Moisture, a Vital but Neglected Factor in the Seasonal Ecology of Insects: Hypotheses and Tests of Mechanisms

Abstract

Of the major physical factors that influence insect seasonal ecology, moisture is least understood and least appreciated. It is our premise that experimental probing of insects from diverse zones and various habitats would reveal general patterns of insect responses to moisture that are as striking as those for photoperiod and temperature. Using the paradigms of photoperiod and temperature as ecophysiological determinants of insect seasonality, we hypothesize that moisture influences insect life cycles via one or more of three mechanisms-as a token stimulus for diapause, modulator of developmental or reproductive rates, or behavioral cue for vital seasonal events. For heuristic purposes, we offer each of these hypotheses in close association with approaches for testing their validity in insects that undergo dry-season dormancy. The approaches appear appropriate for examining the role of moisture in the life histories of terrestrial invertebrates other than insects, as well as plants and microbes that have a seasonal resting stage. Elucidating moisture's role in insect seasonal cycles is critical to the development of comprehensive phenological models, improved insect management systems, and identification of novel evolutionary mechanisms for adaptation to wet-dry seasons, especially in tropical, subtropical, and Mediterranean regions.KEY WORDS phenology, moisture, soil-inhabiting insects, dormancy, development, reproduction PHOTOPERIOD ANDTEMPERATURE are generally considered prime factors in the seasonal ecology of terrestrial invertebrates (Beck 1980, Tauber et aI.1986). In contrast, moisture is usually consigned a secondary or minor role. This subordinate ranking of moisture as a seasonal cue may be grossly misleading for 2 reasons: First, phenological studies are concentrated in the Temperate Zone where large seasonal changes in photoperiod and temperature typically occur. Consequently, investigators are heavily biased toward using photoperiod and temperature as variables, and data on moisture's function and significance are lacking. Second, the natural histories of a large proportion of earth's insects (e.g., those from tropical, Mediterranean, and other regions with conspicuous wet and dry seasons) are highly correlated with changes in moisture conditions (Essig 1926, Edney 1977, Wolda 1978, Shapiro 1979, Mooney et al. 1980, Labeyrie 1981, King 1984, Denlinger 1986, Paarmann 1986, Powell 1986, Brakefield 1987, Dobkin et a1.1987, Janzen 1987, Bradshaw and Holzapfel 1988, Clouds ley-Thompson 1991, Hadley 1994, Somme 1995. Indeed, it appears that cycles of rainfall and moisture in wet-dry regions may rival those of temperature in temperate regions, in both their seasonal predictability and their importance for survival and development of insects. Thus, we contend that moisture constitutes a greatly un-

Soil temperature and diapause maintenance in eggs of the spittlebug, Deois flavopicta (Hemiptera: Cercopidae)

“…The high similarity between simulated and empirical curves from field observations of postdiapause eggs hatching during this study, and observations with other spittlebugs, such as Aeneolamia occidentalis Walk and Prosapia simulans Walk (Cercopidae) in Mexico showing larger and more synchronized populations in the first generations when rains are abundant and sudden starts in the beginning of the season (Oomen, 1975), support the above proposed hypotheses of interaction of temperature and rain distribution to explain the variation of D. flavopicta densities and confirm the importance of moisture as a seasonal cue in the life cycle of insect in tropical and subtropical regions, as proposed by Tauber et al (1998).…”

Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

“…We used regression (PROC REG: SAS 1999) to identify the model with the lowest mean square error and fewest nonsignificant variables. We included as explanatory variables only those climate variables that our literature review indicated were likely to affect thistle development, or weevil activity, survival, or development (Rathcke and Lacey 1985;Tauber et al 1986;Tauber et al 1998;Gassmann and Louda 2001). For R. conicus egg cases per wavyleaf flower head, weevil abundance, and weevil-plant phenological synchrony, we examined both precipitation and GDD in the previous summer, previous autumn, preceding winter and spring; winter and spring relative humidity; and days until both last hard freeze and last freeze in spring.…”

“…Climatic factors, such as temperature and precipitation, can affect both insect activity and plant performance (e.g., Rathcke and Lacey 1985;Tauber et al 1986). For some floral herbivore-host plant interactions, effects of climate on insect herbivores or on their host plants are sufficiently direct and strong to determine the level of herbivore damage (Solbreck and Sillen-Tullberg 1986).…”

Emergence dynamics and bet hedging in a desert bee, Perdita portalis

“…Likewise, Rust (1988), based on eight years of emergence data, found evidence that rainfall in excess of 5 cm was su¤cient to trigger emergence of Nomadopsis larreae Timberlake. Rainfall is likely to be as important a phenological cue as photoperiod and temperature in insects, but has generally been overlooked (Tauber et al 1998). However, melittologists have been reticent to accept the idea of rainfall-triggered emergence (referred to below as`induced emergence') because there has been no direct, experimental demonstration of the hypothesis.…”