Chromatomyia horticola (Goureau, 1851)
The male genitalia of this species are distinct; a further hint might be the missing acrostichal hairs.
IMPORTANT CHARACTERS
Adults
Wing length: 2.2 - 2.7 mm. Frons can vary in color. It is at most completely yellow but sometimes exhibit some brownish darkenings. Margins of frons not distinctly darkened.
Arista appears to be divided into two sections with the apical part distinctly thinner than the basal part (von Tschirnhaus, 1969, compare Chromatomyia syngenesiae). Palps with one large lateral hole (hardly visible in dry specimens) (von Tschirnhaus, personal communication). Acrostichals are missing.
Immature stages
Cephalopharyngeal skeleton cannot be distinguished from the one of the closely related species Chromatomyia syngenesiae.
The most striking difference with some Liriomyza species occurring on the same plants is that the whitish grey puparium regularly remains in the mine. For more characters see the diagnosis of the genus Chromatomyia.
The larval leaf mine is irregularly linear.
BIONOMICS
There is a great number of papers on the life history and host range of Chromatomyia horticola. Literature published before 1970 should be interpreted with care because the species was often referred to as "atricornis" which is a synonym for both horticola and syngenesiae. The latter closely related species has a narrower host range.
Due to its wide distribution and the high number of host plants the seasonal occurrence and number of generations per season can vary considerably. Apparently the activity of the species depends mainly on the availability of suitable food and on the weather conditions. During a vegetational period many subsequent generations can be observed. The prevailing literature suggests that the development of Chromatomyia horticola is highly variable and can be unusually quick. Likewise, the number of eggs per female can be extraordinary high.
Bionomics and life cycle was studied by Melis, 1935 (Italy); Ahmad and Gupta, 1941 (India); Molitas and Gabriel, 1975 (Philippines); Wang and Yan, 1986 (China); Sharma et al., 1997 (India):
The number of eggs one female can lay ranges from 100 to nearly 500. In a single day the maximum number of eggs laid by one female can reach 50. After 1-4 days the young larvae hatch. Under warm conditions the larval period may last only 5-6 days but can be delayed depending on temperature and food quality. In temperate regions a longer time should be expected. Without diapause the pupal stage takes 9-15 days.
The developmental time becomes shorter with increasing temperature but on the other hand the mortality of eggs and puparia is usually lower at moderate temperatures (Sasakawa et al., 1970, Mizukoshi and Togawa, 1999). Thus moderate temperatures and humidity can be favorable for the populations of horticola.
HOST PLANTS
Chromatomyia horticola is highly polyphagous, Spencer, 1990 lists 35 host families (could be listed in a separate table?). Among these are several cultivated plants as reported by Spencer, 1973. It should be noted that it is important to consider the locality where a crop was recorded as host for Chromatomyia horticola and other polyphagous species. Scheffer, 2000 found, that Liriomyza huidobrensis can be split into several distinct lineages with sometimes different host preferences. Because they are only detectable by DNA sequences they are not yet mentioned as subspecies. Furthermore Singh and Bhati, 1996 showed that females preferably feed on those plant species on which they developed.
The following list contains only cultivated crops excluding ornamentals on which horticola often can be found:
Asteraceae (Compositae): Carthamus tinctorius (Safflower, Iran), Helianthus annuus (India), Lactuca sativa L. (India).
Brassicaceae (Cruciferae): Brassica campestris L. (Europe, Eritrea, India, China), Brassica juncea (India), Brassica napus L. (India), Brassica oleracea L. (India, Eritrea), Brassica rapa L. (Egypt), Brassica tournefortii (India).
Cucurbitaceae: Citrullus vulgaris L. (Europe), Cucumis sativus L. (Europe, Japan), Cucumis melo (muskmelon, India), Cucurbita pepo L. (Europe, Japan).
Lamiaceae (Labiatae): Mentha sp. (Europe, India, Japan).
Fabaceae (Leguminosae): Cicer arietinum L. (Europe), Cyamopsis tetragonoloba (Cluster bean, India), Lens esculenta Moench (India), Medicago sativa (Europe), Pisum arvense L. (India), Pisum sativum L. (Europe, Egypt, India, China), Trifolium alexandrinum L. (Egypt, India), Vicia faba L. (Europe, Libya, Japan), Vigna mungo (India), Vigna radiata (India), Vigna unguiculata ssp. sinensis (USSR: Rostov).
Liliaceae: Allium cepa L. (Europe, India, Japan).
Linaceae: Linum usitatissimum L. (flax) (Europe, China, India, Japan).
Papaveraceae: Papaver somniferum (opium poppy, India).
Solanaceae: Lycopersicon esculentum Mill. (Japan), Solanum melongena I. (India, Egypt), S. tuberosum L. (India).
Additionally horticola can damage various ornamental crops mainly within the plant family Asteraceae.
The host suffering most damage appears to be Pisum Spencer, 1973.
DISTRIBUTION
Holarctic, Oriental and Afrotropical distribution, the distribution was most probably widened by human introduction:
Records from Europe; North Africa: Egypt, Eritrea, Libya, Morocco; South Africa, Madagascar, West Africa: Cameroon; East Africa: Kenya; Turkey, India, China, Formosa, Japan (Spencer, 1973), Iran, Philippines, Malaysia, Korea, Nepal.
ECONOMIC IMPORTANCE
A single mine causes not much damage on the host plant but if mass outbreaks occur the plants are weakened significantly and the yield is reduced. Due to their wide distribution and host range Chromatomyia horticola belongs to the more important group of agromyzid pest species.
The most important host plant seems to be Pisum (pea). Recent literature suggests that horticola causes the most problems in India.