By Dave Biddinger
In last the five years or so growers have noticed increases in what appears to be plum curculio damage to apple fruit.
Typical plum curculio (PC) injury appears as a c-shaped or fan-shaped scar that is heavily russeted (Figure 1). Researchers at the Penn State University Fruit Research and Extension Center in Biglerville have also seen an increase in this damage both in research plots on the station and in trials in grower orchards. We have generally attributed this increase in apparent PC damage to the loss of registration of our most effective PC products including Guthion® and Calypso® and a general reduction of insecticide sprays at petal fall. While products like Avaunt® and Imidan® remain effective for PC control at petal fall under the lower PC pressure we typically have in Pennsylvania, they are generally more expensive than the products they replaced and appear to be used less routinely by many growers.
We have recently noticed that some of the damage attributed to plum curculio was atypical and, while still heavily russeted, consisted of large raised round bumps on the fruit surface (Figure 2). This type of damage is most consistent with the type of injury caused by a pest closely related to the tarnished plant bug (TPB) called the apple green bug (AGB). AGB (Lygocoris communis) is an old-time pest of apple and pear from Michigan, New England, and Ontario that largely disappeared from fruit orchards in these regions after the introduction of organophosphate insecticides such as Guthion® back in the 1960s. We have been concerned that, with the loss of many of the older contact insecticides, that AGB and other pests could be returning to our orchards. A study was thus initiated in the spring of 2017 in several grower and research farm apple orchards, 1) to determine extent of this 'atypical' PC damage, 2) if PC numbers were really increasing, and 3) if AGB or some other related pest was causing this injury and becoming more abundant in both commercial or in research orchards that have higher pest pressure.
Pest Survey Method
White and yellow sticky traps known to be visually attractive to TPB and fruit flies were used to monitor for AGB or other related plant bugs from May 3 through September 20 in three commercial grower orchards and in two research orchards at FREC. All orchards monitored had multiple cultivars consisting of 'Golden Delicious' and 'Delicious,' 'Gala,' and 'York' and each location had two traps of each color for a total of 20 traps at the 5 locations.
In addition to the visual traps, a common method for collecting PC using beating trays was utilized to collect insects once weekly from May 10 through August 2, 2017 in the same three commercial grower locations and two research blocks that the visual traps were placed. Six to eight trees were evaluated at each of the five locations. Beating tray samples were taken from a tree row that was two rows beyond the visual traps and by sampling every 3rd tree in that row. This was done to minimize any effects on the pest populations that could have been caused by the visual traps or from disturbance in adjacent trees when using the beating trays. At each tree, we chose five limbs approximately four feet above the ground. We placed a 3' square with a fine mesh screened floor beneath the limb and hit the limb 3-5 times with a padded 1" dowel rod. All insects on the mesh floor were collected and placed in a vial with alcohol and returned to the lab for future identification to determine the levels of PC, AGB, or some new pest that might be present. Fruit harvest evaluations were made by examining a total of 1,000 fruit from four replicated untreated trees that were part of pesticide evaluation trials in three research blocks at the FREC. Fruit injury was categorized into typical PC injuries (Figure 1) and damage we thought was from AGB or a related pest.
Results and Discussion
Damage by PC in commercial apple orchards in this region is seldom above 1% and rarely approaches 5% even in the unsprayed control blocks at FREC. In Michigan and New York, PC fruit damage typically exceeds 70-80% if not controlled. In our2017 fruit evaluations of damage seen in unsprayed control research plots at the Fruit Research and Extension Center, approximately 10-50% of the damage normally attributed to plum curculio was of this atypical type similar to that known from AGB (Figure 3).
Seasonal beating tray samples and sticky traps, however, failed to detect any PC or AGB stages. These sampling methods did find significant numbers of a closely related species known as the Mullein Bug (MB - Campylomma verbasci) which can cause superficial apple fruit damage which appears like small pimples, but is much different from the fruit injury seen in Figure 2. The MB is also a predator of predator of mites, aphids, and scale most of the year so can also be a beneficial insect. Both beating tray samples and the sticky traps in the FREC orchards, however, found another new plant bug closely related to AGB that we had never observed previously. This new bug was not found in either type of trap in grower orchards.
After a review of the literature and finding the appropriate taxonomic keys, this new plant bug was identified as Phytocoris conspurcatus (Figure 4). This species belongs to a very diverse group with hundreds of species in the genus, but a review of the literature initially seemed to indicate that it was just a generalist predator that fed on mites, aphids, scale and other small soft-bodied insects. In more recent economic literature, however, it was found that at least one species of Phytocoris also causes economic damage to pistachio nuts during part of its lifecycle, in addition to being a predator. It now appears that, in addition to being a predator, at least some Phytocoris species must feed for part of their lifecycle on plant tissue to be able to reproduce. Most Phytocoris species overwinter as eggs inserted into plant tissues. These eggs hatch early in the season, which would explain why we found only nymphs early in the season (Figure 5). It is also thought that most Phytocoris species have multiple generations, but this was difficult for us to ascertain since we only caught a few adults and the nymphs were found only in the early season. We now know that some Phytocoris are attracted to lights at night, so will try this in the future. A sex pheromone for Phytocoris relativus attacking pistachios has been identified, but does not appear to be commercially available for monitoring.
It is possible that P. conspurcatus could be a new pest causing this atypical apple fruit damage that is sometimes categorized as PC damage. Due to the similarity to AGB fruit injury, we had originally thought it was due to that pest. Intensive sampling in both grower and FREC orchards, however, failed to find a single AGB adult or nymph using techniques that were designed specifically to find it. P. conspurcatus now seems to be the prime suspect for the type of injury found in Figure 2. From the literature on P. conspurcatus, it is thought to have multiple generations in our area and the adults have been found during most of the season. In our samples, we found peak numbers of nymphs within a couple of weeks after petal fall on apple and speculate that fruit injury to apple fruit from the nymphs would occur at this time as it does with the closely related MB and TPB. We plan to collect live nymphs and adults using the beating tray method in the 2018 season and then cage them on small developing apple fruit to see if we can replicate this damage. Light trapping will also be used to better determine adult phenology during the season.Click here to see more...