Molecular gut-content analysis has revolutionized the study of arthropod predator–prey interactions, enabling detection of large numbers of feeding events with minimal disruption to ongoing population and community processes (Stuart and Greenstone 1990, Symondson 2002). The adoption of polymerase chain reaction (PCR) assays (Zaidi et al. 1999), with their relatively low cost, standardized protocols, and ease of designing primers for any given system, has enabled many more researchers to gather molecular gut-content data for arthropods than in the past. This has led to important ecological insights, such as confirming the hypothesis that generalist predators can subsist on alternate prey before the arrival of a pest, then switch to the pest and thereby retard population buildup before the arrival of more specialized predators and parasitoids (Boreau de Roincé et al. 2013, Firlej et al. 2013, Harwood et al. 2007; see Greenstone et al. 2014 for examples of other insights). However, raw molecular gut-content data are qualitative—they are read as either positive or negative—and because one typically cannot know the size, stage, number, and time since consumption of the prey item or items represented by detected remains, they provide no quantitative information on predator impact on prey populations (Greenstone 1996).

Early serological data suggested large, consistent differences among taxa in the length of time after feeding during which a prey item can be detected in the gut of a predator. Spiders and staphylinid beetles in particular seemed to have longer maximum detectability times (D_max) (Sunderland et al. 1987) than some other predators. If this were true, assay results would have to be weighted to remove the bias, favoring those with long detectability times, which would otherwise confound interpretations of assay results. Although D_max was a useful concept for discussing these differences, it substituted an absolute limit for what is really a continuous variable with significant variation. This provided the impetus for formulating a more realistic tool for weighting gut-content assay data, the detectability half-life, defined as the time after feeding at which prey remains could be detected in only half of the assayed predators. Although this definition first appears in a paper that employed a serological assay to detect prey species-specific protein antigens (Greenstone and Hunt 1993), the concept applies equally well to PCR assay for specific DNA sequences, and was extended to DNA-based assay data with a one-aphid two-predator system, in a paper that also included half-life confidence intervals and statistical testing for differences between them (Chen et al. 2000). Shortly thereafter, Payton et al. (2003) provided further guidance for fitting half-life data to decay models, and for testing sets of half-lives from a given predator–prey system for statistical differences.

The detectability half-life has been widely adopted by the predator–prey research community to characterize gut-content data derived from both serological and DNA-based assays, with around 100 half-lives in the peer-reviewed literature to date (Greenstone et al. 2014). Early DNA-based half-lives, like those first determined by serological methods, suggested that there might be taxonomic trends in the lengths of half-lives. For example, the first DNA-based heteropteran half-lives measured were the longest recorded for insects up to that time (Greenstone et al. 2007), prompting speculation that this might be due to physiological and morphological adaptations that these sit-and-wait predators with sucking mouthparts share with spiders. Because there are thousands of predatory species of diverse arthropod classes, orders, and families in agro-ecosystems alone, any consistent, generalizable taxonomic differences in detectability half-life length might enable biocontrol researchers to use representative half-lives for sets of predator–prey pairs rather than determining each one individually, an arduous and time-consuming process (Greenstone et al. 2014).

Here, we test the hypothesis that heteropteran predators have unusually long prey detectability half-lives by determining them for two previously unstudied heteropterans, Geocoris punctipes (Say) and Orius insidiosus (Say), and comparing them with a previously published set of half lives of seven other predators, comprising four heteropterans and three coleopterans, fed the same prey item (Greenstone et al. 2010).

Materials and Methods

Results and Discussion

The detectability half-life curves for G. punctipes and O. insidiosus are shown in Fig. 1. The half-lives themselves, as well as those previously published (Greenstone et al. 2010) for adults and immatures of two other heteropterans and two coleopterans fed single eggs of L. decemlineata are presented in descending order of length in Table 1. The hypothesis under test is clearly not supported. Although at 18.4 and 17.1 h, the new half-lives are statistically longer than those of the adult beetles, they are also statistically indistinguishable from that of larvae of the coleopteran Coleomegilla maculata (DeGeer), and statistically shorter than three of the four previously published heteropteran half-lives. If only adults are considered, then the heteropterans and coleopterans are separable, but the range is still so large that there are multiple statistical differences among the half-lives, making generalization at the order level unsupportable.

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Table 1. Detectability half-lives and their 95% fiducial limits for DNA of a single prey item in four species of predatory Heteroptera (H) and two species of Coleoptera (C). Data for Geocoris punctipes and Orius insidiosus from this paper; all others from Greenstone et al. (2010). Orius insidiosus fed single nymphs of Bemisia tabaci ; all other predators fed single eggs of Leptinotarsa decemlineata . Half-lives and fiducial limits were determined with a two-parameter probit model (Proc PROBIT; SAS Institute 1999).

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Our intention in designing this research was to make use of our previously published half-life set by adding two more predators of the same pest, L. decemlineata. However, O. insidiosus proved unable to attack eggs of that insect in the laboratory, which is why we substituted B. tabaci. Though it is not the same species, we feel that at 11 mg (A.M. Simmons, unpublished), an adult B. tabaci is not too dissimilar in mass from an L. decemlineata egg (28 mg; Chu et al. 2003) and probably contains a similar quantity of DNA.

Our recent review of the literature revealed a tremendous range in prey detectability half-lives across arthropod predator taxa, one cause of which could have been the variety of prey taxa, prey item size, and feeding protocols (Greenstone et al. 2014). The present analysis shows that even with the same protocol and prey item, heteropterans may exhibit half-lives that range from 17.1 to 84.4 h, many of them statistically different from one another and indistinguishable from that of at least one coleopteran. For now, at least, it appears that rigorous use of the detectability half-life will require assiduous measurement for each predator–prey combination.