We sampled populations of leafminer flies (Liriomyza huidobrensis), in lettuce fields and associated trap catch counts with the sources of those populations. Sources of flies in a newly planted lettuce field are (1) adjacent crops if those crops are good hosts; (2) the previous host-crop in the field; and (3) remote host-crops which flies can be blown from or fly off of. Our data indicate that all three sources operate.|
Sources 1 and 2 can be decreased if growers manipulate crop juxtapositions in time and space. That is, a grower might consider leaving a field fallow for the 2 weeks it takes for soil pupae to emerge as adults before replanting and refrain from planting a new lettuce crop in a field adjacent to a host-crop field that is nearing harvest. Source flies can be found in lettuce, celery, cauliflower, broccoli, spinach, green onion, and mustard, with lettuce being the greatest producer of leafminers. We found no evidence that the flies overwinter as pupae in fallow fields, hence fields held fallow for the few weeks it takes for adult eclosion is sufficient to limit this source of flies in a newly planted field. However, since flies also come from remote fields (source 3), such manipulations alone will not be satisfactorily effective. Hence, understanding mortality is important.
Mortality studies in lettuce in 1995 and in lettuce and celery in 1996 showed that flies had almost no parasitism; only 4% parasitism occurred in the celery and 0% in lettuce. In 1995 mortality was high in the egg stage indicating that host factors were causative. Since high egg mortality was not seen in 1996, we surmise that the 1995 environmental conditions were critical. In 1996 we saw a high pupal mortality indicating that this might be a weak link in the insect's life cycle.
These studies indicate that manipulation of abiotic factors might profitably be tested as a way of increasing mortality. In particular we noted that many pupae became infected with soil-borne fungi when held in moist conditions. In addition, we might attribute the egg mortality to concomitant changes in plant turgor or temperature. Although it is culturally inappropriate to grow lettuce under nonoptimal conditions, we suggest tests with brief dry or wet conditions, especially early in the plant's phenology, as a means of changing turgor in plants without impacting plant growth. During the middle part of plant growth, having conditions wetter than normal may serve to increase entomopathogenic fungal activity against pupae. Such mortality may be experimentally increased by fungal treatments.