ACP, short for Asian Citrus Psyllid, a tiny parasite (brown in color) that poses a huge threat to Californias citrus trees.
Psyllid nymphs are found on new shoots of citrus trees. As they feed, they produce a toxin that causes the plant tips to die back or become contorted, preventing the leaves from expanding normally. Of more importance is the fact that they are vectors for the bacteria that causes one of the most devastating of citrus diseases called Huanglongbing which causes the affected trees bear small, asymmetrical fruit which are partially green and unsalable because of their poor size and quality.
Once a tree becomes infected, there is no cure and it will die hence why it is so important to treat the trees at the first sign of ACP.