Observations on the Biology and Control of the Hawthorn (Two-Circuli) Mealybug, Phenacoccus Dearnessi (King)

Life History Observations

All reported studies were made at the Plant Environmental Research Center (PERC) on the campus of Colorado State University (CSU) in Fort Collins in a planting of mixed Crataegus cultivars that have been established at the site since the early 1980s. The most complete life history observations were made during 1998, at approximately weekly intervals, beginning on 16 March 1998 and continuing until 15 November 1998. Supplementary observations have been made annually from 1994 through 1999.

All insects found feeding on hawthorn mealybug were noted and, if immature, were reared to the adult stage for identification. Also, samples of infested twigs were periodically removed to the lab to determine if any parasitoids were present.

Evaluations of Cultivar Susceptibility

During three years (2 June 1994, 3 June 1997, and 28 May 1998), observations were made on the relative infestation of 14 hawthorn species and cultivars at the PERC site. Evaluations were made by assessing the average number of adult females established on the terminal 45 cm (18 in.) of hawthorn branches. A 0-to-4 scale rating the intensity of infestation was used in all evaluations (0 = no mealybugs present, 4 = 200 or more mealybugs per branch present).

Insecticide Control Trials

Since 1994, insecticide evaluations have been conducted, of which only 3 of the most recent are described in this paper. Standard methodology involved use of individual branches as the experimental unit. Plot design was a randomized complete block (RCB) with 4 or 5 replications. Individual branches were sprayed to point of run-off and in 1998 and 1999 additional trials examined the effect of treatment timing. Evaluations were done by counting all mealybugs on the terminal 30 cm (12 in.) of the treated branch.

Life History Observations

Hawthorn mealybug in Colorado has 1 generation per year and overwinters as nymphs. Wintering sites are concentrated almost entirely in bark crevices of the trunk, with a small percentage on larger branches. Approximately 10% of the overwintered females were observed to have begun first spring activity on 20 March 1998. This spring activity was correlated with onset of bud swell.

The life cycle is somewhat complex because a series of migrations occurs on its host to feeding sites on either foliage and twigs at different times, oversummering diapause sites, and overwintering sites (Miller and Appleby 1971, Kosztarab and Kozar 1988). In 1998, first migration from overwintering sites on trunks and branches was observed on 14 April. Migration to the ultimate feeding sites at the bases of buds and in twig crotches gradually increased with warmer temperature and bud development. Peak migration was observed in the beginning of May and ceased at the end of the month.

Adult males, which are winged, were first observed in 1998 on 8 April among females still remaining on the overwintering sites of the trunk; subsequent mating extended over the next month. Instar 1 stages (“crawlers”) of this ovoviviparous species were first observed on 27 May. Peak egg production occurred around 10 June, at which time females were observed to average 180 maturing eggs. However, egg production and crawler emergence was very extended with a few new crawlers observed 14 October.

Throughout June, crawlers were plentiful on leaves. Beginning on 25 June, some were observed to migrate to cracks in the bark of twigs and small branches and go through a summer diapause. During early fall, beginning 8 September, crawlers returned to the leaves to feed. Subsequently, migration to overwintering sites on trunks was first noted 9 November. After settling into overwintering sites, nymphs produced a small amount of a mealy secretion, giving infested trunk surfaces a speckled appearance.

Notes on Biological Controls

Three species of predaceous lady beetles—Adalia bipunctata (L.), Coccindla septempunctata (L.), and Hippodamia convergens (Guerin-Meneville)—were observed to feed heavily on the overwintered stages of hawthorn mealybug as they emerged from overwintering sites and migrated to twigs. Feeding was observed to begin by late March and peaked during early May, in association with peak migrations. All 3 species of lady beetle, as well as larvae of the green lacewing, Chrysoperla carnea (Stephens), readily fed on P dearnessi crawlers when specimens were provided in the lab, but feeding by the later was not observed in the field. In the field, predation by lady beetles ceased on 20 May, when the settled P. dearnessi females began to swell with eggs. Predation of adult females with maturing eggs has not been observed.

A pilot study in 1995, involving releases of the mealybug destroyer (Cryptolaemus montrouzieri) was not successful. Although there was limited feeding on hawthorn mealybug in the laboratory, there was no observed reduction of hawthorn mealybug numbers on plants where adults were released.

No parasitoids have ever been observed to attack hawthorn mealybug, although efforts have been made annually to attempt to identify their presence.

Evaluations of Cultivar Susceptibility

Hawthorn species and cultivars appeared to show differences in susceptibility to P. dearnessi (Table 1).

All C. laevigata (English hawthorn) cultivars were highly susceptible, and 1 cultivar (‘Toba’) died and had to be removed during the study, following consistently high annual levels of infestation. C. phaenopyrum ‘Cordata’ remained nonsusceptible to infestation throughout this study. Crataegus × mordenensis ‘Snowbird’, C. succulenta macracantha, and C. ambigua also showed substantial levels of resistance.

Insecticide Trials

Some insecticides—permethrin (Astro), imidacloprid (Merit), the related Novartis compound 293,343 (thiomethoxam), and abamectin—appeared to perform better than others (Table 2, Table 3, and Table 4). The addition of horticultural oil (SunSpray) seemed consistently to improve control. Pymetrozine (Relay, Fulfill) provided only modest control.

Most important, however was proper timing of treatments. Applications made after females have migrated to twigs and have begun to produce eggs can be expected to provide poor control, regardless of the insecticide used. This was indicated in the 1998 trial (Table 3) in which there was a fourfold decline in control when Merit applications were delayed 12 days. Furthermore, in a 1999 trial (Table 4), control with Astro declined as applications were delayed from 15 May to 1 June. It is therefore recommended that applications be made to coincide with the period when overwintering stages move from trunks and larger branches to the twigs.