The Hibiscus Bud Weevil (Anthonomus testaceosquamosus Linell, Coleoptera: Curculionidae)

Introduction

The hibiscus bud weevil (Anthonomus testaceosquamosus Linell, Coleoptera: Curculionidae) is a pest of China rose hibiscus (Hibiscus rosa-sinensis L., Malvales: Malvaceae). This weevil originates from northeastern Mexico and southern Texas and was first found in Florida in May 2017 (Skelley and Osborne 2018). Increased weevil population densities in 2019 and 2020 negatively impacted the hibiscus industry in south Florida during the spring shipping period, resulting in large economic losses. Florida leads hibiscus production nationally, and the majority of nursery production occurs in south Florida. Approximately 20 to 25% of plants sold from Miami-Dade County are hibiscus, where the market value of ornamental plants was 697 million (farmgate price) in 2017 (United States Department of Agriculture 2017). The hibiscus bud weevil is a regulated pest by the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (FDACS-DPI). Because of this designation, any nursery found with this weevil has to sign and follow a compliance agreement with FDACS-DPI to reduce the chance of spreading the weevil. The purpose of this article is to provide nursery owners, homeowners, and other interested people with information on this serious pest.

Identification

The hibiscus bud weevil is a beetle (Order Coleoptera) in the weevil family (Curculionidae). It belongs to the Anthonomus squamosus species-group of the Anthonomini tribe. This species-group is characterized by weevils predominantly covered with scales (Clark et al. 2019) (Figure 1). The adult body length is 2.5 - 2.7 mm and the snout or beak is approximately 1 mm long.

Females can be distinguished from males using two characters, one in the protibia (the fourth segment of the first pair of legs) and one in the abdomen. In the protibia, females have an apical uncus and subapical, inner-marginal prominence (mucron) (spur-like structure on the inner side of the tibia) (Figure 2a), which is absent in males (Figure 2b). Additionally, the posterior part of the fifth ventrite (margin of the fifth abdominal segment) in females is straight (Figure 3a right) and in males curved (Figure 3b right). Moreover, females (Figure 3a left) have a small pygidium (the last body part, exposed when the elytra is at rest) in comparison to males (Figure 3b left). The validity of these characters was confirmed by dissecting weevil genitalia (Figure 4).

Eggs (Figure 5) are white when newly deposited and turn yellow as they mature. The larvae of hibiscus bud weevil are transparent to yellow in color, have a well developed head capsule and lack thoracic legs (Figure 6). Larval size varies with the size of the flower bud they are within. In general, larger flower buds have larger larvae.

Host Range and Damage

Weevils belonging to the Anthonomus squamosus species-group are associated with plant species in the families, Asteraceae or Malvaceae. The hibiscus bud weevil, A. testaceosquamosus has been associated with multiple plant species, all from the family Malvaceae (Table 1).

Adult weevils feed primarily on buds, stems, and to a lesser extent on leaves of hibiscus. The females oviposit in flower buds and larvae develop inside the bud, causing bud drop prior to flowering. Symptoms include holes in stems and unopened buds (Figure 7) and severe bud drop in high pest densities. Feeding damage on the leaves is not apparent. In south Florida nurseries, pink and yellow hibiscus varieties appear to be more susceptible to hibiscus bud weevil than red and other varieties (Table 2). The pink variety ‘Painted Lady’ and the yellow variety ‘Sunny Yellow’ are reported to be the most susceptible varieties. The red variety ‘President Red’ is reported to be the most resistant.

In Florida, another species from the Anthonomus squamosus species-group, Anthonomus rubricosus, has been reported infesting cotton and hibiscus plants (Clark et al. 2019; Loiácono et al. 2003). However, there are no reports showing its establishment in Florida. This weevil is similar in size with the hibiscus bud weevil, but it is brown color. The Anthonomus genus includes several pest species of great agricultural importance, such as the cotton boll weevil Anthonomus gandis. The most economically important Anthonomus pests reported in Florida are the pepper weevil Anthonomus eugenii and the acerola weevil Anthonomus macromalus. The pepper weevil attacks plants of the family Solanaceae, particularly peppers (Capsicum spp.) (Capinera 2002), while the acerola weevil attacks Barbados cherry (Malpighia glabra, Family: Malpighiaceae) (Hunsberger and Pena 1998).

Bud drop can also be caused by infestation with the hibiscus bud midge (Contarinia maculipennis, Diptera: Cecidomyiidae), which can be mistaken for hibiscus bud weevil damage (Mannion et al. 2006). Both pests can infest the same hibiscus plant; however, they will rarely infest the same flower bud. Buds infested with the hibiscus bud midge will have multiple white to yellow maggots (larvae) inside them that jump when disturbed. Midge maggots lack a head and legs and need to exit the bud to pupate in the soil while the weevil larvae have true heads and pupate within the buds (Figure 8; Figure 9).

Biology

Adult female weevils oviposit 3-5 eggs in a single hibiscus flower bud close to the anthers (Figure 4). Emerged larvae feed on pollen and remain in the flower bud until reaching adulthood. Due to the high incidence of larval cannibalism, not all eggs will develop into an adult weevil; however, multiple weevils might exit one flower bud. At 26.7°C (80°F), eggs hatch within 2-3 days. The larval stage has three instars and can last, on average, 10 days. The pupal stage (Figure 10) lasts 2.9-4.2 days. Development from egg to adult can range from 12.8-16.3 days and survival has been observed up to 90%. Adult longevity ranges from 13-169 days, with the males living longer than the females. When fed solely on pollen, adults can survive up to 52 days. Adult weevils can survive on average for 28 days without food when they have access to water, and 16 days without water. The sex ratio is 1:1 females to males.

Very low or high temperatures appear to be detrimental to the development of this weevil. In laboratory experiments at the University of Florida, at 10°C (50°F) no eggs hatched, while at 15°C (59°F) eggs hatched 12 days after oviposition, but the larvae did not feed and eventually died. Similarly, at 38.8°C (93 °F), eggs hatched within 5.6 days, but none of the larvae managed to pupate. In south Florida, the peak activity of this weevil has been observed from March through June with lower populations from September until February.

Development of Pest Management and Monitoring Techniques

Integrated pest management programs targeting the hibiscus bud weevil will involve a combination of cultural practices, sanitation, chemical, and biological control. Crop rotation with non-host species has been recommended for breaking population cycles (Bográn et al. 2003). Sanitation includes systematic collection and destruction of all dropped buds from the ground. Although sanitation is labor-intensive, it has been proposed as the most efficient management practice against this weevil since dropped buds contain weevils that will continue to infest new plants (Bográn et al. 2003). Currently there are no insecticides specifically registered for the hibiscus bud weevil in Florida, but growers can legally use insecticides that are registered for use in nurseries. FDACS-DPI has attached a list to the compliance agreement with recommended insecticides that can be used against the hibiscus bud weevil. Efficacy trials of several insecticides registered for weevils/beetles and other specific species on ornamental plants are presently underway. No natural enemies of A. testaceosquamosus have been reported; however, entomopathogenic fungi and nematodes are being explored as potential biological control agents.

Several species within the Anthonomus genus are attracted to a group of commercial lures that consist of male aggregation pheromones and host plant volatiles (Tumlinson et al. 1969; Eller et al. 1994; Innocenzi et al. 2001). There are four components of the synthetic male aggregation pheromone, also known as Grandlures (I-IV). Pheromone traps broadly used for other Anthonomus species are also being explored as a pest monitoring tool. In Texas, pheromone traps developed for the cotton boll weevil (A. grandis) were evaluated for the detection of the hibiscus bud weevil adults, but without success (Bográn et al. 2003). The authors, however, argue that this might be due to the early deployment of the traps in the season. Yellow sticky traps are the most attractive traps for several Anthonomus species (Cross et al. 2006; Szendrei et al. 2011; Silva et al. 2018). Field trials are currently being conducted to test for attraction to the boll (A. grandis) and pepper weevil (A. eugenii) lures and to identify the best trap to catch adult hibiscus bud weevils. An IPM program implementing all the above strategies is needed to manage A. testaceosquamosus populations in Florida and decrease the economic damage caused by this species.

References

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