How do stored product pests get water?

Written by Joe Ballenger

Here’s a question I *really* like:

Indian meal moths and grain weevils get into stored grain products and manage to complete entire life cycles without any access to moisture. How do they manage that?

I really like agricultural pests, stored product pests in particular. They have super cool biology, and they’re really important. Between the farm and the table, 10-15% of the harvest can be lost to bugs which live inside stored products. If that wasn’t bad enough, these bugs can break grains and mess with the humidity inside storage facilities. This damage introduces fungus, which can reduce the value of the product by as much as half its worth. This fungus can also make people very sick, so they’re important to both agriculture and medicine.

If you think about the environment these bugs live in, it’s very extreme. They live their entire lives without seeing a drop of water, all while evading hyper-intelligent animals which are constantly looking for new ways to kill them. There’s life in the driest deserts in the world, but these animals live in an indifferent environment. While harsh, these desert animals do not live in a place which is actively trying to kill them. You could argue that these bugs are the ultimate extremophiles.

So how do these guys get water in such a harsh place?

Balancing Water on the Driest Food

Water balance

Grains tend to contain 10-15% water, but most stored grain pests contain more water despite the fact they never actually drink. Image credit: Tuda, 2011

There are a lot of stored product pests. However, the ones which infest the driest products tend to be beetles, caterpillars, and mites. I started by looking for data on the water content of stored product pests, and I wanted to compare it to how much water was in the beetles.

I wasn’t able to find any information on how much water the caterpillars have, but I was able to find this information on the beetles. I wanted to focus on a beetle which fed on dry food, but had a lot of water. Callosobruchus bean weevils fit the bill very nicely.

I feel that the bean weevil would be a good bug to look at, because they have a pretty large amount of water in their bodies compared to the amount of water in the food they eat. Even though their food only contains about 15% water, their bodies are composed of 55% water. They never see liquid water, but they more than triple the water they get from their food.

That’s a really cool trick if you compare that to human water usage. Our food tends to be about 70% water, and our bodies are about 70% water. On top of this, we’re constantly chugging liquids to keep ourselves from getting thirsty. We use a lot of water, whereas bean weevils use practically none.

Water Without Liquid

There’s three basic sources of water that animals can get. The first is liquid water, and these beetles will drink if given the opportunity. However, water isn’t usually found in food storage places.

Humidity

Pea weevils in cow peas

Bean weevils on beans, their favorite food. Their larvae live inside the beans, eat them from the inside out, and then chew their way out. They’re very significant pests of beans in many parts of the world. Image credit: CSIRO via Wikipedia. License info: CC BY 3.0

First, water is all around us in the air. Water evaporates from everywhere in the environment, and dissolves itself in the air. How much water the air has, called humidity, can influence how many bugs can live in the products. Grain weevils and bean weevils both benefit from higher humidity.

There are insects which can drink directly from humid air. Desert cockroaches, for example, have protrusions which allow them to get water from the air. However, bean weevils don’t seem to be able to do this. They’ll gradually lose weight from dehydration at pretty much any humidity level, so they’re not drinking from the air.

Instead, they rely a lot on conservation to keep them from losing too much water. Their poops tend to be very dry, which is nice because pooping uses a lot of water in most bugs. Dead bean weevils lose water faster than live bean weevils, which means that their body keeps them from losing water too fast. A lot of this seems to be from the way their body works.

Every beetle is covered with a thin layer of wax, which keeps too much water from evaporating from their bodies. In desert beetles, this wax is very solid because a thinner wax will let water escape. The beetle’s exoskeleton, the hard outside covering, also helps keep water in. Their spiracles are located under the wings, which means their wings can help them recapture water lost when they breathe out.

Even though they don’t encounter liquid water, and can’t drink from the air, every aspect of their biology is dedicated to keeping themselves from drying out.

Water From Food

Sugar vs fat

The stuff bugs use to make energy is really important to water conservation. Those H and OH groups are converted to water as the sugar is metabolized. Sugars have a lot of chemically hidden water, while fats don’t. It makes sense these bugs immediately digest sugar, but stock up on fat. Image credit: Wolfgang Schaefer (R), Ben Yikrazuul (L) via Wikimedia commons. License info: Public domain

In addition to really good protection against drying out, the bugs which live in dry foods use their energy reserves very strategically. Bean weevils and grain weevils eat foods that are very starchy, and starch is made up of a lot of sugars hooked up to each other.

Another important fact is that their bodies tend to be low in sugar, and high in fat. In fact, half of the bean weevil’s body is fat by weight. They’re eating starch, but they’re not keeping those sugars around. They appear to be shunting it all into fat.

The general formula for a sugar molecule is some number of carbons linked with some number of water molecules. In fact the term carbohydrate basically means hyrated carbon, or carbon with water. Fat, on the other hand, doesn’t have a lot of water in their structures.

Sugars and fat get broken down to carbon dioxide and water, and this is where most of their water comes from. Even though they don’t ever drink under normal circumstances, they actually get the water they need from breaking down their food.

Sugar breakdown

Storing their energy reserves as fat instead of starch has some additional benefits because this allows them to maximize the amount of water they get from their food. Sugar-based storage molecules will absorb water into their structures while fats don’t do this. Over some really complicated biochemical cycles, they more or less snip the waters off the sugars to maximize the water they get from their food. In essence, these bugs store extra energy in a manner which maximizes the usable water.

Metabolism, breaking down chemicals for energy, is one of the most complicated parts of biochemistry. There’s a number of very complicated cycles here related to breaking down sugars, building up fats, breaking fats down, and burning things for energy that play significant roles in answering this question…and this is a very simplistic view of the process.

The Bottom Line

To understand these beetles, you need to know where they came from. Many pest species which infest stored products came from desert places where there’s not a whole lot of water, but lots of plants which grow during the rainy season. In these places, most of their food comes in the form of dried seeds…and infesting dry food is a very short evolutionary jump. It’s likely the ability of these beetles to live in these dry environments came from their desert history.

These beetles are really good at efficiently using water. On top of having a low amount of water, they also have adaptations to keep that water in. They get all the water they need from breaking down food, and store extra energy in a way that allows the water to be available to them when they need it. They eat all the time as larvae, and don’t feed a whole lot as adults. This means that the adult female just needs to be able to live long enough to lay eggs.

This is a great system which has really helped them to invade some of the harshest environments on earth, and their ability to keep farmers and entomologists flummoxed is testament to their adaptability.

Works Cited

Benoit, J. B., Yoder, J. A., Rellinger, E. J., Ark, J. T., & Keeney, G. D. (2005). Prolonged maintenance of water balance by adult females of the American spider beetle, Mezium affine Boieldieu, in the absence of food and water resources. Journal of Insect Physiology, 51(5), 565-573.
Bhattacharya, B., Barik, A., & Banerjee, T. C. (2003). Bioenergetics and water balance in Callosobruchus maculatus (F.)(Coleoptera: Bruchidae) larval populations. Oriental insects, 37(1), 423-437.
Tuda, M. (2011). Evolutionary diversification of bruchine beetles: climate-dependent traits and development associated with pest status. Bulletin of entomological research, 101(04), 415-422.
Wightman, J. A. (1978). The ecology of Callosobruchus analis (Coleoptera: Bruchidae): energetics and energy reserves of the adults. The Journal of Animal Ecology, 131-142.

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