Thomas:

How can juvenile hormone have several different effects in insects?

Juvenile hormone (JH) is actually a group of hormones (several forms: JH0, JHI, JHII, JHIII) that are produced by the corpora allata (a gland in insects). JH really has two main functions in insects, the primary being morphogenesis, the other being egg production. And of course, there are exceptions. More on the different functions in a sec.

Probably one of the most famous studies using JH was conducted by a prominent entomologist, Vincent B. Wigglesworth. He is considered one of the fathers of insect physiology.

Vincent mainly worked on the kissing bug (Rhodnius prolixus) and he did a classical experiment that showed that JH keeps the larval epidermal cells from producing adult cuticle. The cuticle of last nymphal instar of the kissing bug is dimpled. When the insect becomes an adult the cuticle is smooth. To show that JH was affecting the epidermis, Vincent painted his initials (VBW) with JH on the abdomen of the last instar before the insect molted into an adult. The area that he painted his name in produced another nympal cuticle (dimpled), the rest produced the adult cuticle (smooth).

The picture on the far left is the cuticle of the last nymphal instar, the middle is of the cuticle of the adult, the picture on the far right is where Vincent painted his name with JH showing that the hormone inhibited the epidermis from forming the adult cuticle.

So, JH’s first function is in morphogenesis. The higher the JH titer in the larva, the further it is from becoming an adult. The closer the insect gets to molting into an adult, the lower the JH titer is. In fact, the period before the molt into adulthood is actually called a JH-sensitive (or JH-critical) period, and JH has to be completely absent in order for the insect to successfully molt into an adult.

Why do insects fail to keep molting? JH works in conjuncture with other hormones (such as prothoracic hormone) and it is believed that the absense of JH causes the degeneration and preprogrammed cell death of the gland that produces prothoracic hormone.

After the insect becomes an adult, JH also functions in egg production (ovarian maturation) in many insects. Other behaviors are often linked to egg maturation, such as blood feeding and mating behaviors. Mosquitoes need a blood meal in order to produce eggs. Some researchers have shown that removing the corpora allata (CA) after Culex mosquitoes emerged as adults blocked the initiation of biting behavior. Those same researchers injected JH into mosquitoes that had their CA removed and showed that biting behavior was then recovered. Pheromone production in cockroaches and aggressiveness in honeybees has also been linked to JH.

I know that there are a couple people who visit this site that could answer the original question a lot better (and hopefully they will comment if I’ve missed something), but I think the simplest answer to why JH has several different effects in insects is because it has different actions/functions at different times in the insect’s life (larval vs. adulthood), it has different effects during specific JH-critical periods, there are a couple different forms of JH, and JH has different effects depending on what other hormones it is interacting with (i.e., prothoracic or ecdysial hormone).

As an aside, I tried using methoprene, which is an analogue of JH, in hopes of shutting down biting behavior in horse flies (based off of those experiments with the Culex mosquitoes), but I didn’t get any clear results right away and had too many other confounding factors, so I had to drop that project. Methoprene is an insect growth regulator and is commonly used in pest management programs for fleas, ants, and mosquitoes. In my opinion, it is really cool to see how scientists can use an insect’s natural hormone against it for control purposes. Methoprene isn’t like a traditional insecticide, it doesn’t poison the insect and kill it outright, it disrupts its lifecycle and prevents the insect from ever reaching adulthood and reproducing.

Thats what I find really exciting about insect science; being able to apply the knowledge learned about the mechanisms of basic biology in order to improve the quality of life.

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