Genetically engineered, insect-resistant plants could harm beneficial insects and affect biodiversity. true or false

Absolutely. 

Remember in an ecosystem, plants are usually the main producers of the currency of energy in the form of sugars, proteins, and fat.  They do so via photosynthesis. 

For a lot of insects, plants are their main source of food. 

This means-- if plants are insect-resistant, insects can't eat them. 
Hence, insects can't receive the necessary energy they need to survive. 

This can affect biodiversity in some ways:
It could mean the population of plant-eating insects may dwindle because of less food
It could mean that the population of plant-eating insects would leave the geographic area in search of food

So there's a lot of reasoning behind the harm of insect-resistant plants. 

However, always look at the other side and realize that there are also benefits to insect-resistant plants, particularly for us humans who rely on them religiously to keep up with the demand for food!