There are two popular grain weevil according to the description of National Corn RDE Network.
1) Sitophilus zeamais or the maize grain weevil is the most destructive storage pest. The female lives for an average of four to five months. She can lay 300 to 500 eggs. Range of temperature favorable for egg laying is between 28 to 30ºC and 28 to 32ºC for hatching. No eggs are laid at relative humidity of 30 percent. Total development period from egg to adult is about one month.
2) Rhyzopertha dominica or the lesser grain weevil is 2.3-3 mm long dark brown beetle. It lays egg on the grain or in floury dust. The average incubation period at 70% relative humidity is 7-8 days at 28ºC. The newly hatched, white and parallel-sided larva may enter the damage grain until it pupates. This lesser grain develops on grains with moisture content of 14 percent and below but not less than 9 percent MC at 34ºC, mortality rate is 98 percent. Adults remain in pupal cell for 3 to 5 days after enclosion before beginning to feed and tunnel out of the kernel. Adults mate soon after emergence and begin ovipositing 15 days later and lay eggs for 2-4 months producing 200-600 eggs depending on condition.
The two grain weevil types are real notorious. Imagine producing 300 to 500 eggs or 200 to 600 egg for a short life span of less than five months. Sacks of corn will be wasted in just short span of time. However, natural control is easy.
1) Temperature. Notice that maize grain weevil require 28 to 30ºC for laying eggs and and 28 to 32ºC for hatching. Maintaining a temperature lower than 28ºC will prevent laying and hatching of eggs.
2) Relative humidity. For maize grain weevil, maintain an rh lower than 30% and less than 70% for lesser grain weevil.
3) Moisture Content. Weevil never develops in grain with less than 9% MC. Maintaining a 9% moisture content will prevent weevil infestation. Though the recommended MC for storage is 12%.
first picture courtesy of Agricultural Research Service – US Department of Agriculture through wikepedia
second picture courtesy of USDA-ARS-GMPRC Image Database through ozanimals.com
weevil descriptions courtesy of National Corn RDE Network
Marvin is the lead chocolate maker of Ben and Lyn Chocolate Inc. Has strong background in food research and development. Occasionally conducts training and lectures. Lecturer of Cocoa Foundation of the Philippines. Do coaching and consultancy services on his free time.