Tephritid Larval Keys

INTRODUCTION

Fruit flies (Diptera: Tephritidae) include some of the world’s most serious agricultural pests. They inflict incalculable direct losses to a wide variety of fruit, vegetable, and flower crops. Adult females lay eggs in healthy plant tissue at various developmental stages. The larvae feed on fruit pulp, seeds, or other tissues rendering them inedible to people. Additionally, fruit flies limit the development of agriculture in many countries because of strict trade restrictions imposed to prevent the spread of pest species. Fly-free areas are at risk of invasion and colonization by numerous such pests, and costly eradication programs frequently result from accidental introductions. Usually pests are introduced via infested host plants carrying eggs or larvae whose presence may not be apparent from the external condition of the fruit. At ports of entry into the U.S., most intercepted fruit fly pests arrive as larvae in infested host material that is carried in passenger baggage.

Of the 5,000+ tephritid fruit fly species known worldwide, over 200 species are considered pests. The family Tephritidae includes many cryptic species complexes, and even the adults may be difficult or impossible to identify accurately to species level. The difficulty is much greater for the immature stages since a few of the major pest species have been thoroughly described. The published descriptions that are available usually are only of the third instar. The full range of variation in morphological features is rarely documented, and there is often considerable overlap among species in quantitative and qualitative measures of easily seen, external features (e.g., prothoracic and posterior spiracles). Many diagnostic details can be seen clearly only by using scanning electron microscopy (SEM). 

This multi-entry LucID key uses the best available morphological data to identify third instar larvae to genus level for eight pest genera of fruit infesting tephritids: Anastrepha, Bactrocera, Carpomya, Ceratitis, Dacus, Rhagoletis, Zeugodacus, and Zonosemata. It is based on 12 characters found mostly on the pseudocephalon, thorax, and caudal segmentcaudal segment:
abdominal segment 8 <a href="https://idtools.org/fsm/tools/index.cfm?pageID=3385" target="_blank">(anatomy: figure 13)</a>
. The character states are illustrated with both SEM and optical microscope images as available. Several of the characters are invariant for each genus (occur as only a single state in all known species), while others include significant exceptions. Therefore, it is best to observe and score as many characters as possible to increase the likelihood of a correct genus identification. 

This LucID tool includes a second key to larvae of Anastrepha. Among fruit-infesting tephritids, larvae of Anastrepha have been better studied than most other genera. Anastrepha includes 328 described species and over 100 undescribed species. All occur in the American tropics and subtropics ranging from the southern United States to all countries of Central and South America, except Chile. This key includes the 41 species for which any larval description is available and uses 23 morphological characters. Thirteen species are considered important pest species, for which detailed larva Fact Sheets also are included. Several species are considered minor pests, while others are only known to infest wild, inedible fruits.  

Anastrepha larvae can be separated from other pest fruit fly genera by the following combination of characters: dorsolateral pair of sensilla perpendicular to maxillary sensilla; preoral organ usually fused to apex of single, elongate preoral lobe; preoral teeth (stomal guards) lacking; accessory platesaccessory plates:
small plates, often toothed, immediately lateral to the oral ridges, often poorly differentiated from them, and arranged in one, two or three series <a href="index.cfm?pageID=3385" target="_blank">(anatomy: figures 7, 9)</a>
to oral ridgesoral ridges:
several rows of ridges on each side of the mouth opening which may be entire, incised, emarginate, serrate, scalloped, dentate, or fringed on their lower (posterior margin) edge <a href="https://idtools.org/fsm/tools/index.cfm?pageID=3385" target="_blank">(anatomy: figures 2, 7.8.9)</a>
present and short; mouthhookmouthhook:
a paired, curved, strongly sclerotized, hook-like sclerite found in Cyclorrhapha larvae; it articulates posteriorly with the intermediate sclerite and may have an additional preapical tooth; has also been called mandible. <a href="https://idtools.org/fsm/tools/index.cfm?pageID=3385" target="_blank">(anatomy: figures 2, 5, 6, 11)</a>
with a single primary tooth (without preapical tooth), and basally truncate (without a neck); dental sclerite absent; prothoracic spiracle usually concave centrally; caudal ridgecaudal ridge:
a narrow, transverse ridge of thickened cuticle running dorsad of the I1a, I1b, and I2 sensilla on the intermediate area of the caudal segment in species of Dacinae <a href="https://idtools.org/fsm/tools/index.cfm?pageID=3385" target="_blank">(anatomy: figure 13)</a>
absent; fully developed larvae unable to jump (skip). 

A third, more ambitious and comprehensive key is under development, which will enable identification of larvae of all of the approximately 200 fruit-infesting tephritid pest species. The anatomy and glossary pages already include the additional morphological features that will be encountered in this large set of species as well. First time users should familiarize themselves with the anatomy and glossary pages. 

Much remains to be done to improve our ability to identity fruit fly larvae based on morphology. The user of this tool should be aware that misidentification of fruit fly larvae using this, or any other tool is possible and maybe even likely in some circumstances.