Bagworms have already hatched in recent weeks across Maryland so now would be the time to treat if a pesticide is needed. The title of your question references crapemyrtle but the body of your question is asking about leyland cypress; the latter is more likely as a host plant for bagworms so we'll assume for now that this is the plant of concern.

Leyland Cypress don't turn brown from bagworm feeding so much as they just become bare, because the foliage is being consumed by the caterpillars. (When older, bagworm bags become larger and more noticeable, and they are brown in color, so later in summer a bagworm-infested tree might look somewhat brown and partly bare at the same time.) If some of the foliage on a Leyland is browning, the cause may be a fungal infection. They are commonplace but unfortunately not treatable with a fungicide. Dieback from stress like drought or overly-wet/poorly-drained soil is another possibility since Leylands are sensitive to soil moisture level extremes.

Without photos of the symptoms it's difficult to confirm the presence of bagworms or suspected canker fungi, but if treating the bagworms, we recommend not using Talstar and instead using a product with the active ingredients of either Bt or spinosad. Bt (Bacillus thuringiensis), as mentioned in the linked page above, is the most low-risk approach to start with so few other insects are likely to be harmed by the application. The active ingredient in Talsar is bifenthrin, a type of pyrethroid chemical, which has harsher impacts on insects that are not the target pest, including beneficial insects that might be helping to hunt and consume the bagworms. As with any pesticide, follow label directions about weather conditions (most sprays should not be applied while temperatures are above 85 degrees to avoid the risk of plant tissue damage) and when to re-treat to catch any individuals missed by the first application.

Miri