time series including diverse taxa are urgently required to understand what is going on. However, tropical regions mostly composed of developing countries can only devote limited funds to research on nature conservation. Successful examples of conservation planning and public outreach in temperate regions could be shared with tropical regions and could help to guide insect conservation in those locations. International collaborations involving scientists from both developed and developing nations will be key to expertise sharing, as will be the development of global databases with open access.
Outlook
No matter whether the insect apocalypse is global or not, immediate actions are necessary to mitigate insect decline. Here, more insect-friendly agricultural practices are key. Scientific research into the cost effectiveness of pesticide use will help to reduce unnecessary pesticide applications (9). Redistribution of eco-friendly subsidies to favor insect protection (5) can target integrated pest management, the use of pesticide and fertilizers only when necessary for food security and the protection of remaining natural habitats from land-use conversion. Changes of laws can be implemented quickly using bees or butterflies as the focus of attention, as recently demonstrated in Bavaria, Germany, where a grassroots citizen campaign and a state referendum led to a law necessitating drastic changes in agricultural practice to protect biodiversity.
Efforts to mitigate the effects of climate change, such as the boycott of harmful chemical products by both the public and governments, will also help insect populations to recover. To allow insect populations to prosper in both temperate and tropical areas, scientists and policy-makers need to rethink scientific and public priorities to reach out to the public and develop effective protection measures. We need a bioliterate society that protects insects to ensure humanity’s own survival.
Supplementary Materials
This is an article distributed under the terms of the Science Journals Default License.
References and Notes
Climate change has emerged as one of the most important environmental, social and economic issues today – especially for South Asia, which is highly impacted by these changes. In light of this, an international conference on 
