The biology of Rhynchaenus maculosus provides information and implications for the integrated management of this emerging pest
Life cycle and life history
R. maculosus undergoes complete metamorphosis and develops through four distinct developmental stages, namely egg, larva, pupa and adult stages (Fig. 2).
This pest has one generation per year with univoltine and overwinters as an adult in leaf litter or topsoil under the canopy of trees, or in curled and withered leaves of oak in Liaoning, Jilin and Heilongjiang provinces, respectively. Active overwintering adults (spring adults) emerge from hibernation sites at different times depending on distribution locations (Fig. 3). In Liaoning and Jilin Province, spring adults end hibernation and emerge from their overwintering sites in mid-April to feed on the tender buds and leaves of the host oak tree. The peak period of spring adult activity in Liaoning and Jilin is almost synchronized, both from April 28 to May 2 and from May 13 to May 15 in Heilongjiang. After the emergence peak, there is a sharp decrease in the number of spring adults until the adults gradually disappear in early May in Liaoning and Jilin, and late May in Heilongjiang with a total duration of 25 to 28 days in all three locations (Fig. 4).
Spring adult females lay their eggs on the second day after mating. The eggs take about 4-5 days to hatch into larvae. Newly hatched larvae burrow into oak leaves in late April in Liaoning and Jilin, and mid-May in Heilongjiang, and cause leaf mining damage in the mesophyll throughout the larval stage. The larvae reach the mature stage after 20 to 23 days and gradually pupate from mid to late May in Liaoning and Jilin, and late May in Heilongjiang. Adults of the next generation (summer adults) emerge in mid-May in Liaoning and late May in Jilin Province and continue to feed on oak leaves until early September, when they are beginning to look for wintering sites in both Liaoning and Jilin. In Heilongjiang Province, spring adults appear in late April to feed, mate and lay eggs, then hatch and feed on leaf mesophyll in the larval stage. The larvae mature and pupate from late May to mid-June. Summer adults appear in early June and continue to feed on oak leaves until they stop feeding and begin to overwinter in mid-August (Figure 3).
The egg has a smooth surface and an elliptical shape with a long diameter of 0.5-0.6 mm and a short diameter of 0.3-0.4 mm. The newly laid egg is milky white and gradually turns yellow before hatching. Eggs are laid in mid-April in Shenyang and Dandong, Liaoning Province, Yongji and Dunhua, Jilin Province, and early May in Qiqihar and Jiamusi, Heilongjiang Province, with a duration of 4-5 days. The majority of individually laid eggs are scattered near the veins on the underside of the leaves, the minority on the upper side (Fig. 5). The egg load of an adult female ranged from 7 to 13, and the number of eggs laid by an adult female was 5 to 10 during her lifetime.
The larva is milky white, flat and footless with obvious segmentation. Mature larvae are 5.5 to 6.2 mm long. Newly hatched hoppers emerge in late April in Shenyang, Dandong, Yongji and Dunhua, and in early May in Qiqihar and Jiamusi. After hatching, the tiny larvae burrow directly into the epidermis of leaves and feed on the mesophyll inside oak leaves, forming irregular blister-like, yellow-brown spots. As the stage increases, the larvae gradually feed towards the edges of the leaves, enlarging the spots, the largest diameter of the leaf spot reaching 3–4 cm. Larvae gradually mature in mid-May in Shenyang and Dandong, Liaoning Province, Yongji and Dunhua, Jilin Province, and in early June in Qiqihar and Jiamusi, Heilongjiang Province (Fig. 3). In the mature larval stage, a hollow blister-like spot (pupal chamber) forms on the edge of the oak leaf (Fig. 6). The larval stage lasts 20 to 23 days with three instars.
The nymph is exarate type and spindle-shaped with sparse spines. Mature larvae pupate in the pupal chamber at the edge of the leaves with a pupal duration of 6-8 days.
The adult R. maculosus is fusiform and covered with dense gray hairs. The mouthparts are snout-shaped and opisthognatous type. Adults have a pair of well-developed hind legs that allow jumping movements.
The period of appearance of the adults in summer varies according to the different areas of distribution. Summer adults emerge in mid-May, late May and early June in Liaoning, Jilin and Heilongjiang provinces, respectively (Fig. 3). Adult hatching occurs in the pupal chambers (blister-like spots) from 11:00 p.m. to 1:00 a.m., with a 98% emergence rate in the wild. After emergence, the summer adult does not immediately exit the pupal chamber but remains inside for an additional 1–2 days before biting through the pupal chamber. After emergence, summer adults continue to damage oak leaves by feeding on the upper epidermis and leaf tissues of oak leaves, forming dense, irregularly shaped spots (Fig. 7a). The adult R. maculosus is characterized primarily by well-developed meta-legs with strong jumping ability. When interfered with, adults show a strong ability to jump about 50 cm at a time and a weak ability for long-distance flight. Summer adults stop feeding and overwinter in leaf litter or topsoil under the canopy, or in curled and withered canopy leaves in early September in Liaoning and Jilin, and in mid- August in Heilongjiang.
After overwintering, active spring adults (spring adults) emerge from hibernation sites to feed, mate on winter buds or tender leaves. Copulation lasts about two hours. Interestingly, the symptom of damage caused by spring adults is different from that caused by summer adults. Spring adults cause only slight damage to oak trees by piercing the tender buds (or leaves) and sucking sap with snout-like mouthparts, forming tiny irregular holes in the tender leaves (Fig. 7b). The occurrence of an adult R. maculosus has a total duration of 315.6 ± 3.6–336.4 ± 3.2 days (Mean ± SD), including a feeding period of 110.4 ± 2.0–115.0 ± 2.5 days (Mean ± SD) and a wintering period of 205.2 ± 2.3–221.0 ± 1.6 days (Mean ± SD).
The adult R. maculosus has a strong ability to tolerate hunger. At temperatures between 20-25°C, spring adults can survive 3-6 days without food or water, while summer adults a little longer, 4-8 days.
Larval stage determination
Analysis of the multi-peak normal fit distribution showed three obvious peaks in head capsule width (HCW) indicating three instars throughout the larval stage (Fig. 8a). Each intersection point of the two normal fit curves reveals a two-stage cutoff point.
The health workers of R. maculosus larvae range from 300 to 650 μm (n = 401). The Crosby indices for mean values of HCWs measured at different stages were less than 0.1, and the coefficients of variation for morphological data measured at each stage are less than 15% (Table 1), which resulted in the determination of three instars during the larva arrange. There is a highly significant relationship in the linear regression between the natural logarithms of the mean HCW values and the stages (n = 3, R2= 0.994, F = 170.82, p= 0.05) (Fig. 8b). The results strongly supported the determination of three stages within the entire larval stage of this pest.
Distribution and host plant
The investigations revealed that R. maculosus is distributed in the provinces of Heilongjiang, Jilin and Liaoning, China (Fig. 1). This pest is oligophagous in food and feeds only on oak (genus Quercus) in the larval and adult stages, such as Quercus wutaishanicaand Q.mongolica . Analysis based on leaf damage rate revealed no significant difference between Q.wutaishanicaand Q.mongolica , suggesting no host preference between the two host plant species during the larval period. There was a remarkable increase in the leaf damage rate of both Quercus species from 2017 to 2021, while no damage on Q. acutissima has yet been found (Fig. 9).
A total of three species of parasitoids (Hymentoptera) were observed and collected from larvae or mature pupae in pupal chambers, including Braconidae sp. and Eulophidae sp. in Liaoning, and Ceraphronidae sp. both in Jilin and Heilongjiang (Fig. 10). Most of the maggots of these parasitoids emerge in late May with the R. maculosusthe larvae develop to their mature stage. Data from 2017 to 2021 revealed average parasite rates for Braconidae sp. of 1.3%, Eulophidae sp. of 2.0% and Ceraphronidae sp. 3.7% in Jilin and 4.7% in Heilongjiang, respectively.