Research Projects

Lady beetle larva

Robber fly

Lacewing larva

 
parasitized aphid

NATURAL ENEMIES IN MICHIGAN BLUEBERRIES

As part of a 4-year, USDA-RAMP funded project comparing pest management programs that employ either conventional (GSTD) or reduced-risk insecticides (RAMP), we are measuring abundance of natural enemies, in the highbush blueberry agroecosystem. Reduction in the use of broad-spectrum insecticides is expected to increase natural enemy abundance in RAMP plots, while the potential for secondary pest outbreaks may be greater than in GSTD plots. Beginning in 2003, we have monitored natural enemies of blueberry pests with pitfall traps, yellow sticky traps and during weekly scouting; additionally, we monitored populations of blueberry aphid parasitoids and the host, blueberry aphid (Illinoia pepperi McGillivray) in both management programs. Considered a minor pest, blueberry aphids may respond like a secondary pest to changes in insecticide programs. We have also been monitoring changes in the community and abundance of ants in this system.

Pitfall Traps

In all three years of this study, Harpalus pensylvanicus has been the most common species collected, representing 70.5% of all carabids in 2003, 49% in 2004 and 36% in 2005. The cause of the decrease in relative abundance of this species is not known, but it is not likely due to changes in insecticide use as this species has decreased in abundance in both RAMP and GSTD fields. In 2003 one carabid species, Harpalus erraticus, did respond to the different insecticide programs. Compared to the GSTD program, there was an eight-fold increase in the number of H. erraticus collected throughout the season in the RAMP fields, with the greatest difference occurring in September. This species was also significantly more abundant in RAMP fields in 2004, and an additional species, Amara aenea, was recorded at higher density in RAMP fields. In 2005 however, A. aenea was the only common carabid that was more abundant in RAMP fields and there was no statistical difference in H. erraticus abundance between programs. In 2005 three out of six growers applied the pyrethroid, esfenvalerate, to RAMP fields, and we attribute the diminishing response of H. erraticus to this the use of this compound.

Table 1. Most common ground beetles in highbush blueberry farms of southwest Michigan during 2005 in fields exposed to broad-spectrum (GSTD) or reduced-risk (RAMP) insecticide programs.
      

Natural Enemy Survey

Natural enemies (adult Coccinellids, Chrysopids, Syrphids, parasitoid wasps, social/solitary wasps (Vespids/Sphecids) and spiders) were monitored each year with yellow sticky traps and with weekly scouting. In 2003 natural enemy abundances were similar between programs, and there was little evidence of an improved level of biocontrol in the RAMP fields. Beginning in the second year (2004) a response to the insecticide programs was observed, and the abundance of several groups of natural enemies (blueberry aphid parasitoids, ants, coccinellids, and spiders) was higher in RAMP fields. In 2005 few natural enemy groups were found in higher numbers in the RAMP fields, but there were more ants, spiders and wasps in those fields. Again we suspect the use of a pyrethroid in some RAMP fields may have adversely affected some natural enemies.

Aphid Natural Enemies

In each of the first three years, aphid densities were sampled approximately every two weeks at each farm by surveying 4 randomly selected bushes in each field, and the number of aphids and aphid mummies was recorded. All mummies were removed and kept in individual plastic cups until the parasitoid adult emerged. In 2003, we did not observe a significant difference between the mean number of aphids, aphid mummies or percentage of parasitized aphids between programs. We did observe a blueberry aphid with a parasitic mite attached (likely from the family Trombidiidae, genus Allothrombium). To our knowledge this is the first report of mites attacking this aphid species. During sampling in 2004 we did observed significantly higher aphid parasitism in the RAMP program at the sample taken on 8 August (Figure 1). In 2005 there were no significant differences between programs. The overall rate of parasitism was high in both RAMP and GSTD fields in 2005 and this quantity appears to have increased during this project, and we suggest that aphid control has improved in both programs and this is leading to higher percent parasitism.

            

Figure 1. Aphid parasitism in RAMP and GSTD fields during the first three years of this project.

Summary

In all three years, increases in some of the natural enemies were detected in the RAMP fields compared to the grower’s standard. Aphid parasitoids, some species of ground beetles, lacewings, and ants all were found to increase to some degree. We were not able to demonstrate any translation of this into greater pest control, except for late in the season when higher levels of aphid parasitism were found in RAMP fields. After one year, we have preliminary evidence that use of a pyrethroid in the RAMP field stopped the increasing trend for natural enemy enhancement. We will continue to monitor changes in natural enemy density for one more year. We also will report results of exclusion cage experiments that are being used to determine to what extent ground foraging predators contribute to control of key blueberry pests.