Factors affecting pest abundance in the orchard
When planning treatments for insects attacking pecans, growers should account for bottom-up and top-down factors.
The host range of insect pests attacking pecan spans the range of specialist herbivores attacking only pecan (i.e., pecan nut casebearer) to generalist herbivores attacking a wide array of host plant species (e.g., stink bugs). The same is true for most plant species; they have specialist and generalist pest species attacking them. But why is it that stink bugs, for instance, feed on numerous plant species but the pecan nut casebearer cannot feed on other nut-bearing tree species?
The answer to this question is complex and incomplete. Two factors shaping the host plant range of an herbivore concern the host plant itself and the herbivore’s natural enemies. Ecologists have debated for years which factor, host plant (‘bottom-up’ regulation) or natural enemy (‘top-down’ regulation), puts more selection pressure on the herbivore.
Host plant (i.e., bottom-up) factors include plant chemistry and physical attributes that limit herbivory. From a practical agricultural/horticultural view, aspects of plant chemistry are generally exploited by breeding programs to select for plant traits that lower herbivory. This is because plants produce what are broadly known as secondary plant compounds—chemicals generally not having a physiological function within the plant and are solely for defense. The chemical defenses employed by plants vary between plant species and include substances such as alkaloids, tannins, coumarins, terpenoids, and many others. Specialist herbivores attacking a single, or few closely related, plant species have overcome this defensive mechanism of the host plant. Generalist herbivores attacking many unrelated plant species typically have physiological traits to overcome the defenses of the many plants they feed on.
For example, the recently established brown marmorated stink bug does quite well feeding on plant species never before encountered in its native range in Asia. This is because it is adept at dealing with these new plants’ chemistry, which is somewhat similar to host plants’ in its native range. This broad acceptance of plant chemistry allowed it to establish in North America and other parts of the world when transported there.
In contrast, it is highly unlikely that the pecan nut casebearer would ever become a pest of other plant species if transported out of North America. Even in North America, it only attacks pecan. For as many reasons as I could list why it is better to be a specialist herbivore, a competing list could be made for why it is better to be a generalist herbivore.
Natural enemies (i.e., top-down) limit herbivory by preying upon the herbivores. Just like the herbivores, there are natural enemies that specialize in attacking one or a few species of herbivores and those that attack many species of herbivores. The pecan aphid parasitoid Aphelinus perpallidus attacks the blackmargined pecan aphid and the yellow pecan aphid but not the black pecan aphid. A broad range of acceptable prey species allows many lady beetles and lacewings to feed on all three pecan aphid species in addition to feeding on many other aphid species attacking other plants.
If these natural enemies are adapted to taking out the herbivores of pecan, why do we still have to contend with these pests each year? Quite simply, if they were efficient enough to drive the prey to extinction, they would drive themselves to extinction and the herbivores are not just there for the taking. The herbivores actively use many different tactics to avoid predation.
Consider the silk webbing constructed by the fall webworm, the stinging hairs of numerous caterpillar species, and the toxic cardenolides of the milkweed plant that the monarch butterfly caterpillars commandeer for their own use. Additionally, many natural enemy species disperse across landscapes based upon prey availability, and it may take time for a pecan aphid to build a prey population that attracts and retains natural enemies. Unfortunately, the lag in natural enemies responding and controlling a pest population often allows the pest to reach economically damaging levels, thus requiring intervention.
When looking at examples of bottom-up and top-down regulation in pecan, we may consider which is more important to the survival of the black pecan aphid—the foliage it feeds on or the lady beetle? The interaction of the black pecan aphid with the pecan host plant demonstrates aspects of both bottom-up and top-down population regulation. The relative difficulty the black pecan aphid has manipulating the foliage of some pecan cultivars for feeding (i.e., breaking down chlorophyll) during the early season demonstrates the bottom-up effect of the host plant against the pest.
During the early season, differences in cultivar susceptibility to the black pecan aphid damage are readily observed in orchards, cultivars with or without chlorotic damage. In fact, it is hard to rear the black pecan aphid on ‘Pawnee’ foliage early in the season, whereas the aphids do well on ‘Schley’ foliage at this time of year. During the late season, however, the black pecan aphid performs similarly well on both cultivars, as the apparent defense by the resistant cultivars breaks down (hence the reasoning for using gibberellic acid to enhance plant defense for black pecan aphid management).
As for black pecan aphid survival, it gets hit from both ends—the tree exerts bottom-up control against the black pecan aphid, and at the same time, natural enemies exert top-down control. Because the black pecan aphid requires time to manipulate the host plant for feeding, this exposes it to potential predation for a longer period.
To counter this exposure risk to natural enemies, a proportion of the black pecan aphid nymphs will move to the upper side of the pecan leaf to settle down and begin feeding. Predators such as lady beetles and lacewings spend significantly less time searching for prey on the upper leaf surface compared to the lower leaf surface, thus allowing black pecan aphid nymphs on the upper leaf surface a higher chance to survive predation.
So, which is more important to the black pecan aphid, bottom-up or top-down regulation? It would seem to depend on the pecan cultivar, whether it is early or late in the season, and the relative abundance of aphid predators at the time.
Top-down regulation of a pecan herbivore was clearly demonstrated with PNC during 2018 and 2019. From past trapping experiments, we know that PNC capture is greater the higher the trap is placed in the pecan canopy, and this was true during both 2018 and 2019. In fact, the capture of male PNC in traps placed in the pecan canopy at 5, 20, 35, and 50 feet above ground each year revealed that PNC abundance was remarkably similar in these same orchards each year. With populations being similar both years, it would be reasonable to expect similar levels of damage in these untreated orchards each year. This was not the case. Damage during 2019 was significantly greater than during 2018.
In the southeastern U.S., the amount of damage by PNC is typical of what is shown in the figure for 2018 (as a side note, notice that damage at different heights in the tree was similar even though most male PNC were captured higher in the tree). Factors that could have played a role in damage discrepancy between years could have included misapplication of insecticide (but none were applied) or even a skewed female bias in the PNC population with a higher ratio of females to males during 2019 (highly unlikely).
What was different between years was the apparent abundance of hemipteran predators, specifically members of the genera Orthotylus and Deraeocoris. These bugs may occasionally derive moisture/nutrients from plants, but there is no doubt regarding their propensity to prey on just about any insect they can subdue.
In the Southeast, during a normal year, these predators are plentiful throughout the pecan canopy in the spring. However, during 2019, when sampling from a lift in the pecan canopy, these predators were noticeably rare. I believe the scarcity of these predators led to decreased predation on PNC eggs, thus leading to increased nut damage from PNC during 2019. It would be valuable to know if greater PNC damage to pecans in western pecan growing regions than in the southeastern growing region is related to the abundance of these predators in pecan during the spring. If this observation is correct, then sampling/scouting plans that include these predators would be of benefit to understanding potential PNC damage.
In conclusion, factors affecting pest abundance in pecan orchards include both the tree (i.e., cultivar) and the natural enemies. Being aware of the innate pest susceptibility of cultivars in your orchard and how their susceptibility may change over the season allows for better pest scouting. Additionally, scouting that includes natural enemy assessments can be used to enhance pest control. And if the natural enemies cannot or do not provide economic control of pests, you have the scouting report to make an informed decision for insecticide treatment.