River Islands at Lathrop is gearing up to enter the red-hot South County apartment market.Cambay Group plans to build 600 apartments in the town square area of the 11,000 home planned community. Project manager Susan Dell’Osso said the apartments will be aimed at the middle and luxury markets.A check of apartment rents in Manteca shows rents have gone up nearly 10 percent since January.Manteca’s costliest one bedroom apartments — 737-square-foot units at Paseo Villas billed as a luxury complex on Atherton Drive — are now commanding between $1,490 and $1,590 a month depending upon the unit’s location.Laurel Glenn — a complex on Button Avenue in East Manteca that has been upgrading its interiors with more amenities — is now fetching $1,345 a month for a 597-square-foot one bedroom apartment.The two bedrooms, one bathroom Laurel Glenn apartments that rented for $780 in 2007 are now going for $1,465 to $1,535 depending upon the location if you can get one. There is a waiting list.A 660-square-foot one bedroom unit at Stonegate Apartments on West Center Street is now renting for $1,050 to $1,075 a month.Dell’Osso said River Islands planners will be looking at the best locations for the apartments that will intermixed with retail in the town square off of the main entrance via River Islands Parkway — the extension of Louise Avenue crossing the San Joaquin River using the new bridge.