After exposure to mixed environments with multiple hosts, the frequency of flight take-offs for Plutella xylostella (L.) is known to increase with a concomitant decrease in female fecundity.  The objective of the present study was to elucidate the physiological mechanism underlying the increased flight activity and decreased fecundity of P. xylostella in mixed-host environments.  We measured ovariole length, and triglyceride and vitellogenin (Vg) content in P. xylostella females after they were released into different host environments consisting of either a single host (Brassica campestris, Brassica oleracea or Brassica juncea) or a mixture of all three hosts.  Our results indicated that ovariole length varied significantly between female adults from different host environments.  Females from the B. campestris environment had the longest ovarioles, whereas those from the mixed environment had the shortest ones.  A negative correlation was found between ovariole length and the flight take-off frequency of P. xylostella adults.  Additionally, there were significant differences in the triglyceride content of P. xylostella females from different host environments.  Our data revealed that more triglyceride was consumed by P. xylostella female adults from B. oleracea and the mixed environments than those from environments containing only B. campestris or B. juncea.  In contrast, the relative Vg content in P. xylostella females from the mixed environment was lower than that in females from the B. campestris-, B. juncea- or B. oleracea-only environments.  In conclusion, the mixed environment caused increased consumption of available energy resources (triglyceride) at the cost of retarding ovarian development and decreasing the amount of Vg produced.