Identification of Top-Down Forces Regulating Cotton Aphid Population Growth in Transgenic Bt Cotton in Central China

Abstract

The cotton aphid Aphis gossypii Glover is the main aphid pest in cotton fields in the Yangtze River Valley Cotton-planting Zone (YRZ) in central China. Various natural enemies may attack the cotton aphid in Bt cotton fields but no studies have identified potential specific top-down forces that could help manage this pest in the YRZ in China. In order to identify possibilities for managing the cotton aphid, we monitored cotton aphid population dynamics and identified the effect of natural enemies on cotton aphid population growth using various exclusion cages in transgenic Cry1Ac (Bt)+CpTI (Cowpea trypsin inhibitor) cotton field in 2011. The aphid population growth in the open field (control) was significantly lower than those protected or restricted from exposure to natural enemies in the various exclusion cage types tested. The ladybird predator Propylaea japonica Thunberg represented 65% of Coccinellidae predators, and other predators consisted mainly of syrphids (2.1%) and spiders (1.5%). The aphid parasitoids Aphidiines represented 76.7% of the total count of the natural enemy guild (mainly Lysiphlebia japonica Ashmead and Binodoxys indicus Subba Rao & Sharma). Our results showed that P. japonica can effectively delay the establishment and subsequent population growth of aphids during the cotton growing season. Aphidiines could also reduce aphid density although their impact may be shadowed by the presence of coccinellids in the open field (likely both owing to resource competition and intraguild predation). The implications of these results are discussed in a framework of the compatibility of transgenic crops and top-down forces exerted by natural enemy guild.

Transgenic Bt Cotton Does Not Disrupt the Top-Down Forces Regulating the Cotton Aphid in Central China

“…The foliar fertilization of urea (two times of spray at concentrations of 0.5% and 1% on 20 th May and 14 th June, respectively) and monopotassium phosphate (active gradient with 750 g/ ha) was uniformly applied to both cultivars (Hubei Yihua Chemicals Ltd., China). Other routine practices were performed except for pesticides application [10]. Weed control relied on hand hoeing at four different dates throughout the experiment.…”

“…The cage restriction technique was designed following the methods described in our previous study [10]. All the cages were made of bamboo sticks (2m x 2m x 2m, length x width x height) covered by nylon mesh netting with openings of 530×530μm.…”

“…Top-down force is referred to arthropod pest management in the context of biological control in agro-ecosystems [10]. Within “plant-herbivores insect-natural enemy arthropod” tri-trophic interactions, however, top-down forces are influenced by various biotic factors such as plant features in terms of food/shelter and resistant traits [11,12], or abiotic factors such as fertilization regimes [13–15].…”

“…We have documented strong top-down forces in regulating A . gossypii in Bt cotton field [10]; such top-down forces have been primarily offered by Coccinellids and Aphidiine parasitoids. It still remains elusive whether the top-down forces have been influenced by the insect-resistant traits introduced into Bt cotton cultivars.…”

“…As a departure of our previous study [10], we further explored how Bt cotton affect the strength of top-down forces in regulating A . gossypii populations in the field.…”