*3.2.1 Insecticides*

MSI can be applied to visualize the distribution of insecticides in insects and their negative influence on the target insects. Imidacloprid was used to study its

distribution and accumulation in *D. melanogaster*. Based on laser irradiation, imidacloprid was found to be converted to guanidine-imidacloprid. It eventually accumulated and spread in the abdominal region [36]. *Azadirachta indica* is an economical tree that can be used to distract a biopesticide component, azadirachtin-A. It was only presented in the midgut of *Helicoverpa armigera* after application [61].

#### *3.2.2 Drug/pharmacological test*

Pharmacology model animals are crucial for scientists or pharmacologists to test the side effects of newly developed drugs before clinical trials on human beings. Common pharmacology model animal species include mice, rabbits, dogs, and monkeys. Insects, compared with the above-mentioned animals, have pros such as low costs, high fertility, and moral constraints. Locust has become a new model species for pharmacology test because of its high similarity with mammals. Antihistamine drug terfenadine was tested in locust to study the distribution of secondary metabolites. Terfenadine was gradually degraded from hemolymph to stomach and intestinal wall. However, terfenadine-related chemical compounds such as terfenadine acid, terfenadine glucoside, and terfenadine phosphate were distributed in the unexcreted feces in the intestine, which revealed a rapid discharge of metabolites through defecation [37]. Besides, the spatial and temporal distribution of midazolam was tested in locust. The results showed that midazolam was abundant in 30-min but only found in the feces after a 2-hour application. Midazolam glucoside was found in gut, gastric caeca, and feces after a 2-hour application, indicating that glucose conjugates are a major detoxification pathway to neutralize the effects caused by midazolam in locusts [60].

In addition, *D. melanogaster* was used to test how cocaine, drug removal, and methylphenidate influence the brain lipids. The results showed that cocaine increased the level of phosphatidylcholines and decreased the levels of phosphatidylethanolamines and phosphatidylinositols. Methylphenidate-treated flies failed to rescue the levels of phosphatidylethanolamines and phosphatidylinositols, but enhanced the reversal of phosphatidylcholine levels [49].

#### **3.3 Insect-plant interaction**

Plants and herbivorous insects are co-evolved in nature. Plants activate defense reaction by releasing secondary metabolites when they are under attack by herbivorous insects, while herbivores trigger anti-defense systems for adapting and overcoming the side effects of secondary metabolites produced by plants [68]. Illuminating the changes of secondary metabolites during the interactions between insects and plants can contribute to a better understanding of plant resistance and insect adaptability.

Chemical interaction between soybean (*Glycine max*) and aphid (*Aphis glycines*) was studied. The metabolite changes (e.g., phosphorylcholine and amino acid) were detected in the aphid-infested soybean leaves. The results suggested that secondary metabolites were produced by dead cells after aphid infestation. Moreover, other compounds such as pipecolic acid, salicylic acid, formononetin, and dihydroxyflavone were consistently detected in the plant regions infested by aphids [62]. It was also found that isoflavones can be accumulated in mesophyll cells or epidermis but were not present in the vasculature. The results indicated that isoflavones take part in non-phloem defense response [63].

In addition, MSI can be used for physiological studies such as annihilation of the plant secondary metabolites by herbivores. Glucosinolate gradually changes in the distribution and metabolic sequestration were detected in the body of *Athalia* 

**69**

*Current Advances in Mass Spectrometry Imaging for Insect Physiology and Metabolism*

*rosae* that fed on host plants after different periods. The glucosinolate sinalbin was accumulated in the hemolymph and eventually circulated the Malpighian tubules. The results indicated that the insect gut plays a crucial role as a regulatory func-

Moreover, MSI can be applied to the entire metabolic process of secondary metabolites in the plant-insect-soil system. The fate of the secondary metabolites produced by *Dactylis glomerata* was tracked in the different organs of herbivore *Chorthippus dorsatus*, and finally in the soil solution. After infestation by herbivores, levels of quinic acid, apigenin, and luteolin decreased, while those of flavonoids and rosmarinic acid increased in the leaf wounds of plants in 1 d. Quinic acid can be detected during the digestion process in the grasshoppers' gut and unexcreted feces [38]. Overall, MSI is a useful tool to visualize plant defense and insect defense processes from the responses of plants infested by herbivores to insect defense

MSI has been proved to be an effective and powerful tool to visualize molecules' spatial distribution and temporal changes. In this chapter, we introduce the major types of MSI methodologies and describe the typical experimental workflow for MALDI-MSI. We also retrospect three major applications of MSI in insect physiology, for example, endogenous metabolites, exogenous metabolites, and insect-plant interaction. However, MSI still has some technical cons with limited application range that need to be optimized. In addition, MSI can cooperate with other genetic tools (e.g., proteomics, metabolomics, or lipidomics) for a better understanding of

The work was supported by National Key R&D Program of China

CXZX2016128, CXZX2017321 and 324-1122yb059).

The authors declare no conflict of interest.

(2017YFD0200400), Special Key Project of Fujian Province (2018NZ01010013), Natural Science Foundation of Fujian Province (2019J01369) in China, and Innovation Fund of Fujian Agriculture and Forestry University (CXZX2018092,

*DOI: http://dx.doi.org/10.5772/intechopen.92584*

systems responding to plant-derived metabolites.

tional organ [64].

**4. Conclusion**

sophisticated insect biology.

**Acknowledgements**

**Conflict of interest**

*Current Advances in Mass Spectrometry Imaging for Insect Physiology and Metabolism DOI: http://dx.doi.org/10.5772/intechopen.92584*

*rosae* that fed on host plants after different periods. The glucosinolate sinalbin was accumulated in the hemolymph and eventually circulated the Malpighian tubules. The results indicated that the insect gut plays a crucial role as a regulatory functional organ [64].

Moreover, MSI can be applied to the entire metabolic process of secondary metabolites in the plant-insect-soil system. The fate of the secondary metabolites produced by *Dactylis glomerata* was tracked in the different organs of herbivore *Chorthippus dorsatus*, and finally in the soil solution. After infestation by herbivores, levels of quinic acid, apigenin, and luteolin decreased, while those of flavonoids and rosmarinic acid increased in the leaf wounds of plants in 1 d. Quinic acid can be detected during the digestion process in the grasshoppers' gut and unexcreted feces [38]. Overall, MSI is a useful tool to visualize plant defense and insect defense processes from the responses of plants infested by herbivores to insect defense systems responding to plant-derived metabolites.
