Problem with weevil affected plants (mangoes, lychees, etc.) is that before they are the whitish gray "bug" you see eating your leaves, they are just what they are called, root weevils that are eating your root. That is what usually causes the most harmful damage with insect and why it seems they take over and begin to destroy a plant once it is affected.
This is why a drench of a commercial grade systemic is vital to get rid of the weevils. Forst off, you must get rid of them from eating the roots. Bayer Advanced is an Imidacloprid product it is .235% active ingredient (Imidacloprid), hardly much at all when commercial strength is 25% - 75% depending on the product (yes, the Bayer is ,235 (point 235%)...do you see where I am headed here. The Bayer Advanced, mixed at the label's recommended concentration, is about useless for a plant that has been taken over by the Sri Lankan Weevils. If using the Bayer product, you will need to mix the product at an extremely much higher concentration...or, better yet, visit a chemical supply house (Howard Fertilizer, Winfield Solutions, Helena, etc.) and purchase a commercial product. This will allow yo to have a chance in the battle and take the plant's life back from the weevils. One word of advice, IF you do purchase a commercial strength product, please be sure to follow the label's recommended rates.