Q:

Is there a herb or plant called "Locust" apart from the typical grasshopper or locust? 

The food of John the Baptist was locust and wild honey. 

“And the same John had his raiment of camel’s hair, and a leathern girdle about his loins; and his meat was locusts and wild honey.” ( Matthew 3:4, KJV 1900)

Is there any herb or vegetable that might possibly be the locusts of the verse quoted?  Are there any biblical or extra biblical sources that talk about this? 
I am looking specifically for any evidence that may point that the locust is a herb or shrub and not an insect! 

A:

Is there a herb or plant called “Locust” apart from the actual grasshopper or locust that St. John the Baptist may have eaten? 

4 John’s clothes were made of camel’s hair, and he had a leather belt around his waist.  His food was locusts and wild honey. -  Matthew 3:4

Possibly, it will depend on the interpretation of Sacred Scriptures one desires to follow. 
There are are some, who believe that along with honey, St. John the Baptist ate the fruit of the carob tree also known as the locust bean (tree). 

The carob (Ceratonia siliqua) is a flowering evergreen tree or shrub in the legume family, Fabaceae.  It is widely cultivated for its edible pods, and as an ornamental tree in gardens and landscapes.  The carob tree is native to the Mediterranean region and the Middle East. 
The word "carob" comes from Middle French carobe (modern French caroube), which borrowed it from Arabic خَرُّوبٌ (kharrūb, "locust bean pod"),2 ultimately perhaps from Akkadian language kharubu or Aramaic kharubha, related to Hebrew harubh.3 Ceratonia siliqua, the scientific name of the carob tree, derives from the Greek kerátiοn κεράτιον 'fruit of the carob (from keras κέρας 'horn'), and Latin siliqua 'pod, carob'. 
In English, it is also known as "St John's bread", as well as "locust tree", (not African locust bean) the designation also applied to several other trees from the same family.  Carob (Wikipedia)

Was the locust of Matthew 3:4, really locust? 

John the Baptist's diet has been the centre of much discussion.  For many years, the Greek: ἀκρίδες (akrides) was interpreted as referring not to locusts, the insect, but rather to the seed pods of the carob tree.  But the Greek word is not used this way, and this notion is generally rejected today.  Locusts are mentioned 22 other times in the Bible and all other mentions quite clearly refer to the insect.  Locusts are still commonly eaten in Arabia.  Eaten either raw or roasted they are quite nutritious and a source of many vitamins. ** While most insects were considered unclean under Mosaic law, Leviticus 11:22 specifically states that locusts are permitted.  Portraying John the Baptist as eating seed pods rather than insects is possibly due to squeamishness about having such a revered figure eating insects and also a belief that a true ascetic should be completely vegetarian.  What is meant by honey is also disputed.  While bee honey was a common food in the area at the time, Jones believes that it refers to the tree gum from the tamarisk tree, a tasteless but nutritious liquid, rather than the honey made by bees. -  Matthew 3:4

Dr. James D. Tabor has this to say on the subject: 

The most commonly held view of John’s diet, based on our text in Mark, is that he ate locusts, a migratory form of the grasshopper of the family Acrididae, still commonly consumed by desert peoples in Arabia.  Others have suggested the word translated “locusts” refers to the beans of the carob tree, commonly called “St. John’s bread.”  However, the Greek word translated “locusts,” (akris/ακρις) seems to clearly refer to a species of grasshopper.  The problem is such eating of “flesh,” even if that of an insect, seems to contradict the sources that emphasize his ascetic vegetarian ideal.  Paul, for example, refers to members of the Jesus movement who abstain from eating meat and drinking wine (Roman 14:1-4).  We also have traditions that James, the brother of Jesus, practiced a strictly vegetarian lifestyle, which was also common among the Jewish Christian community that became known as the “Ebionites,” see my post here.  Somehow “locusts” seem out of place. 
A possible solution to this confusion about John’s desert diet is found in the fragments we have of the lost “Gospel of the Ebionites,”  as quoted by the 4th-­century Christian writer Epiphanius (Panarion 30.13.4-5), who hated the group but fortunately, nonetheless, can’t resist quoting them–thus preserving some precious material.  The Greek word for locusts (akris/ἀκρίδες) is very similar to the Greek word for “honey cake” (enkris/έγκρίς) that is used for the “manna” that the Israelites ate in the desert in the days of Moses.  According to this ancient text was not locusts but these cakes cooked in olive oil.  If this is the case then John would have eaten a cake of some type, made from a desert plant, similar to the “manna” that the ancient Israelites ate in the desert in the days of Moses.  This “bread from heaven” is described as “like coriander seed, white, and the taste of it was like wafers made with honey” (Exodus 16:31; Numbers 11:8).  This kind of “pancake” baked in oil, and sweetened with honey, would then reflect and emulate the ideal holiness of the desert wanderings of Israel when the people had to look to God alone for “daily bread.” -  Did John the Baptist Eat Bugs, Beans, or Pancakes? 

After reading the above mention blogger’s post, Fr.  Dwight Longenecker added the follow remarks: 

So it is very possible that John ate grasshoppers in one form or another.  It was the food of poor people and would have been consistent with his ascetically lifestyle, but the reference to honey points to the possibility that  he lived on a simple type of way bread.. This fits nicely with the idea that John in the Wilderness re-capituates the whole of the Old Testament–he is in the wilderness like the people of Israel eating honey cakes like manna.  Combined with the fact that the gospel links him with the Judges (Samson) the miraculous births (Sarah and Samson’s mother) Elijah and Isaiah, it would seem that the third option may be the one which is the most meaningful and real. -  Did John the Baptist Really Eat Grasshoppers? 

A large carob tree in Sardinia, Italy
Here follows more on these biblical mentioned locust:

There are ten Hebrew words used in Scripture to signify locust.  In the New Testament locusts are mentioned as forming part of the food of John the Baptist (Matt.  3:4; Mark 1:6). 
By the Mosaic law they were reckoned "clean," so that he could lawfully eat them.  The name also occurs in Rev. 9:3, 7, in allusion to this Oriental devastating insect.  Locusts belong to the class of Orthoptera, i.e., straight-winged.  They are of many species. 
The ordinary Syrian locust resembles the grasshopper, but is larger and more destructive. " The legs and thighs of these insects are so powerful that they can leap to a height of two hundred times the length of their bodies.  When so raised they spread their wings and fly so close together as to appear like one compact moving mass." 
Locusts are prepared as food in various ways.  Sometimes they are pounded, and then mixed with flour and water, and baked into cakes; "sometimes boiled, roasted, or stewed in butter, and then eaten."  They were eaten in a preserved state by the ancient Assyrians. 
The devastations they make in Eastern lands are often very appalling.  The invasions of locusts are the heaviest calamites that can befall a country. " Their numbers exceed computation: the Hebrews called them 'the countless,' and the Arabs knew them as 'the darkeners of the sun.' 
Unable to guide their own flight, though capable of crossing large spaces, they are at the mercy of the wind, which bears them as blind instruments of Providence to the doomed region given over to them for the time.  Innumerable as the drops of water or the sands of the seashore, their flight obscures the sun and casts a thick shadow on the earth (Ex.  10:15; Judg.  6:5; 7:12; Jer.  46:23; Joel 2:10). -  Locust

