Integration of Natural Enemies for Management of the Sweet Potato Whitefly and Associated Disorders on Mixed Cropped Vegetables

1992 Annual Report for AS92-003

Project Type: Research and Education
Funds awarded in 1992: $0.00
Projected End Date: 12/31/1994
Matching Non-Federal Funds: $77,789.00
ACE Funds: $170,000.00
Region: Southern
State: Florida
Principal Investigator:
David J. Schuster
University of Florida, IFAS

Integration of Natural Enemies for Management of the Sweet Potato Whitefly and Associated Disorders on Mixed Cropped Vegetables


1) Evaluate intercropping marketable crops for the manipulation of sweetpotato whitefly (SPWF) populations and the viruses they vector in Florida and Texas.

2) Estimate the profitability of the methods employed to manage the SPWF on vegetables.

3) Disseminate information to growers and extension personnel throughout the Southern Region.

One site in Southwest Florida and 19 sites in three geographic areas in Texas were sampled periodically for the presence of the SPWF and its natural enemies, principally small wasp parasites. The sites were primarily organic mixed-crop vegetable operations although some of the sites in Texas were nursery operations and ornamental plantings that were not sprayed with conventional insecticides.

Although damaging levels of some pests such as spider mites, pepper weevils, potato aphids, and certain plant diseases caused by fungi were observed at the Florida vegetable organic farm, populations of the SPWF did not increase to damaging levels. Incidence of tomato mottle geminivirus (TMoV), a plant virus vectored by the SPWF, was low in the first year of study but reached nearly 100% of the tomato plants infected in the second year. This occurred because the grower located new plantings adjacent to older, infected plantings. Whiteflies appeared to migrate into the crops in the fall and reach their highest levels early in the season. However, increasing activity of parasitic wasps such as Encarsia pergandiella corresponded to decreasing SPWF populations.

A mathematical model to describe the development and movement of SPWF populations in a small mixed-cropped vegetable farm is under development and will be used as a basis for developing a similar model describing SPWF populations in a mixed-crop region as south Texas or southern California. These models will help in our understanding of potential and real effects of cropping arrangements and sequences on SPWF population development and movement and will be helpful in developing regional and local SPWF management strategies.

In Texas, a situation similar to that in Florida occurred. The SPWF proved nearly inconsequential to organic farms that were located away from the conventional farming systems in south Texas. In fact, the SPWF could not be found on some organic farms; in most cases SPWF populations were so small that host plants required intensive sampling to find specimens. Organic farms located in the conventional farming areas were found to suffer damage from the SPWF which migrated from adjacent crops.

Refuges of sunflower and kale were initiated as companion plantings of watermelon with the cooperating organic farm in the Rio Grande Valley. Adult SPWF migrating from cotton overwhelmed the refugees and melons. Delaying midseason and fall plantings until after flights of SPWF from early melons and cotton have occurred is becoming more widely accepted. Alternative planting times and refuge plantings to conserve the SPWF and its natural enemies are now recognized as a possible means to overcome this recurrent problem.

Even though natural enemies can control the SPWF, the realization of their potential in heavily sprayed conventional farms is difficult. Efforts have been undertaken to enhance the levels of biological control either directly or by reducing initial SPWF populations. Squash and other cucurbits were evaluated as companion and trap crops for tomato for reducing SPWF populations and TMoV incidence. In laboratory and greenhouse experiments, squash was at least twice as attractive than tomato to SPWF adults. In a replicated small plot experiment, tomato was planted either alone or adjacent to squash or mixed cucurbit crops. Although the numbers of SPWF adults and nymphs were not consistently affected on the tomato in companion plantings compared to the tomato planted alone, the incidence of TMoV was less in companion-planted tomato. In one large plot experiment in conventional tomato fields, tomato growing adjacent to six row plantings of squash had fewer SPWF adults and less incidence of TMoV than tomato growing adjacent to tomato; however, in another similar experiment, the reverse was true. Nevertheless, one of the cooperating growers will include a 5 acre planting of squash in the spring crop to further evaluate squash as a trap for migrating SPWF adults.

In replicated small plot experiments, tomato plants growing on UV-reflective plastic soil mulch had fewer SPWF adults and less incidence of TMoV than tomato plants growing on white plastic. Similar results with SPWF adults were obtained in a demonstration on three conventional tomato farms. Incidence of TMoV was low so no difference in yield between plants growing on UV-reflective mulch and plants growing on white mulch was observed.

The insect- pathogenic fungus, Paecilomyces fumosoroseus, was applied to individual plants in a flowering tomato crop. The fungus was recovered up to seven weeks later and peaked three weeks after application when 32% of the whitefly nymphs were infected. A similar application on squash was less effective, presumably because of lower humidity within the plant canopy.

A combination of whey, yeast and sucrose was applied weekly to tomato plants in the field to attract predator insects. The sprays mimicked the sugary honeydew secreted by feeding whiteflies and resulted in increased oviposition by lacewings compared to nonsprayed tomato plants. Similar applications on squash were ineffective, probably because of the large amount of honeydew already deposited by large numbers of SPWF present.

Detailed economic analyses to estimate the profitability of the investigated methods for SPWF management have not been completed; however, a preliminary first year comparison of the organic farm site in Florida with a hypothetical conventional farm was completed. Only items that highlight the cost differences between the farming operations were included.

The results suggest that the organic grower invested more than twice as much for producing and applying compost for fertilization but invested $400 less per acre for pest control. Pest control expenses for the organic grower increased in the second year because of increased pest problems and a resulting greater reliance on organically approved pesticides.

An on-farm field day was held at the organic farm site in Florida. The field day was attended by 35 people consisting of conventional farmers, organic farmers, chemical industry representatives, people interested in organic farming, and University of Florida extension and research personnel.

The cooperating organic farmer shared his philosophy and methodology for growing vegetables in a question and answer format. A broad overview of the SARE project was summarized in formal presentations, handouts, and a poster followed by a tour of the farm.

Summaries of the findings and recommendations from these studies have been presented at numerous local and statewide conferences, seminars, and organic and conventional grower meetings. Training in sampling methods and identification of the parasites associated with the SPWF in south Texas has been conducted. Certain of the results and recommendations of the project have been published in proceedings of statewide grower meetings and in the Proceedings of the Second Conference of Environmentally Sound Agriculture.

December 1994.