American Serpentine Leafminer, Liriomyza trifolii (Burgess)

This leafminer has long been found in eastern North America, northern South America, and the Caribbean. However, in recent years it has been introduced into California, Europe, and elsewhere. Expanded traffic in flower crops appears to be the basis for the expanding range of this species. Liriomyza trifolii (Burgess), sometimes known as the American serpentine leafminer, readily infests greenhouses. As a vegetable pest, however, its occurrence is limited principally to tropical and subtropical regions.

Life Cycle and Description

Leafminers have a relatively short life cycle. The time required for a complete life cycle in warm environments is often 21–28 days, so numerous generations can occur annually in tropical climates. Growth at a constant 25°C requires about

19 days from egg deposition to emergence of the adult. Development rates increase with temperature up to about 30°C; temperatures above 30°C are usually unfavorable and larvae experience high mortality. At 25°C, the egg stage requires 2.7 days for development; the three active larval instars require an average of 1.4, 1.4, and 1.8 days, respectively; and the time spent in the puparium is 9.3 days. Also, there is an adult preovipostion period that averages 1.3 days. The temperature threshold for development of the various stages is 6—10°C, except that egg laying requires about 12 C.

Egg

Eggs tend to be deposited in the middle of the plant; the adult seems to avoid immature leaves. The female deposits the eggs on the lower surface of the leaf, but they are inserted just below the epidermis. Eggs are oval in shape and small in size, measuring about 1.0 mm long and 0.2 mm wide. Initially they are clear, but soon become creamy white in color.

Larva

Body and mouth part size can be used to differentiate instars; the latter is particularly useful. For the first instar, the mean and range of body and mouth parts (cephalopharyngeal skeleton) lengths are

0.39 (0.33–0.53) mm and 0.10 (0.08–0.11) mm, respectively. For the second instar, the body and mouth parts measurements are 1.00 (0.55–1.21) mm and 0.17 (0.15–0.18) mm, respectively. For the third instar, the body and mouth parts measurements are 1.99 (1.26–2.62) mm and 0.25 (0.22—

0.31) mm, respectively. A fourth instar occurs between puparium formation and pupation, but this is a nonfeeding stage and is usually ignored by authors. The puparium is initially golden brown in color, but turns darker brown with time.

Adult

Adults (Fig. 42) are small, measuring less than 2 mm in length, with a wing length of 1.25–1.9 mm. The head is yellow with red eyes. The thorax and abdomen are mostly gray and black although the ventral surface and legs are yellow. The wings are transparent. Key characters that serve to differentiate this species from the vegetable leafminer, Liriomyza sativae Blanchard, are the matte, grayish black mesonotum and the yellow hind margins of the eyes. In vegetable leafminer the mesonotum is shining black and the hind margin of the eyes is black. The small size of this species serves to distinguish it from pea leafminer, Liriomyza huidobrensis (Blanchard), which has a wing length of

1.7–2.25 mm. Also, the yellow femora of American serpentine leafminer help to separate it from pea leafminer, which has darker femora. Oviposition occurs at a rate of 35–39 eggs per day, for a total fecundity of 200–400 eggs. The female makes numerous punctures of the leaf mesophyll with her ovipositor, and uses these punctures for feeding and egg laying. The proportion of punctures receiving an egg is about 25% in chrysanthemum and celery, both favored hosts, but only about 10% in tomato, which is less suitable for larval survival and adult longevity. Although the female apparently feeds on the exuding sap at all wounds, she spends less time feeding on unfavorable hosts. The males live only two to three days, possibly because they cannot puncture foliage and therefore feed less than females, whereas females usually survive for about a week. Typically they feed and oviposit during much of the daylight hours, but especially near mid-day.

Host Plants

Liriomyza trifolii is perhaps best known as a pest of chrysanthemums and celery, but it has a wide host range. For example, at least 55 hosts are known from Florida, including bean, beet, carrot, celery, cucumber, eggplant, lettuce, melon, onion, pea, pepper, potato, squash, and tomato. Flower crops that are readily infested and which are known to facilitate spread of this pest include chrysanthemum, gerbera, gypsophila, and marigold, but there are likely many other hosts, especially among the Compositae. Numerous broad-leaved weed species support larval growth. The nightshade Solanum americanum, Spanish needles, Bidens alba, and pilewort, Erechtites hieracifolia, were suitable weed hosts in Florida.

Damage

Punctures caused by females during the feeding and oviposition processes can result in a stippled appearance on foliage, especially at the leaf tip and along the leaf margins. However, the major form of damage is the mining of leaves by larvae, which results in destruction of leaf mesophyll. The mine becomes noticeable about three to four days aſter oviposition, and becomes larger in size as the larva matures. The pattern of mining is irregular. Both leaf mining and stippling can greatly depress the level of photosynthesis in the plant. Extensive mining also causes premature leaf drop, which can result in lack of shading and sun scalding of fruit. Wounding of the foliage also allows entry of bacterial and fungal diseases. Although leaf mining can reduce plant growth, crops such as tomato are quite resilient, and capable of withstanding considerable leaf damage. It is oſten necessary to have an average of one to three mines per tomato leaf before yield reductions occur. Leafminers are most damaging when they affect floricultural crops due to the low tolerance of such crops for any insect damage.

Natural Enemies

Parasitic wasps (parasitoids) of the families Braconidae, Eulophidae, and Pteromalidae are important in natural control, and in the absence of insecticides usually keep this insect at low levels of abundance. At least 14 parasitoid species are known from Florida alone. Species of Eulophidae such as Diglyphus begina (Ashmead), D. intermedius (Girault), D. pulchripes, and Chrysocharis parksi Crawford are generally found to be most important in studies conducted in North America, although their relative importance varies geographically and temporally. Predators and diseases are not considered to be important, relative to parasitoids. However, both larvae and adults are susceptible to predation by a wide variety of general predators, particularly ants.

Management

Sampling

There are many methods to assess leafminer abundance. Counting mines in leaves is a good index of past activity, but many mines may be vacant. Counting live larvae in mines is time consuming, but more indicative of future damage. Puparia can be collected by placing trays beneath foliage to capture larvae as they evacuate mines, and the captures are highly correlated with the number of active miners. Adults can be captured by using adhesive applied to yellow cards or stakes.

Insecticides

Chemical insecticides are commonly used to protect foliage from injury, but insecticide resistance is a major problem. Insecticide susceptibility varies widely among populations, and level of susceptibility is directly related to frequency of insecticide application. In Florida, longevity of insecticide ef fectiveness is oſten only two to four years, and then is usually followed by severe resistance among the treated populations. Rotation among classes of insecticides is recommended to delay development of resistance. Reduction in dose level and frequency of insecticide application, as well as preservation of susceptible populations through nontreatment of some areas, are suggested as means to preserve insecticide susceptibility among leafminer populations. Insect growth regulators have been more stable, but are not immune from the resistance problem. Insecticides also are highly disruptive to naturally occurring biological control agents, particularly parasitoids. Use of many chemical insecticides exacerbates leafminer problems by killing parasitoids of leafminers. This usually results when insecticides are applied for lepidopterous insects, and use of more selective pest control materials such as Bacillus thuringiensis is recommended as it allows survival of the leafminer parasitoids. Because parasitoids oſten provide effective suppression of leafminers in the field when disruptive insecticides are not used, there has been interest in release of parasitoids into crops. This occurs principally in greenhouse-grown crops, but is also applicable to field conditions. Steinernema nematodes have also been evaluated for suppression of leaf mining activity. High levels of relative humidity (at least 92%) are needed to attain even moderately high (greater than 65%) levels of parasitism. Adjuvants that enhance nematode survival increase levels of leafminer mortality, but thus far nematodes are not considered to be a practical solution to leafminer infestations.

Cultural Practices

Because broadleaf weeds and senescent crops may serve as sources of inoculum, destruction of weeds and deep plowing of crop residues are recommended. Adults experience difficulty in emerging if they are buried deeply in soil.

Figure 42 Adult of American serpentine leafminer, Liriomyza trifolii.