Introduction

Phytoseiid mites (Acari: Mesostigmata) are widely distributed arthropod predators. This family includes more than 90 genera and 2479 species (Moraes et al. 2004; Demite et al. 2017). Phytoseiid mites have received increasing attention worldwide due to their potential in biological control of small arthropod pests, such as spider mites, eriophyid mites, thrips and whiteflies and psyllids (Xu & Zhang 2015). More than 30 species are commercially produced as biological control agents, among which some became very successful and have been widely applied in many countries, including Phytoseiulus persimilis Athias-Henriot, Neoseiulus cucumeris (Oudemans), and Amblyseius swirskii Athias-Henriot, etc. (Van Lenteren 2012). However, these species account for a very small fraction of the big family. Due to increasing requirements for alternatives of chemical control worldwide, it will be valuable to investigate more biological control candidates among predatory mites. The first step to evaluate the biological control potential of a predator is to identify its feeding habits.

There are huge variations in feeding habits of phytoseiid mites, from very specialist species to quite generalist species. McMurtry & Croft (1997) divided predatory mites in this family into four life styles primarily basing on their feeding habits, biological and morphological traits: Type I: specialized predators of Tetranychus spp., including only Phytoseiulus species, e.g. P. persimilis. Type II: selective predators of tetranychid mites, referring to species mainly feed on spider mite species that produces “webnest”, e.g. Neoseiulus californicus (McGregor), Galendromus occidentalis (Nesbitt). Type III: generalist predators, referring to species that consume a wide variety of prey or food such as different kinds of phytophagous mites, thrips, whiteflies, nematodes, pollen, fungus, plant excretes, and honeydew, etc. More than 70% of phytoseiid species belongs to this type. Some representative species are Amblyseius swirskii, Neoseiulus barkeri (Hughes), Neoseiulus cucumeris, etc. Type IV: specialized pollen feeders, referring to generalist predators belonging to the genus Euseius, which have higher reproductive potential when feeding on pollen than other types of food (McMurtry & Croft 1997; Croft et al. 1998a). McMurtry et al. (2013) further divided Type I and Type III into subgroups on the basis of prey preference, and living conditions, respectively. In addition, they added Iphiseius into Type IV.

This classification well grouped known species and directed their potentials in biological control, which is highly valuable in Phytoseiidae because some taxonomically similar species may have quite different prey range. The following question is whether it is possible to estimate the prey range of a species before its feeding habits have been well studied. In other words, are there common features of the species within the same life type group that can be achieved easier than trying to rear them on all possible prey?

Pratt et al. (1999) tried to estimate the life styles of five species (P. persimilis, Neoseiulus fallacis (Garman), Typhlodromus pyri Scheuten, Euseius finlandicus Oudemans and Euseius hibisci (Chant)) based on their oviposition rates. Their analyses separated life styles of Type I and Type II species successfully, but classifications of generalists were not consistent with that of life styles classified by McMurtry and Crofts (1997). Luh and Croft (1999) investigated 24 parameters, including mid-dorsal and margin-dorsal setal measurements, oviposition rates, development rate, and prey type, etc., of 37 species belonging to six genera. They declared that development rate is the key feature in determining life-styles. In their study, 32 out of 37 species were grouped into the same life styles as McMurtry and Croft (1997) (Luh and Croft 2001). Croft et al. (2004) suggested that life styles are related to multiple feature groups, including food types, morphology, reproduction and development, physiology and behavior, interspecies interactions, and dispersal, etc., which actually mean all aspects of phytoseiid biology. Overall, limited practical methods have been provided to estimate life style of phytoseiid mites without detailed biological experiments or wide range of rearing attempts, which will be very useful in preliminary evaluation for biological control potentials of species newly discovered or have not been well studied.

Predatory mites capture prey and feed using their gnathosoma, which includes tectum, a pair of chelicerae and pedipalpi, hypostome, and gnathobase. Kaneko (1988) and Buryn and Brandl (1992) measured multiple chelicerae morphometrics of oribatid mites and mesostigmatid mites, respectively. Differences among species with different prey types were detected in both studies. Flechtmann and McMurtry (1992a, b) described differences in chelicerae of phytoseiid species with different prey types qualitatively. It is valuable to further estimate whether these differences are sufficient in categorizing phytoseiid life styles quantitatively. Previous studies mainly focus on morphological comparisons among chelicerae, but it will be also valuable to compare other organs, such as tritosternum and hypostome, etc., which also play important roles in predating and feeding (Wernz and Krantz 1976).

In this study, scanning electron microscope (SEM) pictures of 10 phytoseiid species belonging to four major life styles were taken. For each species, 23 morphometrics of gnathosoma and the length of dorsal shield were measured. These measurements include lengths, angles, area, and counts that described gnathosoma morphology comprehensively and quantitatively, in order to appropriately estimate phytoseiid life styles using a direct method.

Materials and Methods

Ten species and their life styles used in this study according to McMurtry & Croft (1997) and Wu et al. (2008):

Schicha (1975) considered N. pseudolongispinosus and Neoseiulus womersleyi synonymous, but Chant & McMurtry (2003) and Wu et al. (2010) listed N. pseudolongispinosus as a separate species. So we consider N. pseudolongispinosus as a separate species herein.

Individuals of nine species were collected from lab colonies maintained in the Lab of Predatory Mites for over five years and all species were reared by sufficient Tetranychus urticae (Koch) (Prostigmata: Tetranychidae) on Phaseolus vulgaris L., Institute of Plant Protection, Chinese Academy of Agricultural Sciences (IPP-CAAS). Euseius utilis individuals were collected from Lonicera maackii (Caprifoliaceae) leaves in the yard of the Institute of Food Sciences and Technology, Chinese Academy of Agricultural Sciences (IFST-CAAs)).

For each species, ca. 50 adult females were scalded in 100°C deionized water, to have their chelicerae extended. The individuals were then dehydrated in 70%, 80%, 90%, and 100% ethanol for two minutes successively, and dried in LEICA EM CPD030 drier for two hours. The dried specimens were glued on the stage, either dorsal side, ventral side, or lateral side up. Fifteen replicates of each position were prepared. For each of the lateral side up specimen, its pedipalpi and 1^st pair of legs were removed with 00 insect pin to expose chelicerae under the stereomicroscope.

All samples were gold sputtered in MC1000 ion sputter for one minute, observed and imaged at magnifications of 1000x, 2000x and 3000x with a scanning electron microscope (FEI QUANTA 200F, National Center for Nanoscience and Technology).

For each species, 24 parameters were measured using ImageJ1.42I following the methods provided in Appendix (Table 4 and Figure 2). A minimum of 15 replicates of each parameter were achieved. Principle Components Analysis (PCA) was conducted for dimension reduction. The first two principle components of each replicate were scattered plotted for grouping.

One-way ANOVAs were conducted to compare inter-group differences of major principle components (PCs) and each of the 24 parameters. For each response variable, data were rank-transformed to satisfy normality test. Multiple comparisons were conducted with Turkey HSD test. All mean comparisons with p<0.05 were considered to have statistical significant differences. All analyses were processed with SPSS 19.0.

Results

The first six principle components (PC) explained 83.76% of total variations, among which PC1 and PC2 explained 36.01% and 17.50% variations, respectively. For PC1, loadings of nine parameters were higher than 0.70, including length of chelicerae, length of fixed digit, width of fixed digit, length of teeth row of fixed digit, dorsal perimeter of fixed digit, width of movable digit, ventral perimeter of movable digit, corniculi length, internal malae length. For PC2, only the angle of fixed digit and length of movable digit had high loading (0.76 and 0.80 respectively) (Table 1). When plotted with the two PCs as x and y values, respectively, as indicated in the Figure 1, the 10 species were assigned into three groups as follows:

Inter-group differences were detected for four of the first six PCs (Table 2) and 22 parameters (Table 3). Overall, species in Group I and Group III had smaller chelicerae, chelae and hypostome, with the lengths of these parameters ca. 80% as those of Group II. Group III had ca. 10 and 35 times larger lobe area than Group I and Group II, respectively. And its angle of fixed digit was ca. 2–3 times larger than those of the other two groups.

TABLE 1.

Loadings of the first 6 principal components (PCs).

TABLE 2.

Means and standard deviations of diet categories on six principal components.

TABLE 3.

Means and standard errors of three groups on the 24 mophometrics (µm).

Discussion

In this study, we assigned 10 phytoseiid mite species into three groups based on morphometrics of their gnathosoma. The three groups: Group I, Group II and Group III represents the specialist, generalist and pollen feeder, respectively. This result is broadly consistent with the life styles classification provided by McMurtry and Croft (1997), suggesting it is appropriate to estimate the feeding habits of phytoseiid mites based on their gnathosoma morphology.

FIGURE 1.

Scattered plot of the first two multifactorial axes of 10 Phytoseiidae species on their 24 morphometrics.

McMurtry and Croft (1997) assigned P. persimilis into Type I, and N. californicus into Type II. N. pseudolongispinosus is native species discovered in China. Wu et al. (2008) assigned it into Type II due to its high preference and consumption rates of Tetranychus spp. In general, these three species are all considered as specialist predators of spider mites. However, it has been a debate regarding whether N. californicus is a specialist? Although showing strong preferences for Tetranychus spp., this species also feeds on multiple species of mites and insects, and even on pollens (Pena and Osborne 1996; Easterbrook 2001; Rahmani et al. 2009; Saber 2012). It also shares some other features with generalists, such as being able to complete life cycle on immatures of other predatory mites (Walzer and Schausberger 1999a, b; Schausberger and Croft 1999, 2000), and having short mid-dorsal setae etc. (Croft et al. 1998b). Therefore, Croft et al. (1998b) preferred to assign it into Type III. However, McMurtry et al. (2013) insisted that it belongs to Type II, because it often coexists with spider mites that produces heavy webs and shows strong adaptation to such environments, which is quite different from generalists. In the present study, N. californicus is obviously separated from generalist predators. Among the three specialist species, data points of N. californicus and N. pseudolongispinosus overlapped more. This supported the viewpoint in McMurtry et al. (2013).

All the six species that constitute Group II are Type III generalists, while E. utilis is separated from all other species due to its specific features, such as huge lobe area that covers almost the whole chela and large angle of fixed digit. These features are consistent with descriptions in previous studies (Flechtmann and McMurtry 1992a; Adar et al. 2012). Our results also showed that E. utilis has relatively longer internal malae, equaling ca. 1/2 as long as its corniculi, while the internal malae of other species only are 1/5–1/3 of their corniculi in length. However, it will be necessary to investigate other Type IV species to confirm the pervasiveness of these characteristics in this group.

Overall, we are able to estimate the feeding habits of the 10 phytoseiid mites based on the 24 measurements. These parameters provided comprehensive descriptions of phytoseiid gnathosoma. All the parameters can be easily measured directly from field collected specimens, and showed limited intraspecific variation. Buryn and Brandi (1992) used PC A to estimate feeding habits of gamasid mites based on 10 parameters of chelicerae. Our results showed that PC A is also an appropriate method to estimate feeding habits of phytoseiids.

Only 10 species were measured in this study, however, we expect that further life-style categorization of newly described species and/or some species with limited knowledge about their feeding habits would be possible based on their gnathosoma morphometrics. For example, to separate Type I and II specialists, or further divide Type III generalists into subgroups. Among the six generalists in this study, the three Amblyseius species have smaller values than the three Neoseiulus species in PC2. On the other axis, there are also trends of two subgroups, each containing 3 species whose data points are more overlapped. Although it is too early to discuss these trends in details due to limited number of species, they did suggest some possibilities for further classifications.

The purpose of our study is to develop a method that can be used to estimate feeding habits of phytoseiids directly and easily. Apparently, feeding habits and life styles of these species are also affected by many other factors as well as gnathosoma morphology, such as nutritional requirements, and digestive ability, etc. It is necessary to take measurements of more species in the future to test the reliability of this method and to discuss the possibilities for subgrouping. It will also be very valuable to compare different populations or strains belonging to same species, whether these parameters are variable or consistent at individual, population, and species levels.