It's unlikely that mild winters in California would result in their early hatching. The night time temperatures during the winter do get quite cold, even if the daytime temperatures are warm. Most oothecas I found in the past were not on the surface of a bush, but hidden inside it where it would not have warmed up as quickly or as much from rising day temperatures. I am more inclined to think that the more likely cause of their decline is the increasing use of pesticides.
Both California and Florida are responsible for the production of a good deal of produce and the mild weather makes it easy for both plants and pest species to survive throughout the year. In order to combat the problem of pests--both plants and animals--most growers would find it much easier to either grow plants that have been genetically modified to have the Bt gene or to use insecticides and herbicides--organic and inorganic. Who knows for sure what regular spraying for weeds and household pests has on the surrounding environment? If you read the labels of many sprays, many either claim they become inert or once dry, do not wash away. Reading these same labels, they warn consumers not to use the chemicals near water sources, as many of the chemicals are known to remain in the water for a long period of time and have an adverse affect on aquatic organisms even after they become "inert". Another source of environmental toxins would be from aerial spraying for pests and vectors. For example, everyone is worried about West Nile Virus here in California nowadays, and many cities have opted for aerial spraying for mosquitoes to control the mosquito population and prevent an outbreak of West Nile. Our attempts to control the pest populations brought on by the proliferation of pest habitats through human habitation and crop cultivation can affect mantid populations directly by poisoning them or indirectly by reducing or eliminating their prey.
Edit: I forgot to mention that the neighborhood where my aunt lives has a nearby orchard, so it is possible that use of chemicals there may have contributed to the loss of mantids in the surrounding environment. I have also noticed that the diversity and population density of other insects and arthropods have shown a general decline, which suggests an environmental cause.
