New Aspects of Biological Control of Helicoverpa armigera in Organic Cotton Production

Abstract

Cotton is one of the most important economic crops in Egypt. Egypt started organic cotton production in 1991 in order to meet the demands for organically grown cotton in countries of the Western hemisphere, especially Europe. Cotton is subjected to the infestation of a wide range of insect pests, including the lepidopteran pest Helicoverpa armigera, one of the most damaging pests in cotton. However, control measures other than insecticides in general and biological control measures in particular are not yet properly elaborated for most species. Therefore, this study aimed at testing the effectiveness of biological control measures in organic cotton production by 1. Testing the efficacy of two egg parasitoid species, Trichogramma pretiosum and T. minutum, to parasitize Helicoverpa armigera under laboratory and greenhouse conditions. 2. Evaluating the predation efficacy of lacewing larvae on Helicoverpa eggs and larvae. 3. Testing the efficacy of a combined release of predators and parasitoids. 4. Measuring chemical and physical traits of cotton cultivars related to parasitoid searching efficacy. 5. Evaluating the effect of a jasmonate application on the behaviour of natural enemies in laboratory and field experiments. 1) Parasitism rates of Trichogramma species were significantly influenced by temperature, T. pretiosum being more effective than T. minutum. However, adult lifespan was slightly higher in T. minutum compared to T. pretiosum. Longevity was influenced by the host species, being higher when reared on Helicoverpa compared to Sitotroga eggs. Cotton cultivars significantly influenced the parasitism rates, indicating differences in host location or host handling capabilities by the searching parasitoid species. 2) Predation rates of C. carnea increased during the two first instars and reached a peak in the third one. The third instars consumed about 80% of Helicoverpa eggs and larvae. The developmental time for C. carnea fed on eggs was shorter than those fed on larvae. Position of eggs on the cotton plants significantly influenced predation efficacy; eggs on the 5th leaf were consumed more often than those on the 1st leaf. Cotton cultivars had no significant influence on the predation rate. 3) Preying time (i.e. time Chrysoperla larvae had access to the eggs in the vials) of Chrysoperla both affected predation and parasitism rates; predation rates increased with increasing preying time. Age of eggs parasitized by Trichogramma species affected predation rates; predation rates decreased with aging parasitized eggs, both in parasitized Helicoverpa or Sitotroga eggs. However, a combination of Trichogramma species and C. carnea for controlling H. armigera eggs resulted in higher mortality rates than in Trichogramma alone. In most of experiments Chrysoperla did not prey on parasitized eggs. 4) T. minutum and T. pretiosum differed significantly in searching efficacy on vertically distributed egg batches. Parasitization rates were higher on the upper and lower leaves compared to the middle leaves of different cotton cultivars. Furthermore, parasitization rates were negatively correlated to distance between releasing site of Trichogramma species and egg batches on the cotton plants. Morphological traits of the cotton plants, i.e. presence of glands or trichome densities, significantly influenced Trichogramma behaviour. Parasitization rates on cultivars with glanded leaves and reduced trichome density were higher than on cultivar with non-glanded leaves and high trichome density. Results from GC-MS analysis revealed that volatiles and the phytosterols composition of the leaves differed significantly between cotton cultivars. 5) Females of the parasitoid species Cotesia marginiventris were attracted to plants when treated with jasmonate or damaged by Beet armyworm larvae. Induced-jasmonate-plants were more attractive than plants damaged by 2 Beet armyworm larvae; the least attractive were plants damaged by 1 larva. Cotesia females significantly preferred jasmonate treated arenas compared to larval damaged arenas both on BT-cotton and non-BT-cotton plants. In experiments without jasmonate treatments, females spent more time for searching and selecting and number of visits were higher on BT compared to non-BT-cotton. 6) In a field experiment plants treated with jasmonate had 70% fewer insect cotton pest"s species seven days after application than control plants. The effects of a single jasmonate application increased with time: effects were small on the first and third day after application and moderate to strong at the seventh day after application. BT cotton cultivars (with or without jasmonate treatment) have hosted higher numbers of natural enemies than conventional cultivars. However, sentinel beet armyworm eggs exhibited similar parasitism and predation rates on plants treated with jasmonate compared to control plants. The results of these experiments are discussed with regard to the demands for biological control strategies in organically grown cotton. Large scale field experiments are required to evaluate whether these measures will work under field conditions as well.