Agromyza parvicornis Loew Diptera Agromyzidae

Natural History

Distribution. Corn blotch leafminer is found throughout the United States and southern Canada. This species is native to North America.

Host Plants. This leafminer attacks only grasses. Its principal host is corn, but it also is sometimes found in other crops such as millet and wheat, and weedy grasses such as barnyardgrass, Echinochloa crusgalli; and crabgrass, Digitaria sanguinalis. (See color figure 8.)

Natural Enemies. Corn blotch leafminer is heavily attacked by hymenopterous parasitoids. These beneficial insects seem to keep the leafminer at low levels of abundance. Among the parasitoids known to affect corn blotch leafminer are Achrysocharella diastatae (Howard) and A. punctiventris (Crawford); Chryso-charis ainslei Crawford and C. parksi Crawford; Cirros-pilus flavoviridis Crawford; Closterocerus tricinctus (Ashmead) and C. utahensis Crawford; Diglyphus begini (Ashmead), D. pulchripes (Crawford), and D. websteri Crawford; Notanisomorpha ainsliei Crawford; Zagram-mosoma multilineatum (Ashmead) (all Hymenoptera: Eulophidae); Opius diastatae (Ashmead), O. succineus Gahan, and O. utahensis Gahan (all Hymenoptera: Braconidae). The relative importance of the members of this diverse assemblage of wasps is unknown.

Life Cycle and Description. The number of generations is not well documented because of overlap, but a generation can be completed under warm conditions in less than 30 days. In northern states flies are present from about April to October, which allows for 4-5 generations. In the south, though some pupae apparently diapause, flies can be active throughout the year. Development time is extended during the winter months in Florida, but these insects continue to develop and reproduce.

Egg. The female inserts egg between the lower and upper epidermis. Oviposition may occur from either the lower- or upper-leaf surface. The egg is inserted with the longer axis of the egg parallel to the leaf veins. The white egg is flattened and measures about 0.45-0.50 mm long and 0.17-0.19 mm wide. It is broadly rounded at each end, but constricted slightly at the middle. The eggs are readily visible as white spots in the leaf tissue. Duration of the egg stage is

3-5 days, depending on temperature.

Larva. The larvae live their entire life within leaf tissue, and apparently they are unable to move from leaf to leaf. There likely are three instars, which require

4-10 days for total larval development. The larva attains a length of about 3 mm and a width of 1 mm. It is greenish white early in its life and becomes yellowish-white later. The body is cylindrical, but the

Corn blotch leafminer larva.

Corn blotch leafminer larva.

head region tapers to a point. The mouth hooks are darkly pigmented. At maturity, the larva drops to the ground to pupate.

  1. The larva normally burrows into the soil to pupate, sometimes to a depth of up to 5 cm, but usually about one cm. The puparium is reddish brown. It measures about 3 mm long and 1 mm wide. Unlike the larval stage, in the puparium its body segments are well-differentiated. Also, its anterior spiracles are prominent and protruding, and the posterior pair is less distinct. Duration of the pupal stage is normally 14-21 days, but this is also the overwintering stage.
  2. This fly measures 3-4 mm long. It is predominantly black, though the face and legs are sometimes yellowish-brown. The wings are transparent. Females may live for 14 days or more, and produce eggs during all but the first few days. Adults are most active in the afternoon. Their reproductive capacity is not known.

The biology of corn blotch leafminer was described by Phillips (1914). It was included in the key by Spencer and Steyskal (1986).

Damage

The larvae feed between the lower and upper epidermis of the leaf. The epidermal tissue dies soon after the mesophyll are removed, leaving a thin, brown, and brittle area. Initially, larvae may feed in a straight line, and their path is directed by the parallel veins of the leaf. Later in life, however, larvae feed in an irregular pattern, leaving an large area of dead tissue that may involve the entire leaf of small grains and grasses. In corn, such injury is nominal unless numerous larvae are present.

The female also causes a small amount of injury through the production of feeding punctures. Females use their ovipositor to puncture the leaf, and then they feed upon sap that collects at the wound. Punctures

Diglyphus Websteri

may be quite numerous, but injury is insignificant in all but the smallest plants.

Management

Because these flies rarely become abundant, and injury occurs on foliage rather than the ear (fruit), suppression should not be required. However, application of insecticides to foliage will eliminate these flies.

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