The effects of strawberry cropping practices on the strawberry tortricid (Lepidoptera: Tortricidae), its natural enemies, and the presence of nematodes

Abstract

Cropping practice can affect pests and natural enemies. A three-year study of the strawberry tortricid, Acleris comariana (Lienig and Zeller) (Lepidoptera: Tortricidae), its parasitoid Copidosoma aretas Walker (Hymenoptera: Encyrtidae), and its entomopathogenic fungi was conducted in seven pairs of organic and conventional farms to test the hypothesis that farming practice (organic versus conventional) will affect the level of pest infestation and will affect the natural enemies. In addition, the number of years with strawberries on the farm, field age, and other factors that may affect pests and their natural enemies were considered. Farms were characterized by their cropping practices, cropping history, and other parameters. Field-collected larvae were laboratory reared to assess mortality from parasitoids and entomopathogenic fungi. In 2010, a survey of nematodes was made to assess the response of an unrelated taxonomic group to cropping practice. 2,743 larvae were collected. Of those, 2,584 were identified as A. comariana. 579 A. comariana were parasitized by C. aretas and 64 A. comariana were parasitized by other parasitoid species. Finally, 28% of the larvae and pupae of A. comariana died from unknown causes. Only two of the field-collected A. comariana larvae were infected by entomopathogenic fungi; one was infected by Isaria sp. and the other by Beauvaria sp. The density of A. comariana was on average four times lower in organic farms, which was significantly lower than in conventional farms. A. comariana was more dominant on conventional farms than on organic farms. The effect of crop age (One, two, or three years) on A. comariana infestation was significant, with higher infestations in older fields. Crop age had no effect on A. comariana infestation in a comparison of first- and second-year fields in 2010. Cropping practice did not lead to significant differences in the level of total parasitism or in C. aretas parasitism; however, C. aretas contributed to a higher proportion of the parasitized larvae on conventional farms than on organic farms. Mortality from unknown causes of A. comariana was higher in organic farms than conventional farms, and unknown mortality was two to seven times higher in second-generation A. comariana than in first generation. Entomopathogenic nematodes were found on three organic farms and one conventional farm. Plant parasitic nematodes were found in more samples from conventional farms than from organic farms. The low density of A. comariana in organic farms exposes the specialist C. aretas to a higher risk of local extinction. In organic farms, where the density of A. comariana is low, other parasitoids may play an important role in controlling A. comariana by supplementing C. aretas. Other tortricid species may serve as alternative hosts for these other parasitoids, contributing to conserving them in the habitat. The higher unknown mortality of larvae from organic fields may be the result of non-consumptive parasitoid or predator effects. This study reports an example of the effects of cropping practice on an insect pest, with similar effects on nematodes. An understanding of the responsible factors could be used to develop more sustainable cropping systems.

Keywords: Acleris comariana; Copidosoma aretas; organic farming; parasitoid.

Figure 1.

Field densities of Acleris comariana in organic and conventional strawberry fields from 2009-11. Columns to the left are the conventional fields, columns to the right the organic fields, (a) spring 2009, (b) summer 2009, (c) spring 2010, (d) summer 2010, (e) spring 201 1. High quality figures are available online.

Figure 2.

Mortality factors of A. comariana for organic fields number 1 to 7 and conventional fields number 1 to 7 . The height of stacked columns shows the total number of larvae collected from each field. From below they are: the number emerging as adult A. comariana (50% grey), number dead (hatched lower left to upper right), parasitized by C. aretas (hatched, bold line, upper left to lower right), Ichneumonidae (80% black), Braconidae (chequered), Tachinidae (zigzag lines). (a) 2009 first generation, which was sampled only once; all subsequent generations were sampled twice; (b) 2009, second generation; (c) 2010 first generation; (d) 2010 second generation; (e) 2011, first generation. High quality figures are available online.

Figure 3.

Production potential of strawberry in seven conventional and seven organic farms in 201 1, represented by averaged total number of flowers, fruit and empty stems, i.e. stems per 1 m row, counted in 8-10 randomly selected 1 m rows, equivalent to 3-3.5 plants in late June. This measure replaced a yield estimate because frost damage affected fruit formation of several fields. High quality figures are available online.