Napomyza lateralis

Napomyza lateralis (Fallén, 1823)

N. lateralis is a rare example for a species that is prominent as both pest and potential weed control agent. This will be further explained below.

In older literature the closely related species Napomyza carotae and Napomyza cichorii feeding on different plants were considered as belonging to the same species in that times called lateralis (see Spencer, 1966, Zlobin, 1994 a). Although there are only slight morphological differences between these species, the splitting was not only well supported but also initiated by observations of host preference. Extensive host preference experiments are summarized in van 't Sant et al., 1975.

However, even after the subdivision, it is likely that the remaining N. lateralis, the morphology of which was meticulously investigated by Zlobin, 1994 a might still represent a species complex. Indications are some remarkable behavioural differences and the host range witnessed in the literature.

Wing length: 2 - 3 mm.

Diagnosis after Zlobin, 1994 a:
- orbital hairs sparse, in 2 rows in front.
- third antennal segment elongated, appears almost bare.
- basal part of distiphallus (mesophallus) narrow.
- neck of distiphallus with characteristic ventral curvature.
- head of distiphallus large.
- spermathecae higher than broad.

Immature stages (Nap lateralis ceph.pct)
Larva rather long and slender, body shape conical becoming thinner towards the end. Larval mandibles strong, the larger right mandible bearing two and the left one only one mouth hooks. Lateral sclerites of mandibles well visible. Cephalopharyngeal skeleton of characteristic shape, shown in Nap lateralis ceph.pct.

According to the literature and personal observations both eggs laying females and larvae seem to show a highly flexible behaviour. That, however can be a sign for the presence of cryptic species within the current limits of N. lateralis.
Oviposition can occur either in upper leaves (Spencer, 1976 b), in the stem or in the flower bud. The emerging larva may feed exclusively in the stem (in sun flower, Trenchev, 1979) or mine from a leaf into the stem (Spencer, 1976 b). Other larvae have been observed completing their whole development within the flower head (personal observation). The puparia are found within the mines or during the winter in the decaying plants in the litter. There are at least two generations per year.

Apparently a wide host range among Asteraceae (but see above): Among others Chamomilla (chamomile), Matricaria (scentless chamomile), Lactuca (lettuce) (Spencer, 1990) and Helianthus annuus (sunflower) (Trenchev, 1979) and the ornamental plants Calendula officinalis and Dimorphotheca have been recorded.
Records from other plant families are: Linaceae: Linum usitatissimum (flax), the larvae were found in stems (Spencer, 1990).
Fabaceae: Lupinus (Fabaceae) (Zlobin, 1994 a). A high density of larvae was found in the stems that cause many of these cultivated plants to break and decay.

Choice experiments with adult flies reared from Matricaria discoidea revealed that these flies did not infest Lactuca and Cichorium (van 't Sant et al., 1975). Although Matricaria is regarded as one of the main hosts (Spencer, 1976 b) the authors found that even this plant was rejected by specimens emerged from Chamomilla. However, In the case of Napomyza lateralis, choice experiments should not be generalized because often local populations may show host preferences differing from other populations (see also below).

Common and widespread in Europe and Middle Asia, also known from North America (Zlobin, 1994 a).

Among the pest species previously subsumed under lateralis are Napomyza carotae and Napomyza cichorii. Napomyza lateralis itself in the present narrower species concept is probably of minor importance as a pest, because it mainly infests only rarely cultivated plants. The main plants that can be infested are chamomile and perhaps some asteracean ornamentals (e.g. Calendula, Spencer, 1990). However, the so far only record about lateralis feeding on cultivated sunflower (Trenchev, 1979) deserves further attention and investigations.

Although already introduced in the 19th century, recently the scentless chamomile has been getting an important weed in Canada. Among other phytophagous insects Napomyza lateralis is currently tested as control agent to be introduced in Canada (Hinz and McClay, 2000). According to the authors, first tests of the host specificity of laboratory stocks have been promising. The investigations were hampered by the already mentioned uncertain species status of the agromyzid fly. N. lateralis can be either one plastic species with the ability of easy adaptation to new host plants or a series of cryptic species. Further molecular research is required to clarify the situation.