Assessing the Impact of Beneficial Insect Populations on Organic Farms (AS94-013)
Although organic farmers rely heavily on beneficial insects to control pests, little is known about the impact of naturally-occurring beneficial species or the need for supplemental releases of purchased parasitoids and predators to control pests. This research project was undertaken to provide this information and had as its objectives:
1.) Identify species of natural enemies present in organically grown tomatoes.
2.) Characterize the seasonal patterns of abundance for important natural enemy species.
3.) Identify the important prey or host species for these natural enemies.
4.) Document the impact of naturally occurring biological control on populations of key pest species.
5.) Measure the impact of releases of commercially purchased lacewings and Trichogramma.
Replicated research plots were established on 4 commercial organic farms located in the Piedmont and Coastal Plain of North Carolina. Each of the farmers followed their normal practices throughout the season. Plots were sampled at least weekly at each site. Spring plantings were sampled at all locations in both 1995 and 1996, whereas fall plantings were sampled at 2 sites in both years.
Plants were visually inspected for the presence of pest and beneficial arthropods. Blossoms were collected to sample flower thrips. Lepidopterous eggs and larvae were counted, collected, and held to determine parasitization and for species identification. Aphids were counted and collected to monitor levels of parasitism and to identify parasitoid species. In addition, the impact of natural enemies on pest populations was investigated in greater detail using a natural enemy exclusion procedure for aphids and by placing clusters of fruitworm eggs in each plot to monitor parasitization and predation on fruitworm eggs. The impact of releases of commercially purchased Trichogramma was addressed by comparing levels of fruitworm egg parasitism at the release sites prior to and following the releases and by carrying out quality control tests on Trichogramma purchased from 12 companies.
During both years, biological control by naturally-occurring insect parasitoids and predators was very important in the suppression of insect pest populations in organic tomato production. In particular, egg parasitism of both hornworm eggs by Trichogramma spp. and Telenomus sphingis, and of fruitworm eggs by Trichogramma spp. was generally high and at times approached 100 percent. These high levels of egg parasitism, in combination with egg predation, primarily by green lacewing larvae and lady beetles, appeared to play an important role in making organic tomato production an economically viable enterprise. Larval parasitism was less significant, because it was lower and highly variable, and because it did not prevent the larvae from damaging the plants and fruit.
Commercially purchased Trichogramma were of poor quality. Releases of commercially purchased Trichogramma had mixed results. In 3 of the 6 releases made over both years, no observable change was seen in the incidence of egg parasitism.
To the extent that the Trichogramma purchased for this study are representative, and high levels of egg parasitism are typical on organic farms, the benefits of releasing purchased Trichogramma for control of tomato fruitworm are unpredictable. Our findings clearly identify a need for improved quality control measures by the companies selling Trichogramma. To effectively use Trichogramma releases for insect control, farmers will require a source of high quality Trichogramma and a knowledge of how the parasitism levels change over the season.
Naturally occurring biological control of aphid populations was also important in restraining aphid populations. It reduced the rate of population growth rather than causing a decline in population size. Its importance varied locally within fields. Both plant condition and rainfall seemed to be major factors in the decline of aphid populations. These results suggest that one or more well-timed releases of aphid predators (e.g. green lacewings) could prove valuable in preventing the development of damaging aphid populations in organic tomato plantings.
Impact of Results
The findings presented in this report will equip organic farmers with information required to determine the value of releasing purchased parasitoids and predators to control tomato fruitworm, hornworms, and aphids. The results will also provide the first thorough documentation of the impact of naturally-occurring biological control in organic tomato production in the Southern Region. As such, it will help to identify the level of biological control that can be obtained when conventional production practices are modified to accommodate the important biological control agents.