Whatever happened to cattle grubs?
Very effective cattle endectocides [Tables 1–3] have been credited for the dramatic decline in parasitism of U.S. cattle by cattle grubs (Campbell, 1985; Scholl, 1993). Endectocides are systemic anthelmintics (dewormers) belonging to the macrocyclic lactone group that are also labeled for use against cattle grubs, blood-feeding lice, mange mites, horn flies, and other parasites of cattle. Although endectocides have been effectively controlling cattle grubs and other cattle parasites since the 1980s, cattle grub infestations continue to be reported by ranchers across the country in untreated cattle.
Infestations of cattle grubs on pastured beef cattle were brought to my attention by ranchers in Arizona and South Dakota [Fig. 3] during the 2018 grazing season. It is still important for cattle producers of all ages (ranchers, feedlot operators, dairy farmers) to have a working knowledge of the basic biology, symptoms, and available control products for managing cattle grub infestations in the field.
Cattle grubs are an old pest of cattle (Bishopp et al., 1926). The grub or larval stages can feed directly on the skin, connective tissues, and muscles of live cattle. Cattle grubs can riddle the back muscles and skin with holes; damaged carcasses and hides will need to be trimmed at harvest causing direct losses in weights and quality (Roberts and Lindquist, 1956).
There are two kinds of cattle grubs that infest cattle in the U.S.—the northern cattle grub (Hypoderma bovis) and the common cattle grub (Hypoderma lineatum). Although similar in appearance and biology, they differ in the way they migrate once inside the body of cattle. Northern cattle grubs migrate along the spinal canal; common cattle grubs migrate along the gullet or esophagus. If these migrating cattle grubs are killed by medication (e.g., endectocides) at the wrong time, the cattle can either get paralyzed in the hind legs (northern cattle grubs) or suffer severe vomiting and bloating (common cattle grubs) due to adverse immune responses to the dead or dying cattle grubs in the specific parts of the cattle body (Campbell, 1985). It is, therefore, very important to consult with your veterinarian and MWI Animal Health Territory Manager before treating grubby cattle if there is reason to believe that cattle grubs are actively migrating along the spinal canal or the esophagus or gullet of the animals.
Potential adverse response to treatment and treatment cutoff dates
There are currently very effective endectocides that can be used against cattle grubs [Tables 1–3]. However, mistimed applications of these products can result in adverse reaction by the animals being treated. High numbers of migrating grubs can be killed while passing through the spinal canal (in the case of northern cattle grubs) or the esophagus (in the case of common cattle grubs). Animals with dead northern cattle grubs in the spinal canal may exhibit an adverse immune response that may result in temporary or permanent paralysis of the hind legs; animals with dead common cattle grubs along the esophagus may experience severe vomiting and bloating (Campbell, 1985).
To prevent adverse reactions of cattle being treated for cattle grubs, livestock entomologists and veterinarians have issued local cattle grub treatment cutoff dates after which treatment may no longer be recommended for the cattle grub control for the season. However, because of extensive movements or exchanges of cattle across states (and even across countries) among ranchers, stockers, and feedlot operators, these local cutoff dates may have become increasingly hard to follow. Over 90% of large (8,000 head or more) feedlots in the U.S., for example, treat all of their arriving feeder cattle for parasites (USDA-APHIS, 2012).
The recommended cutoff date for treating cattle for cattle grubs using systemic insecticides or endectocides is no later than “8–12 weeks before the anticipated first appearance of grubs in the backs” of infested cattle (Reichard, 2020). In Florida, for example, the recommended treatment cutoff date is August 31 (Kaufman and Weeks, 2019); in the Pacific Northwest, the recommended treatment cutoff date is “no later than November 30” (Arispe, 2019). Campbell (1985, 1989) recommended November 1 as the cattle grub treatment cutoff date for Nebraska cattle when using systemic insecticides or endectocides; Barker et al. (2017) recommend October 1 as the treatment cutoff date in Oklahoma.
Biology of cattle grubs
The biology of a cattle grub is quite complex; it involves metamorphosis from a fly that does not feed but lays eggs, to maggot or “grub” that feed on live cattle flesh, migration to the back of the host via the esophagus or spinal canal, creation of breathing hole and warble through the skin, dropping to the ground to become a pupa, then transformation back to a fly, all of these taking place in about a year.
- Adult female cattle grubs (other names: warble flies, heel flies, bomb flies, gad flies) lay eggs on the hair shafts of pastured cattle [Figs. 1–2]. Common warble flies lay eggs (several eggs per hair shaft) on the hair covering the legs of resting cattle; northern warble flies aggressively deposit eggs (one egg per hair shaft) on the hair covering the legs of active cattle. The persistent buzzing sound and egg-laying activities of northern warble flies can cause cattle to run around or gad in the pasture risking injuries associated with a herd stampede. Warble flies may only live for less than a week in pastures; they cannot bite or eat (no functional mouthparts) or sting (no stingers). They are active in pastures from spring to summer: February to May in Florida (Kaufman and Weeks, 2019); mid-May to midJuly in South Dakota (Lofgren et al., 1954). Warble flies resemble bumblebees in appearance [Figs. 1–2].
- The warble fly eggs attached to the hair shafts hatch into small (about 0.6–0.7 millimeter long) first instar cattle grubs within 7 days, and then start penetrating the skin through the base of the hair shaft. Cattle grubs are known to penetrate and migrate through live cattle tissues by secreting proteolytic enzymes (Scholl, 1993) that can digest cattle skin, connective tissues, and muscles, and by using their hooked mouthparts.
- After penetrating the skin, the tiny (0.6–0.7 millimeter long) cattle grubs head toward either the esophagus (in the case of common cattle grubs) or the spinal canal (in the case of the northern cattle grubs). The esophagus or spinal canal appears to be resting or staging areas for the final migration toward the back of cattle. It takes about 4–6 months for the cattle grubs to migrate from the legs to the esophagus or the spinal canal while growing up to 13–15 millimeters long (Bishopp et al., 1926; Roberts and Lindquist, 1956; Scholl, 1993). These grubs appear to congregate in the esophagus or spinal canal before making their final migration to the back area of the host cattle.
- From the esophagus or the spinal canal, the late-first instar cattle grubs (13–15 millimeter long) migrate towards the back of cattle to just underneath the skin and immediately create breathing holes through the skin. These holes will gradually become more noticeable cysts or warbles as the cattle grubs grow bigger. It takes about 8–9 months for the grubs to migrate from the legs, to the esophagus or spinal canal, then to the back of cattle.
- Cattle grubs in the warbles increase in size from 13–15 millimeters (about half-an-inch) long to about an inch long by molting twice in 6 weeks. The fully-grown cattle grubs can usually be “popped” out of the warbles with minimal effort to confirm that the warbles or cysts are indeed caused by cattle grubs [Fig. 3].
- Fully-grown cattle grubs turn dark in color, wriggle out of the warbles, then drop to the ground, seek a sheltered area, then transform into pupae [Fig. 2]. This pupal stage lasts for about 1–2 months.
- After 1–2 months under leaf litter or loose soil, the pupae metamorphose into adult warble flies [Figs. 1–2]. Adult warble flies are short-lived (less than a week); they do not feed and must mate and lay eggs on energy reserves derived from cattle during the larval stages (Scholl, 1993).
- It takes about a year for cattle grubs to complete their metamorphosis from eggs, larval stages, pupae, and then adult warble flies. It takes about 4–6 months for the cattle grubs to migrate from the initial entry point in the legs to the esophagus or the spinal canal; it takes about 8–9 months for the grubs to migrate from the initial entry point in the legs, to the esophagus or spinal canal, then to the back of cattle.
[Note: Very effective cattle anthelmintic endectocides [Tables 1–3] (look for “cattle grubs” under the “TARGET PARASITES” column) can be used to control cattle grubs in U.S. cattle. However, it is very important to consult with your veterinarian and MWI Animal Health Territory Manager before treating cattle for cattle grubs if there is reason to believe that cattle grubs are actively migrating along the spinal canal or the esophagus or gullet of the animals. Mistimed cattle grub treatments can result in severe vomiting, bloating, hind leg paralysis, or death of cattle.]
Please read the section entitled “Potential adverse response to treatment and treatment cutoff dates” in this article before initiating cattle grub control treatments.
Recommended cutoff dates for cattle grub treatment