Phytobius vestitus on the bud of parrot feather.
P. vestitus floating on water’s surface.
Phytobius vestitus is a small beetle native to North America that feeds on watermilfoils.
Figures 1. Dorsal and lateral views of Phytobius vestitus. Images by Able Chow.
Phytobius vestitus is small, measuring in from 3/64” (2 mm) to 1/8” (3 mm) in length. Its back is covered in dense black to brown scales with gray to brown scales on its sides and underbelly (Figure 1) . One of the most distinguishing features of this weevil is a small, oval-shaped, lightly colored marking on its back near the head  (Figure 1). This is a sharp contrast with the other weevil found feeding on parrot feather, Pelenomus canaliculatus, which is uniformly black and does not have this lightly colored spot (Figures 11, 12 of the “Plant Biology and Issues” section) (Dr. Robert Anderson, personal communication, January 22, 2019). Pelenomus canaliculatus is also slightly smaller than P. vestitus. Like other weevils, P. vestitus has elongated mouthparts called rostrum that resemble an elephant’s trunk (Figure 1).
Figure 2. Life cycle of Phytobius vestitus.
Figure 3. Life stages of Phytobius vestitus in parrot feather stems. 1. Larvae boring in stem. 2. Pupation site. 3. Teneral adult (stage immediately following pupation) in stem. 4. Fully mature adult chewing exit hole in stem to emerge.
Adult females of P. vestitus lay yellow eggs on the leaves and buds of the plant (Figure 2). Once these eggs hatch, the larvae begin to feed on young, tender leaves on the buds. After several days of feeding, these larvae will bore down through the bud and into the stem. Larger larvae will occasionally enter the plant through the side of the stem and cover their entrance hole with a brown substance, possibly frass (insect feces). They will tunnel through the stem while they feed and search for an adequate pupation site (Figure 3). Once they have found the perfect pupation site, the larvae will create a chamber within the stem and begin to pupate (Figure 3). These insects will often pupate below the waterline and might utilize the oxygen stored within the buoyant, air-filled stems of the plant to avoid drowning. Once the weevils have pupated, they chew small holes through the side of the stem to exit the pupation chamber as fully formed adults (Figure 3). Within several days, these adults will begin mating and laying eggs, starting the cycle anew.
Figure 4. Distribution of Phytobiius vestitus based on museum specimens and literature.
Little is known about the distribution of Phytobius vestitus. However, reports of this weevil from museum records and in literature suggest that these weevils are found throughout the temperate regions of eastern North America, with these insects being reported as far west as Oklahoma, as far north as Michigan and New Brunswick, Canada, and as far south as central Florida (Figure 4) [6, 8].
Figure 5. Stand of variable leaf milfoil (Myriophyllum heterophyllum) in Oakdale, Louisiana.
Phytobius vestitus appears to be host-specific with watermilfoils, meaning they will only feed on plants of the Myriophyllum genus and will not eat other plants, like crops or horticultural plants . There are many species of watermilfoil found throughout North America . At least five species are present in Louisiana, two of which are invasive (Eurasian watermilfoil and parrot feather) . Though very little research has been done on the matter, it is believed that prior to the introduction parrot feather in the 19th century, Phytobius vestitus, a native species, fed on variable leaf milfoil (Figures 5, 6) due to similarities in the growth form between this plant species and parrot feather [4, 10]. However, there is a possibility that this weevil could feed on other heterophyllous species of watermilfoils, such as cutleaf watermilfoil . More research needs to be done to determine the full host range, or the range of plant species an insect will feed on, of this weevil before the insect can be used as a biological control agent.
Figure 6. Phytobius vestitus on variable leaf milfoil.
Figure 7. Weevil-damaged parrot feather buds.
Figure 8. Parrot feather stem with adult feeding scar and an exit hole.
The adult weevils will feed on nearly every part of the plant, especially on the buds, leaves and flowers (Figure 7). Adult feeding damage on the stem results in characteristic black or dark brown circular scars (see Figure 8). The larvae feed the most heavily on the buds and interior of the stem . Larval feeding will often result in gray or brown, rotting or dried, wilted buds, and brown, rotting, hollow stems (). Circular holes resulting from adult weevils exiting their pupation chambers are another clear sign of damage (see Figure 8).
Figure 9. The parrot feather-infested canal in Gramercy, Louisiana. The image on the left depicts the parrot feather infestation before the peak of weevil densities in mid-April, 2019. The image on the right shows the infestation after this peak in weevil densities.
High densities of Phytobius vestitus in parrot feather stands result in dried or rotten buds, which effectively prevents plant growth. Larval feeding damage to the stem will often cause the plant to wilt, eventually leading to its collapse and death (Figure 9 and 10). In southern Louisiana, LSU researchers have discovered a massive outbreak of this weevil in a large infestation of parrot feather . During the spring of 2019, feeding damage from these weevils seemed to drastically reduce the density of parrot feather, nearly eradicating all emergent stems by midsummer (Figure 11). Though this is a promising discovery, more research needs to be done to determine the host range of the weevil, its efficiency as a biological control agent, and other factors influencing the survival of parrot feather throughout the growing season before this weevil can be used as a management tool for this invasive aquatic weed.
Figure 10. Early impacts of weevil damage on parrot feather in Gramercy, Louisiana (March 2019).
Figure 11. Late impacts of weevil damage on parrot feather in Gramercy, Louisiana (May 2019).
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