We monitored 18 natural ponds with surface areas that varied between 1 m2 and 8 m2 and a water depth that varied between 3 cm and 25 cm. No vegetation was present inside the ponds, but trees and bushes surrounded them. A natural community of mosquito larvae and other insects had colonized the ponds before treatment. However, these insect populations consisted mainly of mosquito larvae (Culicidae). Other insects, such as members of Ephemeroptera, http://www.selleckchem.com/products/Vincristine-Sulfate.html Chironomoidae, and Notonectidae, were found only sporadically, and thus were excluded from the analysis. The results obtained from ponds that provided less suitable breeding conditions for mosquitoes as indicated by less than 50 mosquito larvae per liter on the day of treatment (N = 3) are not reported. Three different experimental treatments were tested. For the control condition, no treatment was applied. For the first treatment, which is referred to as “Competition,” a natural community of crustaceans was introduced. The organisms that formed these crustacean communities were collected from a lake near Rosslau (51°53′08″ N, 12°19′11″ E) using a plankton net (mesh size, 500 μm). Organisms were introduced such that their final density in the treated ponds was approximately five individuals per litre. The introduced community comprised Ceriodaphnia spp. (74.7%), Simocephalus spp. (7.5%), Daphnia spp. (3.6%), Scapheloberis spp. (2.7%), Ostracoda (9.7%), and Cyclopoida (1.7%). For the second treatment, which is referred to as “Bti,” ponds were treated with 1,000 μg Bti/L, which is the concentration used routinely in mosquito control programs (Becker 2003). The liquid stock solution used (Vectobac 12 AS, Valent BioScience Corporation, Lyon, France) had an activity of 1,200 International Toxin Units (ITU) per milligram. For the third treatment, which is referred to as “Bti+Competition,” Bti was applied at a concentration of 1,000 μg Bti/liter, and a community of natural crustaceans was introduced as in the treatment “Competition.” Four replicates were considered in the data analysis for the treatments “Bti,”“Bti+Competition,” and control, and three replicates for the treatment “Competition. A water sample with a total volume of 0.5–2 liters (2 liters for ponds with a surface area >5 m2, 1 liter for ponds with a surface area 1–5 m2, 0.5 liter for ponds with a surface area <1 m2) was collected twice a week from each pond. Each water sample consisted of several 300 ml subsamples, which were collected with a scoop from different parts of the pond. The samples were filtered through a plankton net (55 μm mesh size) and preserved in 70% ethanol (approximately 30 ml). The abundances and composition of the zooplankton were then analyzed using a binocular Leica S6D microscope (Wetzlar, Germany). Mosquito larvae were characterized to the species level using the determination key of (Becker et al. 2010).