Start with the Insect First

Start with the Insect First

By David K. Mueller, BCE


One insect is found more often than any other stored food and grain insect in the United States, Japan and Europe. This is the Indianmeal moth (a.k.a. Miller moth, Mealy moth, and Grain moth). This small moth alone is responsible for much of the problems associated with seed, popcorn, natural health food, pet food, cereal based mixes, candy, nuts, and stored grain products. Let’s take a look at this “Dirty Rat” that spends your money so freely and causes 100’s of millions of dollars in finished food to be discarded each year:

• The IMM female lays between 350-500 eggs in her short life span.

• The adult IMM only lives for 7-10 days and doesn’t eat but may drink (often found in the sink, vase, or toilet).

• She lays her eggs at night singly or in clusters up to 40.

• She lays her eggs where they have a better chance of surviving. This is near creases in bags containing food or seed where they are hidden from egg parasites.

• The IMM larva can survive outdoors during our worst winters only to start a new generation in the spring. The larval stage of the IMM is the damaging and the overwintering stage.

• The larva stage lasts for two weeks or longer and goes through 5-6 molts before reaching the pupa stage.

Penetrating Packaging

The first instar larva is small enough to crawl through this period (.). It vigorously searches for the smell of food and searches packaged materials until it finds a small defect in the package and then penetrates. If it doesn’t find food in two days, it will die. It is amazing to most humans how this insect finds a way into a package. However, millions of years of evolution have taught it and its offspring to find a meal or die.

After finding food, the larva eats and starts to grow. It now spins a single silken thread from spinnerets under its mouth. This webbing has several purposes. It helps the larva crawl across surfaces. This could be the surface of a grain or a burlap bag. The IMM webbing left by the larva acts as an oviposition site (egg laying) for females to cue in on an area than other moths have used to survive and grow. In the winter months the thick webbing acts as a blanket and can help the moths stay warm, continue growing, propagate; and it forms a protective layer against its natural parasitic enemies.

During the sixth instar (molting stage) the IMM larvae needs to wander from this site. This inherent need to wonder causes it to chew out the bag that it once penetrated as a small first instar larva. The IMM larvae will graze across a plastic bag, testing it occasionally for weakness. When it finds a weak spot, it will begin the laborious task of slashing at it with its rasping mouthparts (like a sickle). After hundreds of slashes it may break through or go on to another location. This is much like a man trying to dig a hole in the ground. Some locations are hard and rocky and some are soft and easy. The larva will then crawl though the round hole and may find a nice safe cardboard fluted box or a 90-degree angle to secure itself. The pupa stage is a defenseless quiescent stage that is vulnerable to attack by natural predators. The larva carefully chooses safe perch to start pupation. This will be the location where the IMM adult will dry its wings when it emerges from its transformation to take the first flight of its life, perhaps into a sticky pheromone trap.

The first generation of IMM in much of North America emerges from the overwintering (large) larva in April-May. In the tropical regions of the United States it can stay active most of the year. This moth doesn’t like to fly when the temperatures are below 62-65° F or less. Pheromone traps should be placed when temperatures reach 60° degrees F/ 17° C or higher.

The IMM goes through a new generation every 4-6 weeks during the warm summer months and 5-8 weeks in the cooler months. In the insect rearing lab at Insects Limited, Inc. we can shorten a generation of IMM to 18 days when the temperatures are set for 29 C/ 85 F and 60% relative humidity on a special diet.

In the Midwest we normally have 3 generations per year (400 x 400 x 400 offspring). In warm summers like the ones we have experienced in the past five years, we are getting 4 generations per year (400 x 400 x 400 x 400 offspring). The reproductive potential then is 26 billion IMM from one pair. With the number of 90-degree weather days doubling last year in many parts of the country, one can see how this moth has been such a nuisance.

The IMM does not carry diseases known to man or causes health problems like mosquitoes, bees, or flies. It is a nuisance pest that contaminates food in your home and your factory with its presence or its webbing. It lives outdoors naturally and feeds on grass seed and cereal protein.

In a survey conducted by Dr. M. Hirao of Japan, over 95% of the 200 households survey from around Japan had IMM indoors and also outdoors.

The IMM does not like hot weather. They are seldom found in the tropical areas like Southeast Asia or Hawaii. Found there are related cousinslike the Rice moth, or the Cocoa moth. The temperate region of the world is where the IMM survives best.

One characteristic that Alain Van Ryckeghem of Insects Limited, Inc. recently discovered is that the IMM adults like to stay near the walls. The pheromone traps near the center of the room capture fewer adult males than the ones near walls. The adult moth seems to prefer resting on the walls in a vertical posture. Pheromone traps are very effective in locating IMM. The traps should be placed in a convenient location away from children or fork truck operators. Write the date on the trap when they are first placed and again when the lures are changed. The lures should be changed every 8-10 weeks indoor and 4 weeks outdoors. The traps should be changed when dust accumulates on the sticky surface or the numbers of moths make change necessary. Check the traps weekly if possible and remove all captured insects. Keep excellent records and maps of trap location. Predicting future population trends will be possible by closely evaluated and mapping the collected data.


Pest management begins with prevention and monitoring. The pheromone traps for stored product insects shouldn’t be considered a control tool but rather sensitive detection and monitoring tool. The accumulated data will help predict future populations for pest insects and better determine the best time for directing a pesticide control program. After several years of collecting data a pest manager can fine-tune his/her pest management program to compensate for unusually warm weather or other circumstances that arise. The Indianmeal moth is predictably programmed through 1000’s of years of evolution. It is a controllable nuisance pest of store products. It all starts with knowing the pest… Knowing the pest is half the battle in controlling it.


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