DDT is a class 2 insecticide, meaning it is moderately toxic. It is important to note that it was banned in the US in 1972, because of the health risks that emerge from it's usage. Many tests have been conducted in order to determine it's effect on the human body. Low to moderate exposure (10mg/kg) may result in nausea, diarrhea, irritation of eyes, nose or throat, while higher doses (16mg/kg) can lead to tremors and convulsions. In experimental animals, such as mice, rats, and dogs, DDT has shown to cause chronic effects on the nervous system, liver, kidneys, and immune system. It has also been found that humans, who were occupationally exposed to DDT, suffered chromosomal damage. It is also known that DDT accumulates in fatty tissue and is excreted in milk. These are just the human health implications that this moderately toxic insecticide bears, the environmental threats haven't even been mentioned.
DDT is very insoluble in water and very persistent in the environment, making it a highly polluting hazard. It's half life has been reported to be between 2 and 15 years. After extensive exposure to soil, DDT will break down into two products, DDE and DDD, with quite similar properties and the same highly persistent characteristics as found in the original DDT. It is this metabolite known as DDE, which is thought to be responsible for the eggshell thinning phenomenon observed birds. Studies have been conducted in regard to this alarming issue, and a decrease in calcium in the eggs laid by birds whose diet included small levels of DDT (100ppm) has been shown.
DDT's devastating effect on the aquatic environment has been thoroughly studied. Due to it's low solubility, it has a greater rate of bioaccumulation in water, and thus poses a great long-term threat to aquatic wildlife. It's highly toxic to both aquatic invertebrate species, like stoneflies and crayfish, as well as fish. Being able to affect all kinds of wildlife, and given it's low decay rate, it proliferates through the entire web of life until it reaches the species inhabiting the throne of the great chain of life, Homo Sapiens.
Bacillus thuringiensis, a microbial insecticide, has no negative effect on the environment. Since it's a naturally occurring bacterium, it doesn't effect the environment of which it is a normal part of. It's biodegradability and low toxicity poses absolutely no threat to ground water. To this day, no studies have proven any environmental drawbacks to its use and from an environmental perspective it can be thought as a miracle insecticide.