Aphids And Natural Enemies Relationships In Wheat And Cotton Eco-system In Northern China

China is the biggest wheat and cotton producing country in the World with121million metric tons (18.47%of the World) and35metric tons to480-1b bales (28.87%of the World) of production, respectively. Wheat is second primary crop in China where winter wheat accounted for about94%of total China’s output. Cotton is one of the most important cash and fiber crops in China. Both these crops cultivated on wide area in northern China. Aphids are major pests in wheat-cotton rotations in northern China and current management methods of this pest rely primarily on broad-spectrum insecticides, though importance of biocontrol service provided by natural enemies in cotton has been documented lately. It is crucial to identify key natural enemies of aphids and to assess potential side effects of insecticides on these natural enemies in the two crops for optimizing integrated pest management (IPM) in northern China. For this reason, we carried out six studies:(1) Characterization of aphid and natural enemies population dynamics in wheat fields,(2) Evaluation of biocontrol service provided by natural enemies in wheat fields,(3) Characterization of aphid and natural enemies population dynamics in cotton fields,(4) Evaluation of biocontrol service provided by natural enemies in cotton fields,(5) Assessment of side effects of insecticides on population dynamics of aphids and their natural enemies in wheat and cotton fields, and (6) assessment of side effects of insecticides on natural enemies. All field experiments (field survey methods and exclusion cages experiments) were conducted during2011,2012and2013using repetitive fixed sampling sites at Langfang Experimental Station of Institute of Plant Protection, Chinese Academy of Agricultural Sciences (IPP-CAAS), Beijing, China. All laboratory experiments were conducted during2012-2013using Coccinella septempunctata as biological model of natural enemy. The predator was fed on insecticide-treated aphids infesting wheat seedlings at Institute of Plant and Environment Protection, Beijing Academy of Agricultural and Forestry Sciences, Beijing, China.1-Characterization of aphid and natural enemies population dynamics in wheat fieldsSitobion avenae population dynamics and its natural enemies were studied using open field survey in wheat fields. S. avenae colonized wheat in November to December but very low winter temperatures prevented aphids to pullulate. During the spring, aphid populations increased slowly and peaked at～2300(May23rd2011),～5000(May17th2012) and～5400(May26th2013) aphids per100wheat tillers. We observed a drastic decline in aphid populations later in2012and2013while it peaked again in the1st week of June in2011just before harvesting. The main natural enemies were aphid parasitoids along with few predators, mainly coccinellid species(Harmonia axyridis and Coccinella septempunctata). Parasitoid mummies and coccinellids population peaked in end of May to start of June each season.2-Evaluation of biocontrol service provided by natural enemies in wheat fieldsUsing natural enemy exclusion cages in wheat fields in2011, we observed that S. avenae populations in exclusion cages-SMC (exclusion of all natural enemies) peaked1.84-fold higher than aphid populations in restriction cages-LMC (exclusion of large predators e.g. coccinellids) and 41.10-fold higher than in uncaged plants (no exclusion of natural enemies). In2012,, we observed that S. avenae populations in exclusion cages peaked1.42-fold higher than aphid populations in restriction cages and2.52-fold higher than in uncaged plants. In2013, results were similar to last year but of different magnitude. Our results indicated that natural enemies, notably parasitoids along with multiple predatory coccinellids (H. axyridis and C. septempunctata), did limit aphid population growth.3-Characterization of aphid and natural enemies population dynamics in cotton fieldsAphis gossypii population dynamics and its natural enemies were studied using open field survey in cotton fields. During2011and2013cotton aphid populations were higher at the beginning but in2011season, it declined suddenly whereas in later season, it started to increase slowly. Each season, A. gossypii populations after increasing slowly, peaked at～13800(June24th2011),～319300(July9th2012) and～16000(June28th2013) aphids per100cotton plants. We observed a drastic decline in aphid populations later in each season while it peaked again in the mid-August in2013just before boll openings. There was seasonal variation in the dominance of natural enemies. The main natural enemies were coccinellids (Propylea japonica, Harmonia axyridis, Coccinella septempunctata and Adonia variegata) in2011, aphid parasitoids(Trioxys spp., Lysiphlebus spp. and A. gifuensis) in2012and spiders (Erigonidium graminicolum, Misumenopos tricuspidata and Pardosa t-insignita) along with other natural enemies, mainly multiple anthocorids, chrysopids and parasitoids each season. Parasitoid populations (mummies recorded) peaked in early June (2011) and mid-late July during2012-2013. Among predators, coccinellid populations peaked late-July to early-August during2011-2012while in the2013season, peaked in mid-June. The spiders populations peaked in late-end of July (2012-2013) while early September in2011.4-Evaluation of biocontrol service provided by natural enemies in cotton fieldsBy selectively restricting or allowing predator access (using various exclusion cage types) we studied the response of the cotton aphid, Aphis gossypii to top-down influence. During the three seasons, the cotton aphid populations peaked in mid-late July in cotton fields. In2011, we observed that A. gossypii populations in exclusion cages-SMC (exclusion of all natural enemies) peaked1.54-fold higher than aphid populations in restriction cages-LMC (exclusion of large predators e.g. coccinellids) and2.16-fold higher than in uncaged plants (no exclusion of natural enemies). In2012results, we observed that A. gossypii populations in exclusion cages peaked2.30-fold higher than aphid populations in restriction cages and8.10-fold higher than in uncaged plants. In2013results, we observed that A. gossypii populations in exclusion cages peaked2.47-fold higher than aphid populations in restriction cages and4.80-fold higher than in uncaged plants. The results demonstrated that the main natural enemies of the cotton aphids during2012and2013were aphid parasitoids, mainly Trioxys spp. and A. gifuensis while anthocorids(Orius laevigatus, Geocoris punctipes, G. bullatus, G. pollens, G. uliginosus and G. floridanus) in2011. In addition, we also observed multiple predatory coccinellids, chrysopids, spiders and parasitoids in cotton fields each season. Overall, we observed strong evidence for top-down control of the aphid population growth in cotton crop.5-Assessment of side effects of insecticides on population dynamics of aphids and their natural enemies in wheat and cotton fieldsBy open field survey method, population dynamics of S. avenae, A. gossypii and their natural enemies were assessed three and four times in wheat and cotton fields, respectively after imidacloprid (highest field rate) applied using knap-sack sprayer at langfang experimental station in northern China. The results demonstrated that mean seasonal population dynamics of A. gossypii was higher in treated cotton fields while lower in case of S. avenae (treated wheat fields) as compared to untreated field results. The results for natural enemies were similar to cotton aphids but with different magnitude. We also observed that highest field rate of imidacloprid greatly decreased the population dynamics of parasitoids only on2nd day after spray application both in wheat and cotton fields. In addition, we observed that natural enemies treated with tested insecticide in wheat and cotton fields were decreased in numbers on2nd and4th day,respectively as compared to untreated fields. Overall, results demonstrated that the highest field rate applied in wheat and cotton fields adversely affect population density of predators and parasitoids.6-Assessment of effects of insecticides on natural enemies and aphidsThe potential side effects of lowest and highest field rates of imidacloprid on grain aphid and its natural enemies were assessed using open field survey in wheat field. Results demonstrated the remarkable difference in population dynamics of S. avenae, parasitoid mummies and predators (coccinellids, chrysopids and spiders) when imidacloprid (highest and lowest field rates) and water as control treatment applied with nap-sack sprayer in wheat field. We also observed the lowest seasonal mean population dynamics of grain aphid when applied highest field rate and in reverse for lowest field rate if compared with control (water) treatment. Similar results were observed on above mentioned natural enemy species with different magnitude when applied with tested rates of imidacloprid and control treatment. Overall, field results demonstrated that highest field rate of imidacloprid is toxic for nontarget organisms (natural enemies) in field level. In another experiment, highest field rates of both imidacloprid and thiamethoxam were assessed for their side effects on C. septempunctata using tap water as control treatment under laboratory conditions. Pesticides were applied on grain aphid infested wheat seedlings through power-pack aerosol hand sprayer and adult beetles were exposed to feed on them for first three days after treatment. Both pesticides induced significant mortality in the predator; they also showed negative effects on predator offspring production and adult longevity.On the basis of these results, we reached the following conclusions for each section:(i) Each season, before mid-late May, management practices should be started and keep on monitoring the natural control agents to overcome the aphid associated problems in wheat fields in northern China,(ii) Aphid parasitoids, A. gifuensis and coccinellids (H.. axyridis and C. septempunctata) are equally important natural enemies in wheat fields. Overall, strong evidence for top-down control of the grain aphid population growth observed in wheat crop by these natural enemies in northern China,(iii) Each season, before mid-late July, management practices should be adopted and also continue to monitor the natural enemy populations to overcome the aphid related problems in cotton fields in northern China.(iv) Aphid parasitoids, Trioxys spp. and A. gifuensis along with anthocorids(Orius laevigatus, Geocoris punctipes and few other Geocoris spp.) observed as key natural enemies. Overall, strong evidence for top-down control of the cotton aphid population growth observed in cotton crop by these natural enemies in northern China. It concludes that aphid parasitoids, A. gifuensis and coccinellids (H. axyridis and C. septempunctata) are common natural enemies found in both wheat and cotton crops in northern China.(v) Imidacloprid have potential to reduce the population density of natural enemies, parasitoids2-4days after application and further studies to assess its impact on natural enemies at field level could be more informative.(vi) Imidacloprid resulted in more detrimental effects from our both field and laboratory tests. It suggested that be careful to select proper concentrations in the wheat field after investigating the right time and stage of the crop to be applied. Our results concluded that the effects of insecticides on parasitoid mummies and predators differed with insecticide, concentrations and time after application that might be of critical importance in the maintenance and reinforcement of integrated pest management programs or to the development of an effective biological control program against aphids in winter wheat in northern China.Overall, farmers must be careful in the selection of pesticides at appropriate time and field rates to conserve the population dynamics of natural enemies that are providing biological services in northern China. Thus, it will increase key natural enemy populations in wheat and will help to increase biological control of aphids in wheat fields, and subsequently on cotton fields when wheat is harvested. These findings will make an easy way to assess side effects of pesticides on the key aphid parasitoids, major predator groups including different species of coccinellids, anthocorids and spiders to enable incorporating biocontrol services provided by these natural enemies in improved IPM strategies for wheat and cotton crops in northern China.