Insecticide Resistant Female Cockroaches Are Picky Over Males’ Gifts

​Insecticide resistance can have troubling costs for insects. What happens when resistant insects start rejecting potential mates? Will the population revert back to susceptibility? Not necessarily. Read on to find out more about a unique, unintended consequence of behavioral insecticide resistance in a common urban pest. 

A female receiving a nuptial gift from a male German cockroach. These gifts contain sugars and fats that susceptible cockroaches enjoy. However, females that avoid glucose might find that the mixture tastes bitter and will flee instead of mating. Photo credit: Ayako Wada-Katsumata.

​Previously, Bug Lessons mentioned how insecticide resistant German cockroaches started to avoid glucose (a type of sugar). Sugary, sweet baits used for managing these insects are also insecticidal. By avoiding sugar, the cockroaches ultimately also avoid the bait. Now, researchers have discovered that female cockroaches that avoid glucose (GA-females) often reject males attempting to mate.
 
WHAT DOES SUGAR HAVE TO DO WITH COCKROACH MATING?
 
Nuptial gifts are small “treats” that animals give to one another to help increase their chance of mating. (Think about that next time your partner gives you a box of chocolates.) This “gift-giving” practice is common in the insect world. For German cockroaches, males present females with a nuptial gift of body secretions that contain sugars and fats. This gift holds the female’s attention long enough for the males to mate.

​The German cockroach mating ritual is a multi-step process that involves this type of nice snack. Males will approach a female, raise their wings up, and start producing chemical signals via a small gland on their backs. If females are receptive to the signal, they will climb on the males back and begin feeding on his nuptial gift. This step initiates the mating process. The whole mating event can take up to 90 minutes from initiation (nuptial gift acceptance) to transfer of sperm to the female.

​However, the nuptial gift that males provide to females contains sugar. Researchers noticed that GA-females turn down the nuptial gift presented to them by most males. As a result, they are rejecting successful mating too. This is counter to what we would expect, since most insects must mate to survive. Why do these females reject the gift, especially since the sugars in the male nuptial gifts are not glucose?
 
SOLVING THE PUZZLE
 
To understand why females were leaving mating events early, researchers started matching up different pairs of cockroaches and observed what happened. Females that avoid glucose most often avoided mating with normal males. (Normal males do not avoid glucose.) However, GA-females feeding on nuptial gifts from males that also avoid glucose (GA-males) did not interrupt mating as quickly. 

A picture directly above two cockroaches shows the male cockroaches' lifted wings and female investigating his nuptial gift. Photo credit: Ayako Wada-Katsumata.

​After more experiments, researchers showed that the sugar in the nuptial gifts from GA-males (maltotriose) did not convert to glucose as quickly as the normal males’ sugar (maltose). As a result, the GA-males have longer to mate and, thus, an advantage over normal males. Through a series of different experiments, researchers ultimately determined that the saliva of GA-females degrades male nuptial gifts to glucose very quickly (especially gifts that contain mostly maltose), and glucose tastes bitter to those females that avoid sugar. 

​FINAL THOUGHTS
 
The insecticides we use for pest management can affect both the behavior and physiology of insect populations. We need to understand exactly how we are modifying populations with insecticides because unintended consequences that make pest management more difficult can occur. 
 
According to Dr. Coby Schal, an author on the study, “We are constantly in an evolutionary battle with cockroaches. Evolution can be sped up tremendously in the urban, human environment because the selection force imposed on insects, especially inside homes, is so intense.” This study provides yet another timely reminder of the importance of science-based pest management to delay insecticide resistance and other possible unintended consequences.

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