Taxonomic Revision of Zelus Fabricius


The Zelus monograph is published here.

Background - Taxonomic status of Zelus, use in biological control, and sticky trap predation

Zelus Fabricius, 1803 is a large New World genus in the subfamily Harpactorinae, the largest subfamily within Reduviidae. Zelus consists of 60 valid species. The majority of species of Zelus occurs in Central and South America. Five species are present in North America. Dr. Hart in his PhD dissertation (1972, Texas A&M) provided a revision of this genus, and published in two articles the North American, Northern Mexican and West Indian species (19 species), while most of the work remains unpublished. In the same dissertation, Hart also proposed 16 new synonyms and combinations, described 21 new species, and they have not been published and thus remain invalid or unavailable. In the current revisionary project, a total of 75 species are expected and ca. 10,000 specimens will be examined.

Species of Zelus such as Z. longipes (Linnaeus, 1767), and Z. renardii Kolenati, 1856, have been explored as natural enemies in integrated pest management. Zelus preys on a wide range of insects, e.g. boll weevils and leafhoppers, but up to 40% of their diet may consist of lepidopteran larvae including cotton bollworm, tobacco budworm, and pink cotton bollworm. They are beneficial predators especially in cotton, soybean, alfalfa crops and tree fruit in California and elsewhere, may reach population densities of up to 75,000/ha and prevent outbreaks of lepidopteran larvae. In order to use Zelus spp. as an effective biological control agent, we need additional studies. Recently, it was found that males of Z. tetracanthus Stål, 1862 are attracted to aggregation pheromones of bostrichid beetles and preliminary studies confirm the presence of a pheromone in females of this species. The nature of the pheromone is not yet determined. Knowledge on pheromones will improve the use of Zelus spp. as a biological control agent by allowing to manipulate their behaviors.

Zelus possesses a unique sticky trap predation strategy. Species of Zelus (Harpactorini) such as Z. leucogrammus (Perty, 1833), Z. longipes, Z. luridus Stål, 1862 and Z. renardii have been found to secret sticky substances from unique dermal glands on the front tibiae, which are smeared onto setae that resemble leaves of sundew. The ontology of this behavior is interesting. Newly hatched nymphs do not posses dermal glands or produce secretions. Instead, they collect material from the sticky coating of eggs that the female produced and use it for prey capture. Tibial dermal glands start developing from the second instar on. The use of endogenous secretion may be a derived trait of Zelus. Other means of sticky trap predation such as collecting plant resins with front legs for prey capture have been documented in other tribes such as Ectinoderini, Apiomerini, and Diaspidiini. Morphology and predatory behavior of the majority of species of Zelus remain unexamined and the phylogenetic pattern of such traits unexplored. 

Research objectives

1. Conduct a monographic revision of Zelus - A monographic revision treating all species of Zelus will be conducted. Species limits proposed by Hart will be examined, a phylogeny based on both molecular and morphological characters will be constructed, and immature stages for common species will be described. Tools in modern taxonomy will be employed such as online database, high-resolution digital photography, georeferencing, and online interactive keys. Habitus images will be made available online. Important taxonomic characters such as male genitalia will be documented with GT EntoVision, providing high accuracy. Female genitalia will be explored and documented with Confocal Microscopy. Also, techniques of inflating the endosoma of the male genitalia are currently being explored, which will allow for documenting characters on the endosoma.  

2. Reconstruct phylogeny and investigate biogeography of Zelus - A phylogeny of Zelus will be constructed with both molecular and morphological characters. A molecular data set consisting of ca. 30 species and 5 genes will be targeted. The morphological dataset will draw characters from male and female genitalia and as well as external morphology. Analytical methods will comprise the conventional multi-gene concatenation method and also a recently developed method of constructing species trees from gene trees. The phylogeny of Zelus can be used to investigate evolution of interesting morphological characters such as tibial glands and setation involved in sticky trap predation. Biogeographical pattern of Zelus will also be examined. Specifically, we will investigate if vicariance or dispersal contribute to the current distribution pattern of Zelus. Also, we will propose a hypothesis on the origin of the West Indian species?  

3. Study sticky trap predation and associated morphology - Predatory strategies are very diverse in Reduviidae. Yet, none has been investigated under a phylogenetic framework. The more phylogenetically basal tribes of Harpactorinae collect resins for prey capture. Using endogenous sticky secretion is currently only known from Zelus. Is this a unique character to Zelus? Do other genera or species of the same tribe (Harpactorini) possess similar morphological structures on the front tibiae? Do all species of Zelus possess glandular structures on the forelegs? What is the phylogenetic history of sticky trap predation within Zelus? These are some of the questions we would like to answer.  

Preliminary results

1.  Specimen sorting, databasing and georeferencing - About 1,000 pinned specimens have been obtained through museum loans, representing about 30 species of Zelus, of which 250 specimens have been databased. Georeferencing has also been performed and the results are constantly updated through Discover Life.   

2. Imaging and documentation of morphological characters - Habitus images of 27 species (incl. manuscript names) have been uploaded on Discover Life. Male genitalia of 6 species were photographed with GT EntoVision and uploaded on MorphBank . This effort will expand to 15 species by April, 2009. All images are made available for public access.

3. Molecular phylogeny - A preliminary molecular phylogeny for 11 species based on 3 ribosomal genes (16S, 18S, 28SD2, D3-D5) shows reasonable resolution and statistical support. With upcoming trips and donations of specimens from collaborators, we expect to generate a phylogeny including 30 species, i.e. about half of the species of Zelus. The monophyly of Zelus was recovered in the current analysis. Atopozelus Elkins was shown to be the sister genus of Zelus, conforming to the hypothesis by Hart (1972). Parental care has been found in Atopozelus, and its sister group relationship with Zelus sparks speculations on the potential existence of parental care in Zelus.  

4. Investigation of sticky trap predation - Setation and gland openings on the front tibiae of Z. renardii have been documented using scanning electron microscopy. Laboratory observations of Z. renardii and Z. tetracanthus provide evidence of the effectiveness of sticky secretions on front legs for prey capture. Behavioral experiments will be important for testing the significance of the secretion quantitatively.

Broader impact

1. Monographic revision in the 21st century - Monographic revisions will continue to be an importance aspect of documenting the biodiversity of the planet in the 21st century (Wheeler 2008 in The New Taxonomy ).  One cannot discover new species without knowing what is already known and knowing it well. With the advancement of information technology, the field of revisionary taxonomy is also undergoing transformation in the 21st century. Online databases, electronic access to information, and collaborative tools such as MorphBank are playing more and more important roles.  This project will be a showcase of using and experimenting with these tools. The results will be exemplary for future revisionary projects.  

2. Facilitation of using Zelus spp. as a biological control agent - Sound taxonomic knowledge of Zelus is imperative for using it as an effective biological control agent. The wide distribution of Zelus in the New World makes it a possible biological control candidate in various climates and countries. The taxonomic revision of Zelus will provide tools for accurate identification. The pheromone work will facilitate manipulation of the behavior of species of Zelus.

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