Phytoseiidae

de MoraesK, Gilberto J., CastilhoK, Raphael de Campos, FlechtmannK, Carlos H. W., DemiteK, Peterson R. & HallidayK, Bruce, 2025, Progress in understanding the world mesostigmatic mites, with emphasis on the family Phytoseiidae (Acari: Mesostigmata), Acarologia 65 (3), pp. 647-676 : 666-670

Phytoseiidae

This group was first reported by Berlese (1916) as the tribe Phytoseiini of the family Laelapidae ,

and was raised to the family level by Baker and Wharton (1952). A detailed account of the initial studies of the morphology, taxonomy, biology and possible importance of the Phytoseiidae for pest control was published by Chant (1959a) in the first issue of the first volume of

Acarologia. Phytoseiidae is composed of species considered beneficial for their predatory behaviour on harmful mites and insects ( McMurtry et al. 2013, 2015). Hence, in her evaluation of the economic importance of Mesostigmata in agriculture, Bregetova (1966) mentioned the expectation then held for the possible use of phytoseiids as biological control agents. A few years later, a revision of the importance of phytoseiid and other predatory arthropods in the biological control of harmful organisms was published by Huffaker et al. (1970).

In contrast to most other mesostigmatic families, phytoseiids are mainly plant inhabitants,

and only a minority can be found in the soil. The inability of mites of most other mesostigmatic families to remain on plants is probably related to their requirement for high humidity levels.

This seems consistent with the presence of some members of other families on (at least) low growing plants at night ( Esteca et al. 2021), when air saturation deficit reaches the lowest levels. This trend favours the phytoseiids, by releasing them from competition with other mites of the same guild. Some other arboreal groups such as the Stigmaeidae ( Trombidiformes ) have been shown to interfere with and compete with phytoseiids ( Fan and Flechtmann 2015).

Presently, phytoseiids constitute the most important group of predatory mites used commercially for the biological control of mite and small insect pests, in all three types of biological control strategies: classic, conservation and augmentation. Over 35 species of this family have been commercially mass produced and sold to growers for pest control around the world

( van Lenteren 2012 ; Knapp et al. 2018 ; van Lenteren et al. 2018), especially of spider mites

( Tetranychidae ), whiteflies ( Hemiptera ) and thrips (Thysanoptera). These mites are naturally found on both cultivated plants and natural vegetation. They move quickly when disturbed or in search of prey, but during the day they usually hide most on the underside of the leaves,

along the main veins. Others protect themselves by hiding in the growing tips of plants or in domatia ( McMurtry et al. 2015), or within natural refuges in the structure of the plant ( Beard and Walter 2001). They are rather uniform morphologically, but some unusual forms have been recently reported from different habitats, such as the Neotropical forests (Figure 12), bearing setae that are uncommon in this family, or unpaired setae, among other differences.

Table 9 shows records of the numbers of described phytoseiid species from different sources. Despite a reduction in the rate of new species discovery, the absolute number of species described is growing continuously. An evaluation of the number of phytoseiid species described in each decade (Figure 13) shows a pattern similar to that reported for mesostigmatic mites in general. A peak in the number of species described in 1960–1969 was followed by a slight reduction in the number of new species described in recent decades. Yet, the data suggest a continuously growing trend for the number of publications about taxonomy since

1950. Hence, efforts to understand these mites have increased, potentially contributing to improvements in their conservation and use.

What are the factors contributing to that growth? Certainly, the first two revisions of the phytoseiid mites ( Nesbitt 1951 ; Chant 1959b) were fundamental for the establishment of the basis on which the extensive successive works could be conducted. Only 26 species of the group presently known as Phytoseiidae (then as a subfamily of Laelapidae ) were mentioned by

Nesbitt (1951), most of which had been described previously by other authors. Just a few years later, Chant (1959b) reported 148 species, including 38 new species, as well as a few species that were overlooked by Nesbitt (1951) and many species that had been published since 1951.

The subsequent series of revisions by Chant and Yoshida-Shaul during the 1980’s and

1990’s, followed by the publication of the two editions of the Phytoseiidae Catalogue ( Moraes et al. 1986, 2004) were important in facilitating the work of new taxonomists interested in studying these mites. The comprehensive revision of the family by Chant and McMurtry from

Species Genera References 26 4 Nesbitt (1951) 148 9 Chant (1959b) 1,500 79 Moraes et al. (1986) 2,200 67 Moraes et al. (2004) 2,416 91 Demite et al. (2014) 2,985 102 Demite et al. (2024)

Despite the great diversity of the Phytoseiidae , about 83% of all known species belong to one of the ten most speciose genera ( Table 10). However, the search for new biological control agents should not be limited to those genera. At least two of the most commonly used phytoseiids internationally belong to the genus Phytoseiulus , which has a relatively restricted natural distribution in southern South America, and low diversity, with only four species.

Suggestions to increase the understanding and practical use of Mesostigmata

Priority studies aimed at better understanding the Mesostigmata and expanding their practical use are certainly tied to the particular geographic location under consideration. In some regions, virtually nothing is known about the local fauna, whereas in others, at least some knowledge about the species composition and perspective for their use is available. Taking that into consideration, we present a list of subjects considered relevant at this stage to promote the practical use of those mites ( Table 11).

In terms of research, we emphasise the need to continue the efforts to synthesise the data that has already been published, including databases of taxonomy and biology. The present easy access to searching tools in the internet has tremendously facilitated this type of work. Some work has been done in this area but much more can be achieved, as some mesostigmatic groups or some geographic regions have still not received the attention they deserve. In terms of taxonomy, not only written information should be synthesised, but effort should also be dedicated to the establishment of image libraries, especially of type specimens, making the images available electronically, eliminating the need for the eventual shipment of type specimens by regular mail for taxonomic evaluation. That could prevent not only their eventual damage or loss during transport, but also a more thorough and permanent registration of their morphological features. One such image library is available as VIRMISCO — The Virtual Microscope Slide Collection, at the Senckenberg Museum of Natural History, Görlitz ( Decker et al. 2018).

For the same reason, and of similar importance, is the preparation of more detailed original descriptions or redescriptions of taxa, based on a reasonable number of type specimens and measurements of taxonomically relevant structures of all types, to allow a study of their morphological variability ( Tixier 2012). Detailed illustrations, whenever possible including good quality photographs, are also desirable.

It has become progressively easier to include molecular data in descriptions of new taxa.

According to availability of appropriate specimens, this should also be considered, using one of the adequate markers for the more plesiomorphic characters, for the study of phylogenetic relationships, or more recently derived, to distinguish species. This type of information has become progressively more common in the description of new species, but a critical analysis of what has been achieved in this area is not yet available.

In seeking adequate representation of the members of the group, in attempting to produce a meaningful classification system, efforts could be dedicated to the conduct of surveys in inadequately explored regions around the globe. For example, little is known about the soil mesostigmatic mites from most of South America and Africa. Endemic species collected in those areas may significantly affect the concepts of their taxonomic groupings. Many examples of considerable changes can be found in the literature. As an example, Berlese (1906) referred to the need to change his own previous concept ( Berlese 1892) of the family Gamasidae

( Parasitidae ), which considered it necessary and sufficient for males to have spurs on leg II,

because of the exceptions he became aware of inside and outside of the group. A more recent example concerns the modification of the concept of the family Ameroseiidae , given the finding of a few new species in a new genus of this family ( Endopodoseius Abo-Shnaf & Moraes ),

which was distinguished from the other genera of the same family by the presence of seta J5

and divided dorsal shield, among other important features that were until recently taken as distinguishing characteristics of the whole family ( Abo-Shnaf et al. 2023).

In terms of practical applications, biological control of pest organisms with predatory mites has been used mostly in protected crops, or in small open fields of crops of high economic return (e.g., strawberry, some fruit trees). However, the bulk of food around the globe is produced in open fields, where crops are subject to attack by several mite species that are not currently controlled biologically, most probably because of cost and difficulty in handling the natural enemies under those conditions. Efforts could be dedicated to change this pattern. The use of biological control with predatory mites in extensively grown crops such as soybean and cotton could lead to important reductions of the use of chemicals in agriculture. Hence, the development of techniques to change that status quo is highly desirable.

Present regulations concerning biological control and conservation efforts of local fauna and flora have made the importation of exotic biological control agents more difficult. Hence,

better use of local species becomes more desirable. For that purpose, for each promising

· Synthesis of information: taxonomic databases, photographic databases, revisionary works · Provision of quality descriptions

· Complementing molecular characterisation (DNA barcoding)

· Survey in underexplored areas

· Training of new mite taxonomists

· Main challenges for practical use:

• Develop viable use of predatory mites in large cultivation areas

• Conservation of predatory mites by supplying alternative food

• Selection of predator strains with higher efficacy in the control of pest organisms biological control agent, the selection of populations with higher efficacy in the control of a particular pest could be sought, making use of selection practices developed by plant and animal breeders. The artificial selection of genotypes of promising species within the range of its natural genetic variability has been proposed for quite a long time for different groups of natural enemies. A recent revision on this subject was published by Lommen et al. (2017). For predatory mites, this type of study has been conducted mostly seeking for enhanced resistance to some types of pesticides ( Hoy 1986) or, at some degree, for reduced capacity of the predator to diapause ( Veerman 1992 ; van Houten et al. 1995). However, several other traits could be studied in attempts to improve the performance of predatory mites or their more economic commercial production, such as higher predation rates ( Lommen et al. 2017). For example,

Beard and Walter (2001) showed that species of Neoseiulus can be highly host plant specific in their natural habitat, and are not the free-ranging generalist predators they were thought to be. This finding has far-reaching implications for the efficient use of phytoseiids as control agents. Aspects of the host plant influence the behaviour and distribution of phytoseiids independently of prey availability and distribution ( Bakker and Klein 1992 ; Beard and Walter 2001), so phytoseiids with specific needs in terms of host plants may not be amenable for use as generalist predators in biological control. The phytoseiid most appropriate to a particular host plant should be sought, and emphasis in phytoseiid studies should be redirected from diet alone and placed on the range of specific requirements of that particular species.

This type of study, which depends heavily on genetic tools, can be adapted for the inclusion of mites as the subject to be studied. That would require the precise determination of attributes to be considered for selection as well as of the possible effects that such endeavour could have on the natural populations of the same and other organisms, envisioning the better performance of the predator with no environmental disturbances. It is also important is to study the development of conservation practices fostering the efficacy of locally occurring predatory mites, especially in extensively cultivated crops, when the release of commercial predators is less suitable ( Azevedo et al. 2020).