Life stages

 

Which life stage should I use for identification?

The life stages that are useful for identifying mites are adults and specialized dispersal stages called phoretic deutonymphs. If you sample from mites on an adult bee, almost every specimen will be either a phoretic deutonymph or a female, both of which are useful for identification. In some Prostigmata that are permanent parasites, adult males and juveniles may also be found. If you sample from an adult bee's internal tracheae or air sacs, adult mites are the most important stage for identification. And if you sample from a bee nest, the best strategy is to obtain adults by collecting as many mites as possible. While adults, especially females, are the largest stage, the immature stages are usually more numerous in nests, and it is not always possible for non-acarologists to confidently distinguish adults (especially males) under a dissecting microscope. A large sample should ensure that adults are included.

How do I recognize each stage?

Mites may pass through up to six instars: prelarva, larva, protonymph, deutonymph, tritonymph, and adult. These developmental stages may look different or may be omitted depending on the mite group. All mites have an adult stage; the female is the dispersal stage in some Mesostigmata and Prostigmata. Deutonymphs may also be the dispersal stage in Mesostigmata and Astigmata. The deutonymph is unspecialized or slightly specialized in Mesostigmata.

In Astigmata, the larva, protonymph, tritonymph, and adult are usually feeding ontogenetic stages, though exceptions occur, while the deutonymph serves for dispersal or survival in severe conditions, lacks mouthparts, and cannot feed (though non-oral feeding is possible in several taxa). Deutonymphs have many morphological specializations for phoresy, and they are strikingly different from other life stages.

In the fact sheets we describe and illustrate adults and give specific information on what the dispersal stage is in the life cycle for each mite lineage. If this stage is not the adult female, then a dispersal stage is also described and illustrated. Consult the fact sheets to determine which life stages are relevant for identifying each genus. Below we provide general information about mite developmental stages.


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Life stages of Chaetodactylus krombeini (Astigmata); male not shown; individual LT-SEM photos (excluding phoretic deutonymph photo) by Ron Ochoa and Gary Bauchan, USDA-ARS.
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Life stages of Varroa destructor (Mesostigmata); four individual photos by Gilles San Martin/Flickr; male deutonymph and larva photos by MrJEberhardt/Youtube.

Prelarva

This is the first ontogenetic stage in mite development. In bee-associated mites, the prelarva is inactive, non-feeding, and sac-like (no legs or mouthparts). It develops inside the egg chorion where it consumes yolk and it molts into a larva. The prelarva is absent in Mesostigmata.

Larva

This is an active, feeding instar invariably present in mite life cycles. After eclosion, larvae shed both prelarval skin and chorion. In most bee-associated mites, this is the first active stage. It is usually a weak, miniature stage, but in some groups, larvae are aggressive parasites (Parasitengona) or predators (family Cheyletidae). In contrast, in most Mesostigmata (e.g., Varroa), the larval stage is short-lived and non-feeding, sometimes remaining within the egg chorion.

IDENTIFICATION
  • Mite larvae have three pairs of legs.
  • Larvae lack functional external genital structures related to reproduction and oviposition. In taxa with genital papillae (Oribatida and Astigmata), larvae often have Claparède organs.
  • Males of Locustacarus buchneri (Podapolipidae) are an exception: they are born sexually mature and have external genitals developed; in this species all larvae are females that can copulate with males but cannot lay eggs.

Nymphal stages

There are three possible nymphal stages separated by molts: protonymph, deutonymph, and tritonymph. One of the stages may be either permanently or optionally suppressed, resulting in only two nymphal stages. Rarely, there is only one nymphal stage. Nymphal stages are suppressed in some Trombidiformes (Podapolipidae, Tarsonemidae, and Pyemotidae). A nymphal stage can be facultative, depending on particular circumstances.

In bee-associated Astigmata, deutonymphs are heteromorphic with respect to other stages and may also be polymorphic. There are two types of deutonymphs for these taxa: (1) phoretic deutonymph, an active and typically non-feeding stage serving for attachment and dispersal on insect hosts; and (2) non-phoretic (or immobile) deutonymph, a highly regressive, cyst-like stage serving for survival in adverse conditions. The phoretic deutonymph is a facultative stage; it only appears when there is a need for dispersal. Many bees only use nests for a single generation, so mites feeding on pollen inside those nests are doomed unless they can disperse to other nests that have adult bees. The life cycle of mites is therefore synchronized with that of their bee hosts, and the dispersal stage appears when adult bees are about to emerge and leave the nest, thus dispersing mites to new nests. In bee-associated mites, non-phoretic deutonymphs are known only in Chaetodactylus and certain species of Glycyphagus and Acarus. In Chaetodactylus, which is associated with solitary bees constructing nests only for a single generation/season, the deutonymphs remain in the nest cavity waiting for the nest to be re-used by other bees rather than dispersing.

IDENTIFICATION
    Cultured life stages (feeding stages only) of Chaetodactylus krombeini (Astigmata). Pollen grains visible.
  • Nymphs have four pairs of legs and lack functional external organs related to reproduction and oviposition. In some groups, the nymphal stages can only be distinguished by experts.
  • In Oribatida, different nymphal stages can be relatively easily identified by the number of genital papillae: the protonymph has 1 pair, deutonymph 2 pairs, and tritonymph and adult 3 pairs.
  • In Astigmata the protonymph has 1 pair of genital papillae, deutonymph 2 pairs, and tritonymph and adult 2 pairs. The deutonymph is phoretic and is very different from any other stage. It is easily identified by the presence of the attachment organ and the lack of functional mouthparts. A few taxa in the families Chaetodactylidae, Glycyphagidae, and Acaridae have non-phoretic, immobile deutonymphs. This stage is a featureless sack with greatly reduced legs, attachment organ, and mouthparts, and it usually stays under the protonymphal skin.

Adult

The adult is a sexually mature stage that cannot molt anymore, though rare instances of adult molts are known.

IDENTIFICATION
  • Adults have four pairs of legs and functional external organs related to reproduction. The exception is Locustacarus buchneri (Podapolipidae): this species' adult females have 1 pair of legs and the larviform males have 3 pairs (like the larva of many other mites). Larviform males of this species can copulate with larviform females, though only adult females lay eggs.
  • The adult female has an ovipore, and the male has either an aedeagus, a gonopode, or a spermatopositor. A sperm access system not connected with the ovipore is present in Astigmata, Prostigmata (Tetranychidae), and some Parasitiformes (Dermanyssina and Heterozerconina). In these cases, the ovipore and copulatory opening are separate and disconnected.

Where can I find each life stage?

For bee-associated mites, there are typically two primary habitats where various life stages of mites can be found: adult bees and their nests. There are three different combinations of these habitats.

  1. Mite life-cycles occur both on adult bees and in their nests. On adults bees, you'll find stages specifically adapted for dispersal or parasitism. All life stages can be found in the nests.
  2. EXAMPLES
    • In most Astigmata, phoretic deutonymphs are found on adult bees. This is a non-feeding stage serving for dispersal from one nest to another. All other stages occur in the bee nest, where they feed on nest waste or pollen, or they can attack developing bee larvae.
    • In Parasitengona (e.g., Leptus), the larvae are parasitic on adult bees. All other active stages live either in the nest or the environment, where they prey on microarthropods.
    • In some Mesostigmata (Macrocheles and Pneumolaelaps), the entire life cycle (larva, protonymph, deutonymph, and adult males and females) occurs in the nest, but fertilized females can also be found on adult bees. So here, the female is both a phoretic stage (found on bees) and a feeding/reproductive stage (found in the nest). The life cycle of Varroa is similar, except it is always parasitic, both during dispersal on adult bees and in the nest.
  3. All stages occur on adult bees. This is most common for internal parasites that live in bee tracheae (Acarapis) or air sacs (Locustacarus). The fertilized female is also the dispersal stage. Fertilization occurs inside the bee, then fertilized females travel outside of the bee to infect new bees from the same nest. These fertilized females can also occasionally be found in the nest infecting other bees or externally on bees, where they can infect other bees on contact. Other stages of these mites are only found internally on the host.
  4. All stages occur in the nest (four genera within Astigmata: Tyrophagus, Aleuroglyphus, Tyroborus, and Tyrolichus and all Oribatida). These are habitat generalists; they do not have a specialized dispersal stage, and they invade nests from the environment. Rarely, some of these mites (e.g., Tyrophagus) can be found dispersing on adult bees.

A note about feeding stages

In this tool, we use the terms "feeding stage" and "non-feeding stage" only for the members of Astigmata to distinguish the phoretic deutonymph from other stages. In Astigmata, this stage is always non-feeding, in contrast to the other stages in this group's life-cycle (i.e., adults, tritonymphs, protonymphs, and larvae), which, for bee-associated mites, are always feeding stages. In other groups of bee mites, other stages may be non-feeding. For example, larvae are non-feeding in Varroa, and some Heterostigmata adult males may be non-feeding. Similarly, phoretic stages may be non-feeding during phoresy (as in females of Heterostigmata and some Mesostigmata, and deutonymphs of some Mesostigmata), but are feeding when not phoretic. We recognize that using the terms "feeding stage" and "non-feeding stage" may be confusing for other groups, so in this tool we use these terms only for Astigmata.