Whited’s milkvetch (Astragalus sinuatus) is found only within a ten-square mile (26 sq. km) area in central Washington. Although much of the historic range of this species has been converted or destroyed by agriculture and grazing, there is quite a bit of suitable habitat remaining. Scientists do not know why Astragalus sinuatus has not spread to these areas of seemingly suitable habitat. Whited’s milkvetch presents quite a challenge to researchers and seed conservationists. The seed pods of this species are small and incredibly tough. It may take a pair of pliers to crack the fruit open and extract the seed. For all the work involved, the pods have only one or two (rarely more) seeds inside (and sometimes they are completely empty). In the wild, pods open at one end, releasing seeds as they shake in the wind or roll down the hill. Often, the pods are so tough that they do not open until weathered and rolled along the ground. Despite the tough pod, the seeds often fall victim to seed predation or larval damage. Certain insects can bore a tiny hole through the pod and the seed coat and deposit an egg within the seed. As the larvae develops it consumes the contents of the seed. (From CPC website)