Geospatial analysis of seed removal rates and arthropod diversity and activity density in adjacent annual cropping studies.
Shuler, Rachel*,1, DiTommaso, Toni1, Losey, John2, 1 Crop and Soil Sciences, Cornell University, Ithaca, NY2 Department of Entomology, Cornell University, Ithaca, NY
ABSTRACT- Weed seed predation and the activity densities of ground dwelling arthropods were investigated in adjacent annual cropping studies. Study 1, planted in continuous corn, examined conventional strategies to control corn rootworm (Diabrotica spp ): the transgenic Cry3Bb Bt corn and the broad-spectrum insecticide tefluthrin. Study 2, transitioning from conventionally managed continuous corn to a rotation of organic corn, soybeans, and spelt/clover, compared four distinct weed and fertility management designs. Pitfalls were used to assess arthropod activity density in each study at the same time as seed cards were used to measure removal rates of three annual weeds: velvetleaf (Abutilon theophrasti Medic.), foxtail (Setaria faberi Herm.), and lambsquarters (Chenopodium album L.). Despite major differences in management of these studies, arthropod activity density, Shannon’s H diversity index, and species richness were not significantly different in any month. However, populations of beneficial seed-eating carabid beetles were significantly higher in the organic transition system. Seed removal rates, however, were not correlated with arthropod populations. Substantial seed predation (>25%) was twice as likely to occur in the conventional corn plots than in the transitional organic system. This may be due to lower system-wide seed availability in the conventional corn, making the seeds offered as part of the study a more important food source. Vertebrate seed predation was also significant only in the conventional corn study. Significant spatial autocorrelation of certain key seed predators (e.g. Poecilus lucublandus Say; Harpalus caliginosus Fabricius) was observed intermittently in both studies, likely due to clustering in wetter field microsites during dry months or to edge effects. The complexity of managing arthropods to enhance this ecosystem service is illustrated by the inversely correlated activity density of important carabid seed predators with seed removal rates. Finally, the value of testing spatial autocorrelation cannot be underemphasized given that clustered insect populations or those asymmetrically colonizing a field can be missed easily and skew traditional statistical analysis.
Key words: seed predation, insects: carabidae, annual cropping systems
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